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VES soundings Kajiado Town Technical Interpretation and inversion results of all available VES soundings in and near Kajiado Town up to
date April 2017
by Harry Rolf , SamSamWater Foundation, Netherlands
November, 2017
SamSamWater Foundation http://www.samsamwater.com/index.php
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Introduction This document comprises the technical interpretations (inversions) of all VES soundings -as far as available up to date - in and
around Kajiado town.
It includes:
• 22 VES soundings from fieldwork for ISGEAG and Sponge city projects February 2017 and April 2017.
• Re-interpretation of VES Soundings as found in existing survey reports .
Unfortunately only a very limited number of seven (7) survey reports could be made available of which 3 are lacking
measured field data. There must have been many more geophysical surveys, but the availability reports often
appears to be a major organisational challenge in Kenya.
Each interpretation includes all relevant administrative data (location, reference to report, etc.), the field measurements and the
interpretation (inversion) results using SchlumBG software1. In case of the existing survey reports it concerns a re-interpretation
of the data and a comparison to the interpretation as given in the report.
The inversion, layer-model results are given with their 90% uncertainty range as calculated in the SchlumBG software. Some of
the existing survey reports do not comprise the measured data; in those cases the measurements are generated by forward
modelling but the re-interpretation is questionable.
Reading guide First, the locations of all VES soundings are presented on a map (next page Figure 1) and the inversion (layer model) results are
summarised in Tables 1,2 and 3.
Next, for each VES sounding a short (2-3 pages) interpretation report is included. These reports are presented in three blocks:
1) all soundings of the 2017 Sponge City/ ISGEAG surveys (up to page 53)
2) the re-interpretations of soundings as found in existing survey reports that include the measured data and
3) the soundings from reports that lack the measured data.
Interpretation report content Each VES interpretation report contains 6 sections:
General:
• giving the aim, date and location (map) and reference to the survey report2
VES information:
• WGS84 coordinates, line direction, electrode reach, VES array, land elevation and estimated depth to groundwater
Measured data:
• Table and graph of measurements and data remarks
VES interpretation:
• giving the equivalent inversion model(s) having the best fit with the minimum nr. of layers
Results:
• giving the interpreted layer model parameters (depth of the top of the layer and layer resistivity) including their 90%
confidence range
• note: the layer depth gives the depth of the top (!) of the layer
• ‘basement’ is defined when layers have resistivity > 1000 Ωm
Major conclusion
1 SchlumBG for Windows, C.J. Hemker and V.E.A.Post, http://www.microfem.com 2 For the Sponge City soundings the reader is referred to ‘Geophysical surveys for Sponge City project Kajiado, Kenya;
SamSamWater, Harry Rolf, November 2017’
For the ISGEAG soundings in Kajiado a research report is being prepared by Acacia Water (probably available in February 2018)
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Figure 1 VES locations in and around Kajiado Town. The ‘existing survey report’ locations are given by name, the
ISGEAG/Sponge city VES locations are numbered.
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TABLE 1 summarized results of Sponge City/ ISGEAG VES soundings
Sponge City/ ISGEAG soundings February and April 2017
coordinates (WGS84) Electr RMS REMARKSname/ VES LAT LONG Elev sp. error TOP Rho confidence TOP confidence Rho confidence TOP confidence Rho confidence TOP confidence Rho
location nr. (S) (East) (mas) AB/2 (m) x10-2 (m) (Ωm) (m) (Ωm) (m) (Ωm) (m) (Ωm)
Gulley C VES1 1 01°49'39.5'' 36°46'31.2'' 1690 320 3.6 0 26 (23 - 29) 2 (1 - 4) 50 (40 - 80) 15 (12 - 30) >>>1000 discontinuities in meas.
Gulley C VES3 3 01°49'26.8'' 36°46'27.8'' 1710 320 3.5 0 90 (80 - 120) 1.7 (1,5 - 2) 30 (23 - 40) 50 (40 - 60) discontinuities in meas.
Gulley C VES4 4 01°49'34.7'' 36°46'22.9' 1685 320 9.2 0 2 (1 - 5) 2.5 (2-8_ 50 (30 - 1000) 4 < 8 >>1000 discontinuities in meas.
Gulley C VES7 7 01°49'33.0'' 36°46'24.3' 1690 400 1.2 0 30 (25 - 60) 2.5 (2 - 3) 300 (290 - 320) 90 (80 - 100)
Gulley C VES13 13 01°49'34.4'' 36°46'23.4' 1682 63 5 0 10 (6 - 12) 4.5 (4 - 5) >>1000 on gulley river bed
Gulley C VES16 16 01°49'31.9'' 36°46'25.2'' 1687 500 3 0.1 250 (220 -280) 11 (9 - 15) 57 l(arge) 15 (large) 500 (250-1000) 76 (40-160) (1000-1500) some discontinuity
VES2 (road) 2 01°49'55.1'' 36°46'22.0'' 1720 320 unreliable sounding unreliable (reason ?)
Gulley B VES9 9 01°50'17.3'' 36°46'17.2'' 1710 500 1.2 0 290 (250 - 300) 1 (1 - 2) 145 (130 - 150) 9 (8 - 13) 90 (85 - 110) 55 (50 - 70) (700 - 900)
Town VES5 5 01°50'21.7'' 36°46'59.1'' 1720 130 1.7 0 170 (150 - 200) 0.5 (0 - 1) 65 (60 - 70) 9 (8 - 12) 190 (150 - 300) ? >40 short AB line
Town VES6 6 01°50'24.1'' 36°47'28.8'' 1735 130 1.6 0 5 (1 - 12) 3 (2 - 4) 43 (40 - 55) 45 (20 - 90) short AB line large disc.
Town VES8 8 01°50'27.8'' 36°47'25.3'' 1732 63 1.8 0 53 (52 - 56) 11 (9 - 13) 100 (90 - 110) ? >75 short AB line large disc.
Kajiado River:
Kaj. R VES10,12 10,12 01°48'50.9'' 36°46'02.3'' 1680 500 2 0 35 (35 - 50) 1 (1 - 2) 48 (44 - 70) 12 (10 - 15) 400 (300 - 500) 130 (120 - 200) >>>1000 two perpendicual VES's
Kajiado R VES11 11 01°49'46.3'' 36°45'50.6'' 1670 500 1.5 0 320 (150 - 500) 0.7 (0- 1) 4 (large) 0.8 (large) 130 (100 - 200) 35 (25 - 43) (>3000)
VES11 layer4: 27 (large) 25 ( 1000
Kajiado Ri VES15 15 01°49'03.5'' 36°45'59.8'' 1692 500 1.5 0 480 (340 - 700) 7 (2 - 12) 200 (185 - 210) 80 (65 - 95) >800 large discontinuity
Kajiado R VES17 17 01°49'11.0'' 36°46'03.8'' 1675 250 4 0 60 (large) 0.3 (0 - 3) 20 (12 - 40) 13 (11 - 15) >>>1000
Kajiado R VES18 18 01°49'11.1'' 36°46'03.5'' 1675 160 2.3 0 600 (300 - 1500) 1.2 (1 - 3) 3 (1 - 10) 1.7 (large) 240 (50 - 600) 80 >30 discontinuity
Kajiado R VES19 19 01°49'06.2'' 36°46'11.6'' 1675 500 1.9 0 1200 (large) 0.7 (0 - 5) 80 (30 - 150) 9 (3 - 30) 500 (300 - 800) 33 (>25, uncertain) major disconinuity
Kajiado R VES20 20 01°49'03.7'' 36°46'04.3'' 1680 200 1.7 0 800 (250 - 2600) 1.2 (1 - 4) 5 (large) 2.4 (large) 300 (140 - 700) 90 (>45, uncertain) short AB reach
Interpreted geo-electric resistivity model
surface layer Layer 2 Layer 3 Basement (Rho≈≥1000
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TABLE 2 summarized results re-interpreted VES soundings from existing survey reports
TABLE 3 summarized results VES soundings from existing survey reports, lacking measured data
re-interpreted from existing survey reports
coordinates (WGS84) Electrode RMS remarks
name/ VES nr. LAT LONG Elev spacing error TOP Rho confidence TOP confidence Rho confidence TOP confidence Rho confidence TOP confidence Rho
location (S) (East) (mas) AB/2 (m) x10-2 (m) (Ωm) (m) (Ωm) (m) (Ωm) (m) (Ωm)
MTTI VES1 MTTI 01°51'16.0'' 36°46'55.3'' 1720 200 1.87 0 350 (300-400) 1.5 (1 - 2) 100 (20 - 500) 25 (10 - 40) 200 (100 - 300) ?? > 80 detail coordinates missing
MTTI VES2 MTTI 01°51'16.0'' 36°46'55.3'' 1720 160 0.95 0 10 (8 - 12) 1.5 (1 - 2) 75 (70 - 80) 25 (20 - 30) 340 (220 - 460 ?? > 80
MTTI VES3 MTTI 01°51'16.0'' 36°46'55.3'' 1720 200 1.88 0 15 (10 - 20) 3 (1 - 5) 85 (70 - 100) 10 (8 - 14) 210 (190 - 240) ?? >130
MTTI VES4 MTTI 01°51'16.0'' 36°46'55.3'' 1720 200 1.72 0 120 (90 - 160) 1.5 (1 - 2) 35 (28 - 40) 9 (5 - 10) 200 (130 - 230) ?? >50 or deep correctness of last 3 points ?
Masai Stores Masai St 01°50'52.9'' 36°47'24.7'' 1730 200 1.28 0 75 (70 - 80) 1.5 (1- 2) 52 (50 - 55) 20 (15 - 25) 175 (100 - 400) 80 (50 - 130) >>1000 survey report model gives BAD FIT
Osman town Osman 01°50'45.0'' 36°47'25.0'' 1730 250 1.31 0 14 (8 - 27) 0.7 (0- 3) 22 (21 - 23) 10 (10 - 11) 105 (93 - 130) ? >90
Osman Kaj River Osman 01°49'26.0'' 36°45'54.5'' 1670 250 1.9 0 4 (3 - 5) 5 (4 - 6) 40 (15 - 50) 30 (15 - 40) >>1000 inconsistent interpretation in report
Interpreted geo-electric resistivity model
surface layer Layer 2 Layer 3 Basement (Rho≈≥1000
Interpreted geo-electric resistivity model
coordinates (WGS84) Electrode remarks
Name/ VES nr. LAT LONG Elev
spacing RMS
error TOP Rho TOP Rho TOP Rho TOP Rho TOP Rho TOP Rho
Location (S) (East) (mas) AB/2 (m) x10-2 (m bgs) (Ωm) (m bgs) (Ωm) (m bgs) (Ωm) (m bgs) (Ωm) (m bgs) (Ωm) (m bgs) (Ωm)
Itinyika 01°50'40.0'' 36°45'40.0'' 1660 ? ? 0 70 2 30 8 100 16 200 50 80 100 700 field data missing
Empeut VES1 01°50'3.54'' 36°47'42.0'' 1720 ? ? 0 79 0.8 17 1.4 53 19 1150 36 81 100 1000 field data missing
Empeut VES2 01°50'0.97'' 36°47'40.4'' 1720 ? ? 0 16 1 30 11 93 42 1000 field data missing
Parmet 01°49'05.0'' 36°45'40.0'' 1685 ? ? 0 90 2 15 7 100 40 250 80 125 125 1200 field data missing
Layer 4surface layerLayer 2 Layer 3 Layer 5 Basement ≥1000 Ωm
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Content
Interpretation report index
Page nr.
Sponge City and ISGEAG soundings:
VES 1 --------------------------------------------------------------- 7
VES 2 --------------------------------------------------------------- 9
VES 3 --------------------------------------------------------------- 11
VES 4 --------------------------------------------------------------- 13
VES 5 --------------------------------------------------------------- 15
VES 6 --------------------------------------------------------------- 17
VES 7 --------------------------------------------------------------- 19
VES 8 --------------------------------------------------------------- 21
VES 9 --------------------------------------------------------------- 23
VES 10/12 --------------------------------------------------------------- 25
VES 11 --------------------------------------------------------------- 28
VES 13 --------------------------------------------------------------- 31
VES 14 --------------------------------------------------------------- 33
VES 15 --------------------------------------------------------------- 35
VES 16 --------------------------------------------------------------- 37
VES 17/18 --------------------------------------------------------------- 39
VES 19 --------------------------------------------------------------- 44
VES 20 --------------------------------------------------------------- 47
VES 21 --------------------------------------------------------------- 50
mini VES --------------------------------------------------------------- 52
From existing survey reports:
MTTI --------------------------------------------------------------- 55
Masai Stores --------------------------------------------------------------- 60
Mr. Osman (Town) --------------------------------------------------------------- 63
Mr. Osman (Kajiado River)--------------------------------------------------------------- 65
---------------------------------------------------------------
Existing reports lacking field data:
Itinyika --------------------------------------------------------------- 68
Empeut --------------------------------------------------------------- 70
Mr. Parmet --------------------------------------------------------------- 73
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Kajiado VES sounding nr.1 Technical interpretation report
Harry Rolf for SamSamWater Foundation 17/02/2017
Client : Acacia Water
General
VES sounding for Sponge City project
Aim: finding locations for infiltration (highly weathered or fractured rock)
Location name: South bank of Gully C. This is one of the VES’s and HEP lines conducted close to this gully, see report Rolf 2017,
Geophysical surveys for Sponge City project Kajiado, Kenya.
Sounding Date : 15/02/2017
Location
VES sounding information
Location: (WGS82): 360 46’31.2” E and Latitude 010 49’39.5” S
Direction of sounding: 110 gr SE (the VES line has crossed the gully which may have caused anomaly in the measurements)
AB reach: 320 m
Array: Schlumberger
Elevation: 1690 mas
Estimated depth to Water: 30 m
Measured data
Remarks: There obviously was a lateral change, which in this case may be caused by the gully (geology change) that was crossed
at the South east part of the line (at 65 m from the centerpoint)
DATA table VES 1
MN/2 AB/2 Rho Rho shifted
0.5 1.6 27 27
2 27 27
2.5 28 28
3.2 32 32
4 31 31
5 32 32
6.3 38 38
8 43 43
10 46 46
0.5 13 52 52
5 13 58
16 63 56
20 63 59
25 69 61.7
5 32 95 85
10 32 71
40 97 114
50 126 150
10 63 163 193
25 63 126
80 182 280
100 220 335
25 130 268 407
50 130 194
160 230 483
200 226 475
250 225 472
50 320 190 399
100 320
400
500
VES measurements
10
100
1000
1 10 100 1000AB/2 (m)
Rho (ohm
m)
Rho
Rho shifted
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VES interpretation Inversion results (SchlumBG modelling)
Possible model concepts
3 layers shifted data (leaving out the last bending down measurements) very high resistivity basement
Results including 90% confidence levels
Depth (m) Rho (Ωm )
topsoil ------ 23 - 29
Overburden 1 -4 40 – 80
Shallow aquifer ------- ---------
Basement (ρ>1000) 12 - 30 >>>1000 (high resistivity)
Conclusion
• Overburden on top of fresh basement rock at 12 – 30m depth
The bending of the measurements beyond AB/2 = 100 is not (yet) explained.
The VES line crossed a major anomaly, probably when crossing the gully.
1 10 100 L/2
10
1e+2
1e+3
1e+4
1e+5
1e+6
Rho
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Kajiado VES sounding nr.2 Technical interpretation report
Harry Rolf for SamSamWater Foundation 17/02/2017
Client : Acacia Water
General
VES sounding for Sponge City project.
Aim: finding locations for infiltration (highly weathered or fractured rock)
Location name: along the road down to the Kajado River (in between gulley C and gulley B, at the beginning of HEP1 and HEP2
profiles)
Sounding date : 13/02/2017
Location
VES sounding information
Location: (WGS82): 360 46’ 22.0” E and Latitude 010 49’ 55.1” S
Direction of sounding: SE -N W along the road
AB reach: 640 m
Array: Schlumberger
Altitude: 1718 mas
Depth to groundwater
(if any) estimated at: 70 mbgl
Measured data
Remarks: The measurements are discontinuous. The difference between the shifted and unshifted measurements is
considerable. There are some clear outliers e.g. at AB/2 =30 m. The cause of these varying measurement quality is not clear. It
may be caused by crossed powerlines and fences along the road.
DATA table VES 2
MN/2 AB/2 Rho Rho shifted
0.5 1.6 276 276
2 270 270
2.5 264 264
3.2 274 274
4 274 274
5 277 277
6.3 284 284
8 257 257
10 228 228
0.5 13 191 191
5 13 225
16 192 164
20 185 157
25 209 178
5 32 221 187
10 32 220
40 282 240
50 243 207
10 63 257 220
25 63 219
80 252 252
100 281 282
25 130 318 319
50 130 292
160 326 299
200 364 333
250 404 370
50 320 404 371
100 320
400
500
VES measurements
100
1000
1 10 100 1000AB/2 (m)
Rho (ohm
m)
Rho
Rho shifted
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VES interpretation
Inversion results (SchlumBG modelling)
Possible model concepts
3 or 5 layer model with shifted or unshifted data
Basement layer (taken at ρ>1000 ohmm) is uncertain
\
5 layer unshifted
3 layer unshifted
4 layer shifted data
Results including 90% confidence levels
Depth (m) **) Rho (ohmm)
topsoil (1000) >40 (actually unknown)
*)Please note the topsoil Rho can be just a phony representing electrode contacts, electric leakings
**) the top of the layer
Conclusion
• High uncertainty, actually the sounding is unreliable
• The (dry) overburden layers are in the order of 160 - 250ohmm.
• At the level of in between 25 – 75 m there is a change to a somewhat higher resistivity (380 – 500 ohmm)
• No basement rock within 40 m depth
1 10 100 L/2
1e+2
1e+3
1 10 100 L/2
1e+2
1e+3
1 10 100 L/2
10
1e+2
1e+3
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Kajiado VES sounding nr.3 Technical interpretation report
Harry Rolf for SamSamWater Foundation 01/03/2017
Client : Acacia Water
General
VES sounding for Sponge City project.
Aim: finding locations for infiltration (highly weathered or fractured rock)
Location name: on the Gully bed (Northern runoff gulley C) close to a solar borehole (Mr. Rehoba)
Sounding Date : 16/02/2017
Location
VES sounding information
Location: (WGS82): 36046’27.8” E and Latitude 01049’26.8” S
Direction of sounding: NE at the north end of HEP line 1 (in the HEP line direction!)
AB reach: 640 m
Array: Schlumberger
Elevation: 1710 mas
Depth to groundwater (if any)
Estimated at: 50 m
Measured data
Remarks: Major lateral changes and outliers. The sounding direction was across the direction of geology changes
Data shifted acc. to Earth Water Consultant’s methodology.
DATA table VES 3
MN/2 AB/2 Rho Rho shifted
0.5 1.6 62 62
2 73 73
2.5 85 85
3.2 89 89
4 90 90
5 77 77
6.3 78 78
8 68 68
10 61 61
0.5 13 49 49
5 13 76
16 57 37
20 48 31
25
5 32 48 31
10 32 58
40 64 34
50 70 37
10 63 53 28
25 63 74
80 104 36.64
100 123 47.2
25 130 177 67.6
50 130 134
160 152 76.5
200 189 95.1
250 202 101.5
50 320 275 138
100 320
400
500
VES measurements
10
100
1000
1 10 100 1000AB/2 (m)
Rho (ohm
m)
Rho
Rho shifted
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Interpretation
Inversion results (SchlumBG modelling)
Possible model concepts
4 layer model with shifted data
4 layer unshifted data
Results including 90% confidence levels
Depth (m) **) Rho (Ωm )
topsoil (1000) 40 - 60 >1000
*) the topsoil Rho can be phony representing electrode contacts, electric leakings
**) the top of the layer
Conclusion
• Overburden relatively low resistivity 23 - 40
• Basement at 40 – 60 m
Strong lateral changes indicating geology changes in NE direction
1 10 100 L/2
10
1e+2
1e+3
1 10 100 L/2
10
1e+2
1e+3
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Kajiado VES sounding nr.4 Technical interpretation report
Harry Rolf for SamSamWater Foundation 17/02/2017
General
VES sounding for Sponge City project.
Aim: finding locations for infiltration (highly weathered or fractured rock)
Location name: across Gulley C at HEPline1 (in the direction of HEP line!!)
Sounding Date : 15/02/2017
Location
VES sounding information
Location: (WGS82): 36°46'22.90"E and Latitude 1°49'34.70"S
Direction of sounding: NNE (across the structure, centre is just South the gulley bed)
AB reach: 320 m
Array: Schlumberger
Elevation 1685 mas
Groundwater (if any)
Depth estimated at: 20m
Measured data
Remarks: There obviously are large lateral changes, both from structural changes in geology, but also because the VES
centerpoint was stationed close to the bed of the gulley, having clay and loamy sand deposits (explaining the low resistivity at
small AB/2 range. The shifted curve is remarkable due the method of shifting. Therefore , the shifted line was only used to up to
AB/2 = 20m in the inversion.
DATA table VES4
MN/2 AB/2 Rho Rho shifted
0.5 1.6 18 18
2 18 18
2.5 17 17
3.2 15 15
4 13 13
5 12 12
6.3 12 12
8 13 13
10 14 14
0.5 13 20 20
5 13 29
16 37 26
20 49 34
25 65 44
5 32 82
10 32 57
40 95 63
50 89 59
10 63 91 60
25 63 131
80 131 63
100 145 69
25 130 151
50 130 374 69.21429
160 385 71
200 380 70
250 464 86
50 320 584 108
100 320
400
500
VES measurements
10
100
1000
1 10 100 1000AB/2
Rho
Rho
Rho shifted
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VES interpretation
Inversion results (SchlumBG modelling)
Possible model concepts
4 layer model with shifted (up to 20 m) data
The model fit is not good (RMSE 9.28E-02) due to the many lateral changes
4 layer model , unshifted data
Results including 90% confidence levels
Depth (m) Rho (ohmm)
topsoil (1000)
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Kajiado VES sounding nr.5 Technical report
Harry Rolf for SamSamWater Foundation 16/02/2017
Client : Acacia Water
General
VES sounding for Sponge City project.
Aim: finding locations for infiltration (highly weathered or fractured rock)
Location name: street in Kajiado town (originally suggested for HEP line3)
Sounding Date : 15/02/2017
Location
VES sounding information
Location: (WGS82): 36046’59.1” E and Latitude 01050’21.7” S
Direction of sounding: 2620 West-SouthWest
AB reach: 260m (short due to buildings and risky power lines)
Array: Schlumberger
Elevation: 1720 mas
Groundwater depth (if any)
Estimated at: 55 mbgl
Measured data
Remarks: Shifting is reasonable (K3= 1.177) , minor lateral changes within this (short) reach
DATA table VES5
MN/2 AB/2 Rho SHIFTED
0.5 1.6 122
2 102
2.5 93
3.2 79
4 72
5 68
6.3 72
8 70
10 72
0.5 13 75
5 13 72
16 77 81
20 100 104
25 111 116
5 32 122
10 32 125 127
40 133 135
50 148 150
10 63 170 167
25 63 142
80 185 219
100 199 234
25 130 237 279
50 130
160
200
250
50 320
100 320
400
500
VES measurements
10
100
1000
1 10 100 1000AB/2
Rho
Rho
SHIFTED
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VES interpretation
Inversion results (SchlumBG modelling)
Possible model concepts
3 or 4 layer model with shifted data
Basement layer (assuming ρ>1000 ohmm) very uncertain
3 layer model
4 layer model
Results including 90% confidence levels
Depth (m) Rho (Ωm)
(top)
topsoil (1000) >40
*)Please note the topsoil Rho can be just a phony layer, representing electrode contacts, electric leaking
Conclusion
• Overburden is like 65 (60 – 70) Ωm.
• Less weathered/fractured rock starts at ≈10 m
• Fresh basement rock unknown, but at least deeper than 40m
• No indications for substantial structural changes within the (short) AB reach
1 10 100 L/2
10
1e+2
1e+3
1 10 100 L/2
10
1e+2
1e+3
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Kajiado VES sounding nr.6 Technical interpretation report
Harry Rolf for SamSamWater Foundation 24/02/2017
Client : Acacia Water
General
VES sounding for Sponge City project.
Aim: finding locations for infiltration (highly weathered or fractured rock)
Location name: at the location in Kajiado Town, close to a Sponge City test infiltration pit along roadwater runoff gully
Sounding Date : 15/02/2017
Location
VES sounding information
Location: (WGS84): 36047’28.8” E and Latitude 01050’24.1” S
Direction of sounding: East-Northeast
AB reach: 160 m (short due to buildings/powerlines)
Array: Schlumberger
Elevation: 1735 mas
Estimated depth to groundwater (if any): 75 mbgl
Measured data
Remarks: Short AB electrode reach due to building restrictions. Measurements show large discontinuity at potential change at
13m. The inversion of the unshifted data is too inaccurate. Inversion only done on the shifted data. This however is tricky
because over the large shift in the data. The shift may be caused by serious fracturing, or….. ??
DATA table VES 6
MN/2 AB/2 Rho Rho shifted
0.5 1.6 10.2 10.2
2 9.8 9.8
2.5 9.9 9.9
3.2 10.3 10.3
4 11 11
5 12.4 12.4
6.3 15 15
8 15.5 15.5
10 19.6 19.6
0.5 13 20.8 20.8
5 13 36.7
16 43.5 25
20 53.3 30
25 58.3 33
5 32 54.7 35
10 32
40 72.2 41
50 84 48
10 63 87
25 63 94 53
80 123 70
100
25 130
50 130
160
200
250
50 320
100 320
400
500
VES measurements
10
100
1000
1 10 100AB/2 (m)
Rho (ohm
m)
Rho
Rho shifted
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VES interpretation
Inversion results (SchlumBG modelling)
Possible model concepts
3 or 4 layer model with shifted data only
3 layer model
4 layer model
Results including 90% confidence levels
Depth (m) Rho (Ωm )
(top)
topsoil ------ 9 - 12 *)
topsoil2 0.5 – 4 1 - 10
Overburden 2 - 4 38 - 54
Basement 20 – 90 >> 1000
*)Please note the topsoil Rho can be just a phony representing electrode contacts, electric leaking
Conclusion
• Overburden is like 45 (38 - 54) Ωm, top starting at 2 – 4 m
• Fresh basement rock below 20 m, not deeper than 90 m
• VES is rather uncertain due to large shifts in the measured data
• Considerable shift in the measurements may indicate fracture zone??
1 10 100 L/2
1
10
1e+2
1e+3
1e+4
Rho
1 10 100 L/2
1
10
1e+2
1e+3
Rho
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Kajiado VES sounding nr.7 Technical interpretation report
Harry Rolf for SamSamWater Foundation 01/03/2017
Client : Acacia Water
General
VES sounding for Sponge City project.
Aim: finding locations for infiltration (highly weathered or fractured rock)
Location name: parallel to the Gully C (North bank) on HEPline1
Sounding Date : 15/02/2017
Location
VES sounding information
Location: (WGS82): 360 46’ 24.3” E and Latitude 010 49’ 33.0” S
Direction of sounding: 300gr SEast
AB reach: 800 m
Array: Schlumberger
Elevation 1690 mas
Groundwater (if any)
Depth estimated at: 25m
Measured data
Remarks: only minor lateral changes
DATA table VES 7
MN/2 AB/2 Rho Rho shifted
0.5 1.6 96 96
2 82 82
2.5 76 76
3.2 80 80
4 87 87
5 95 95
6.3 111 111
8 138 138
10 146 146
0.5 13 170 170
5 13 195
16 224 196
20 238 208
25 251 219
5 32 269
10 32 273 236
40 278 274
50 277 273
10 63 289 285
25 63 276
80 299 308
100 309 318
25 130 370 381
50 130 344
160 410 455
200 458 508
250 497 552
50 320 623 691
100 320 548
400 613 772
500
VES measurements
10
100
1000
1 10 100 1000AB/2 (m)
Rho (ohm
m)
Rho
Rho shifted
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P a g i n a | 20
VES interpretation
Inversion results (SchlumBG modelling)
Possible model concepts
4 layer model with both shifted and unshifted data
4 layer model, shifted data
4 layer model, unshifted data
Results including 90% confidence levels
Depth (m) Rho (ohmm)
topsoil (1000) 80 – 100 1300 – 1800
*)Please note the topsoil Rho can be a phony layer representing electrode contacts, electric resistance
Conclusion
• Basement at 90 m (80 – 100) overlain by overburden 300 Ωm (290 -320)
• No structural changes in 300 gr SE direction
1 10 100 L/2
10
1e+2
1e+3
1 10 100 L/2
10
1e+2
1e+3 VES7
north of the gully
(parrallel )
-
P a g i n a | 21
Kajiado VES sounding nr.8 Technical interpretation report
Harry Rolf for SamSamWater Foundation 16/02/2017
Client : Acacia Water
General
VES sounding for Sponge City project.
Aim: finding locations for infiltration (highly weathered or fractured rock)
Location name: in Kajiado town, behind the Mosque at school premises
Sounding Date : 15/02/2017
Location
VES sounding information
Location: (WGS82): 36047’25.3” E and Latitude 01050’27.8” S
Direction of sounding: 2900 Northwest
AB reach: 126 m (short due to school buildings)
Array: Schlumberger
Elevation: 1732mas
Depth to groundwater (if any)
Estimated at: ??
Measured data
Remarks: Discontinuity at AB/2 32m is considerable, maybe caused by e.g close-by buildings?
DATA table VES 8
MN/2 AB/2 Rho Rho shifted
0.5 1.6 34 34
2 29 29
2.5 33 33
3.2 37 37
4 41 41
5 43 43
6.3 49 49
8 50 50
10 51 51
0.5 13 52 52
5 13 53
16 56 56
20 62 62
25 66 66
5 32 71
10 32 83 71
40 85 73
50 94 80
10 63 93 80
25 63
80
100
25 130
50 130
160
200
250
50 320
100 320
400
500
VES measurements
10
100
1 10 100AB/2 (m)
Rh
o (
oh
mm
)
Rho
Rho shifted
-
P a g i n a | 22
VES interpretation
Inversion results (SchlumBG modelling)
Possible model concepts
3 layer model with shifted or unshifted data
Basement layer (assuming ρ>1000 ohmm) beyond Depth of Investigation
3 layer model, unshifted data
3 layer model, shifted data
Results including 90% confidence levels
Depth (m) Rho (Ωm )
topsoil (1000) >75
*)Please note the topsoil Rho can be just a phony layer representing electrode contacts, electric leakings
Conclusion
• Overburden is like 54 (52-56) Ωm.
• Weathered/Fractured rock (≈ 100 Ωm) starts at 10 (9-13) m.
• Fresh basement rock unknown, but at least deeper than 75m
• VES is too short to see any evidence of fracturing/structural lateral changes, though the measured shift at AB/2=32m may
indicate a geological anomaly.
1 10 100 L/2
1
10
1e+2
Rho
1 10 100 L/2
1
10
1e+2
Rho
-
P a g i n a | 23
Kajiado VES sounding nr.9 Technical interpretation report
Harry Rolf for SamSamWater Foundation 01/03/2017
Client : Acacia Water
General
VES sounding for Sponge City project.
Aim: finding locations for infiltration (highly wheathered or fractured rock)
Location name: end of HEP line2, SW of the Gully B orthogonal to HEPline
Sounding Date : 15/02/2017
Location
VES sounding information
Location: (WGS82): 360 46’ 17.2” E and Latitude 010 50’ 17.3” S
Direction of sounding: SEast- NW
AB reach: 1000 m
Array: Schlumberger
Elevation 1710 mas
Groundwater (if any)
Depth estimated at: 50m
Measured data
Remarks: The shifting is wrong due to an error outlier measurement at AB/2 = 63. Beyond that point (63) , the unshifted data
were used in the inversion.
DATA table VES 9
MN/2 AB/2 Rho Rho shifted
0.5 1.6 231 231
2 228 228
2.5 201 201
3.2 176 176
4 169 169
5 151 151
6.3 153 153
8 144 144
10 141 141
0.5 13 130 130
5 13 153
16 143 121
20 135 116
25 123 105
5 32 125
10 32 131 107
40 132 107
50 135 110
10 63 83 67
25 63 126
80 151 81
100 171 92
25 130 210 112
50 130 189
160 244 145
200 274 162
250 317 187
50 320 377 223
100 320 357
400 430 268
500 466 291
VES measurements
10
100
1000
1 10 100 1000AB/2 (m)
Rho (ohm
m)
Rho
Rho shifted
-
P a g i n a | 24
VES interpretation
Inversion results (SchlumBG modelling)
Possible model concepts
4 layer model with shifted or unshifted data
4 layer shifted data
4 layer model, unshifted data
Results including 90% confidence levels
Depth (m) Rho (ohmm)
topsoil ------ 250 - 300 *)
overburden 1 - 2 130 - 150
Shallow aquifer 8 - 15 85 - 110
Basement 50 - 70 700 – 900 (just below 1000)
*)Please note the topsoil Rho can be phony layer representing electrode contacts, electric leaking
Conclusion
• Basement at ≈60 m (50 – 70) overlain by shallow aquifer and overburden
• Basement resistivity is relatively low (just below 1000 Ωm).
• Minor/ no structural changes in SE direction
1 10 100 L/2
10
1e+2
1e+3
Rho
1 10 100 L/2
1e+2
1e+3
-
P a g i n a | 25
Kajiado VES sounding nr.10 and 12 Technical interpretation report
Harry Rolf for SamSamWater Foundation 26/02/2017
Client : Acacia Water
General
Two perpendicular VES soundings ISGEAG geophysical project .
Aim: reconnaissance to find interesting locations to test contemporary geophysical methods
Location name: NW of Kajiado town Two perpendicular VES soundings in between 2 branches of Kajiado river (Kajiado river
branch and Imutatin river branch in the West)
Sounding Date : 17/02/2017
Location
VES sounding information
Location: (WGS82): 360 46’ 02.3” E and Latitude 01048’ 50.9” S
Direction of sounding: North - South (Ves10) and East West (Ves 12) ,
AB reach: 1000 m
Array: Schlumberger
Elevation: 1680 mas
Depth to groundwater (if any)
Estimated at: 15 - 20 m
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P a g i n a | 26
Measured data VES10
Measured data VES12
Remarks: Apart from the (unexplained) discontinuity in VES12 (at AB/2=13 m) there is no big difference in lateral changes
between the two soundings.
It is not clear whether the discontinuity in VES12 actually due to a change in geology or a measurement error
DATA table VES 10
MN/2 AB/2 Rho Rho shifted
0.5 1.6 42.8 42.8
2 38.7 38.7
2.5 43.2 43.2
3.2 44.4 44.4
4 51.2 51.2
5 45.9 45.9
6.3 49.5 49.5
8 50.9 50.9
10 51.1 51.1
0.5 13 49.1 49.1
5 13 45.5
16 51.2 55.3
20 59.8 64.6
25 72.7 78.6
5 32 91.6
10 32 84.1 99
40 103.9 122.3
50 127.5 150.2
10 63 164 193
25 63 131
80 157 231
100 192 283
25 130 247 364
50 130 256
160 307 436
200 378 537
250 445 632
50 320 513 728
100 320 553
400 654 861
500 928 958
VES measurements
10
100
1000
10000
1 10 100 1000AB/2 (m)
Rh
o (
oh
mm
)
Rho
Rho shifted
DATA table VES 12
MN/2 AB/2 Rho Rho shifted
0.5 1.6 64 64
2 60 60
2.5 64 64
3.2 69 69
4 75 75
5 75 75
6.3 82 82
8 84 84
10 87 87
0.5 13 98 98
5 13 56
16 60 104
20 70 122
25 82 144
5 32 101
10 32 93 177
40 113 215
50 143 272
10 63 179 341
25 63 173
80 198 391
100 218 430
25 130 227 448
50 130 230
160 279 544
200 344 671
250 381 743
50 320 497 969
100 320 485
400 628 1254
500 765 1528
VES measurements
10
100
1000
10000
1 10 100 1000AB/2 (m)
Rh
o (
oh
mm
)
Rho
Rho shifted
-
P a g i n a | 27
VES interpretation
Inversion results (SchlumBG modelling)
Possible model concepts
5 layer model with shifted or unshifted data
VES10 Unshifted 5 layer
VES12 5layer model , first measurements shifted down
Results including 90% confidence levels
Depth (m) Rho (Ωm )
topsoil (1000) 120 - 200 >>>>1000 Ωm
Conclusion
• Overburden layer accurately 44 – 70 Ωm
• Shallow 300 – 500 Ωm aquifer and
• deep, high resistive basement > 120m (like 150m)
• Apart from the unexplained shift in (the first - shallow - part of VES12) there are no indications for large structural changes
in both directions.
1 10 100 L/2
10
1e+2
1e+3
Rho
1 10 100 L/2
10
1e+2
1e+3
1e+4
1e+5
1e+6
Rho
-
P a g i n a | 28
Kajiado VES sounding nr.11 Technical interpretation report
Harry Rolf for SamSamWater Foundation 26/02/2017
Client : Acacia Water
General
VES sounding for VIA Water ISGEAG project.
Aim: finding locations , fracture zones for the ISGEAG geophysical methods project
Location name: on the Kajiado river bed South of Haraf Borehole
Date : 17/02/2017
Location
VES sounding information
Location: (WGS82): 360 45’ 50.6” E and Latitude 010 49’ 46.3” S
Direction of sounding: SSWest on the river bed ,
AB reach: 1000 m
Array: Schlumberger
Elevation: 1670 mas
Depth to groundwater (if any)
Estimated at: 20 m
VES interpretation
Measured data
Remarks: just minor discontinuity (lateral changes)
DATA table VES 11
MN/2 AB/2 Rho Rho shifted
0.5 1.6 110.95 110.95
2 80.9 80.9
2.5 73.6 73.6
3.2 69.8 69.8
4 72.5 72.5
5 78 78
6.3 89.8 89.8
8 96.3 96.3
10 101.3 101.3
0.5 13 111 111
5 13 109
16 112 114
20 118 120
25 116 118
5 32 116 117
10 32 118
40 121 120
50 125 125
10 63 139 139
25 63 134
80 162 168
100 197 204
25 130 291 301
50 130 268
160 319 358
200 400 448
250 490 550
50 320 606 680
100 320 565
400 705 850
500 794 957
VES measurements
10
100
1000
10000
1 10 100 1000AB/2 (m)
Rh
o (
oh
mm
)
Rho
Rho shifted
-
P a g i n a | 29
Inversion results (SchlumBG modelling)
Possible model concepts
4 or 5 layer model with shifted or unshifted data
4 layer model , unshifted data
5 layer model with shifted data
Results including 90% confidence levels
Depth (m) Rho (Ωm )
topsoil (1000) 35 (25-43)*) 5000 ((>3000)
*) see sensitivity analysis below
Conclusion
• The 5 layer model concept (basement overlain by a less resistive layer) has a significantly better fit, but the parameters are
uncertain
• Accepting the 5 layer concept, sensitivity analysis indicates basement depth in between 25 a 43 m
• No indications of fracturing (in this SSW direction)
1 10 100 L/2
10
1e+2
1e+3
Rho
1 10 100 L/2
1
10
1e+2
1e+3
1e+4
Rho
-
P a g i n a | 30
Sensitivity analysis
Sensitivity analysis on the 5 layer model (shifted data) shows the following:
The optimal RMSE fit is 1.51*10-2. Any solution within a 10% less good fit (RMSE < 1.66 *10-2) is acceptable
Minimizing (and fixing) the basement depth shows a (still mathematically acceptable) solution as:
Minimized basement depth (at 16m, RMSE = 1.56*10-2) mathematically possible, but it is most unlikely that the sounding can
‘see’ such a thin low resistivity layer.
And maximizing basement depth gives a (maximal) solution as follows:
Maximum basement depth solution (basement at 43m RMSE = 1.65*10-2)
From this we conclude that basement depth is somewhere in between ≈25m and 43m
1 10 100 L/2
1
10
1e+2
1e+3
1e+4
Rho
RMSE = 1.56
1 10 100 L/2
10
1e+2
1e+3
1e+4
Rho
RMSE = 1.65
-
P a g i n a | 31
Kajiado VES sounding nr.13 Technical interpretation report
Harry Rolf for SamSamWater Foundation 26/02/2017
Client : Acacia Water
General
VES sounding for Sponge City project.
Aim: finding locations for infiltration (highly weathered or fractured rock)
Location name: on the Gully bed (Northern runoff-gully c) in the centre of HEP profile 1
Sounding Date : 16/02/2017
Location
VES sounding information
Location: (WGS82): 360 46’ 23.4” E and Latitude 010549’ 34.4” S
Direction of sounding: SEast on top of the gulley bed ,
AB reach: 126 m
Array: Schlumberger
Elevation: 1682 mas
Depth to groundwater (if any)
Estimated at: 20 m
Measured data
Remarks: Only minor discontinuities (at AB/2=32m)
DATA table VES 13
MN/2 AB/2 Rho Rho shifted
0.5 1.6 34 34
2 15.3 15.3
2.5 14 14
3.2 14.2 14.2
4 14.1 14.1
5 15.2 15.2
6.3 17.3 17.3
8 21.9 21.9
10 26.2 26.2
0.5 13 35.5 35.5
5 13 32.9
16 40.5 43.7
20 52.5 56.7
25 76.6 82.7
5 32 106.1
10 32 95.1 114.6
40 126.9 154.1
50 171 206
10 63 229.5 276.6
25 63
80
100
25 130
50 130
160
200
250
50 320
100 320
400
500
VES measurements
10
100
1000
10000
1 10 100AB/2 (m)
Rho (ohm
m)
Rho
Rho shifted
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P a g i n a | 32
VES interpretation
Inversion results (SchlumBG modelling)
Possible model concepts
3 or 4 layer model with shifted or unshifted data
3 layer Unshifted data
4 layer Shifted data
Results including 90% confidence levels
Depth (m) Rho (Ωm )
topsoil (1000) 4 – 5 >>>1000
Conclusion
• Topsoil is representing the clayey sand deposits on the gully bed
• Thickness of these deposits is 3 – 4 m
• Right below the gully bed there is high resistive fresh basement rock at ≈ 4 m depth
• No signs of lateral changes is this direction , no indications of fracturing (in this direction)
Remark:
Other experiments (visual and logger data) prove that water infiltrates at this location from the gully into the subsoil. There
must be major permeable fractures right to the North or South gulley bank allowing this vertical infiltration
1 10 100 L/2
10
1e+2
1e+3
Rho
1 10 100 L/2
1
10
1e+2
1e+3
1e+4
1e+5
Rho
-
P a g i n a | 33
Kajiado VES sounding nr.14 Technical interpretation report
Harry Rolf for SamSamWater Foundation 18/04/2017
Client : Acacia Water
General
VES sounding for VIA Water ISGEAG project.
Aim: underground at West side of HEP8 line (having a constant resistivity of ≈60 ohmm)
Location name: Kajiado River, close to Osiligi farm at the West end of ISGEAG HEP profile 8
Date : 18/04/2017
Location
VES sounding information
Location: (WGS84): 360 45’ 53.4” E and Latitude 010 49’ 03.4” S
Direction of sounding: N-South
AB reach: 400 m
Array: Schlumberger
Elevation: 1675 mas
Depth to groundwater (if any)
Estimated at: 20 m
Measured data
Remarks: Only minor discontinuity at AB/2=130m
DATA table VES14
# MN/2 AB/2 Rho Rho shifted
0.5 1.6 18.6
2 20.4
2.5 20
3.2 18.1
4 15.3
5 12.6
6.3 10.4
8 8.4
10 7.5
0.5 13 8.5
5 13 8
16 9.7
20 12
25 13.9
5 32 18.2
10 32 17.1
40 23.1
50 30.9
10 63 43.2
25 63 46.6
80 65
100 85.4
25 130 116.6
50 130 100.8
160 128.5
200 174.3
250
50 320
100 320
400
500
VES measurements
1
10
100
1000
1 10 100 1000AB/2 (m)
Rho (ohm
m)
Rho
Rho shifted
-
P a g i n a | 34
VES interpretation
Inversion results (SchlumBG modelling)
Possible model concepts
4 or 5 layer model unshifted data
4 layer model , clay right on top of very high resistive basement
5 layer model, having an ‘in between’ layer below the clay overlaying basement
Results including 90% confidence levels (
Depth (m) Rho (Ωm)
(top of the layer)
topsoil (>>1000
Conclusion
• Clay layer on top of shallow very high resistive rock (hardcap?)
1 10 100 L/2
0.1
1
10
1e+2
1e+3
1e+4
1e+5
1e+6
Rho
-
P a g i n a | 35
Kajiado VES sounding nr.15 Technical interpretation report
Harry Rolf for SamSamWater Foundation 21/04/2017
Client : Acacia Water
General
VES sounding for VIA Water ISGEAG project.
Aim: to confirm ERT2 results at Kajiado River
Location name: 65 m East of the ERT profile centre
Date : 21/04/2017
Location
VES sounding information
Location: (WGS84): 360 45’ 59.8” E 010 49’ 03.5” S
Direction of sounding: 195 azimuth S-N
AB reach: 1000 m
Array: Schlumberger
Elevation: 1692 mas
Depth to groundwater (if any)
Estimated at: 30 m
Measured data
Remarks: major discontinuities. The south line was close to powerline at AB/2=250 m (or even passed the river bed?)
DATA table VES15
MN/2 AB/2 Rho Rho shifted
0.5 1.6 308 355.432
2 331 381.974
2.5 367 423.518
3.2 394 454.676
4 380 438.52
5 389 448.906
6.3 384 443.136
8 382 440.828
10 352 406.208
0.5 13 337 388.898
5 13 389
16 348 348
20 334 334
25 278 278
5 32 249 249
10 32 258 257
40 248 248
50 227 227
10 63 237 237
25 63 222 222
80 242 242
100 265 265
25 130 303 303
50 130 293 293
160 325 325
200 389 389
250 384
50 320 404
100 320 441 539.784
400 532 651.168
500 591 723.384
VES measurements
100
1000
1 10 100 1000AB/2 (m)
Rh
o (
oh
mm
)
Rho
Rho shifted
-
P a g i n a | 36
VES interpretation
Inversion results (SchlumBG modelling)
Possible model concepts
4 layer unshifted or shifted
4 layer model unshifted data
4 layer model shifted data
Results including 90% confidence levels (
Depth (m) Rho (Ωm)
(top of the layer)
topsoil 0 (340 – 700)
Overburden 7 (2 – 12) ( 185 – 210)
Basement (ρ>1000) 80 (65 – 95) >800
Conclusion
• Shallow aquifer
• Basement (not very high resistive) at 80m
Discontinuities may be caused by anomalies or by passing powerline or river bed
1 10 100 L/2
1e+2
1e+3
1 10 100 L/2
10
1e+2
1e+3
-
P a g i n a | 37
Kajiado VES sounding nr.16 Technical interpretation report
Harry Rolf for SamSamWater Foundation 23/04/2017
Client : Acacia Water
General
VES sounding for VIA Water ISGEAG project.
Aim: to confirm ERT1 anomaly results at Gully C
Location name: , Kajiado Osiligi , Northern run-off Gully C about 130 m North East of the Gulley bed, on HEPline/ERT line 1
Date : 23/04/2017
Location map
VES sounding information
Location: (WGS84): 360 46’ 25.2” E 010 49’ 31.9” S
Direction of sounding: 295 GR azimuth SE-NW
AB reach: 1000 m
Array: Schlumberger
Elevation: 1687 mas
Depth to groundwater (if any)
Estimated at: 25 m
Measured data
DATA table VES16
MN/2 AB/2 Rho Rho shifted
0.5 1.6 37 37
2 42.4 42.4
2.5 49.2 49.2
3.2 58.9 58.9
4 73.5 73.5
5 88.9 88.9
6.3 104 104
8 116 116
10 131 131
0.5 13 147
5 13 129 147
16 153 174.35
20 161 183.47
25 161.4 183.92
5 32 174
10 32 203 198.28
40 215 210
50 239 233.44
10 63 277
25 63 279 270.56
80 315 305.47
100 353 342.32
25 130 422
50 130 349 409.23
160 389 456.13
200 421 493.66
250 494 579.26
50 320 606
100 320 527 710.59
400 584 787.44
500 599 807.67
VES measurements
10
100
1000
1 10 100 1000AB/2 (m)
Rh
o (
oh
mm
)
Rho
Rho shifted
-
P a g i n a | 38
VES interpretation
Inversion results (SchlumBG modelling)
Possible model concepts
4 or 5 layer model shifted data
(the unshifted data have too many discontinuity)
4 layer model shifted data
5 layer model , shifted data
There is reason to believe, and to accept the 5 layer model (it corresponds with the findings in the ERT tomography), the
uncertainty though is high.
Results including 90% confidence levels
Depth (m) Rho (Ωm)
(top of the layer)
topsoil (1000) 76 (40 – 160) 1250 (1000 – 1500)
*) large: meaning that uncertainty is high and not quantifiable
Conclusion
• Basement depth uncertain; shallow aquifer in the order of 500 ohmm possibly below a (57 ohmm) sub-layer
1 10 100 L/2
1
10
1e+2
1e+3
Rho
1 10 100 L/2
1
10
1e+2
1e+3
Rho
-
P a g i n a | 39
Kajiado VES sounding nr.17 and 18 Technical interpretation report
Harry Rolf for SamSamWater Foundation 25/04/2017
Client : Acacia Water
General
VES sounding for VIA Water ISGEAG project Kajiado.
Aim: to confirm fracturing at HARAF Traders borehole at Kajiado River
The HARAF borehole is probably the best yielding borehole in Kajiado. It is believed to abstract water from a major regional
fracture zone close to Kajiado river. Unfortunately no documents (from the borehole survey, nor from borehole completion and
drillers’ log, could be made available.
Two VES soundings were conducted in two directions, close to the borehole, mainly to detect fracture structures. The positioning
of the VES lines was restricted by local powerlines and river beds.
The co-operation the owner Mr. Osman is highly appreciated
Location name: , Kajiado (Olkuajado) river close to HARAF Traders borehole
Souning date : 25/04/2017
Location
Location details. The center-points of the two VESs were 10 m apart, about 40 m NE of the HARAF traders borehole
-
P a g i n a | 40
VES 17 and 18 sounding information
Location: (WGS84): VES17: 360 46’ 03.8” E 010 49’ 11.0” S
VES18: 360 46’ 03.5” E 010 49’ 11.1” S
Direction of sounding: VES17: azimuth 360gr. S-N
VES18: azimuth 67gr ENE
AB reach: VES17: 500 m
VES18: 320 m
Array: Schlumberger
Elevation: 1675 mas
Depth to groundwater
Estimated at: 15 m (SWL from data logger observations in the borehole)
Measured data
Remarks:
Although the centerpoints were just 10 m apart, there is a striking difference in the field curves, both in the absolute level and
the discontinuity (AB/2 = 63m) in VES18.
VES17 was done parallel to the believed N-S fractured structure direction and VES18 was across the structure direction.
VES17 has passed the river bed on the south side at AB/2 = 150 m.
DATA table
MN/2 AB/2 VES17 VES18
0.5 1.6 18.3 139
2 17 166
2.5 16 191
3.2 16 200
4 17 189
5 17.9 170
6.3 19.2 141
8 20.4 98
10 21.9 84
0.5 13 24.9 80
5 13 26.7 73
16 30.1 77
20 35 91
25 42.3 108
5 32 52.1 122
10 32 56.3 112
40 64 134
50 75.4 148
10 63 93.6 179
25 63 89 127
80 125 171
100 165 171
25 130 236 214
50 130 236 211
160 282 249
200 349
250 558
50 320
100 320
400
500
VES measurements
10
100
1000
1 10 100 1000AB/2 (m)
Rh
o (
oh
mm
)
VES17
VES18
-
P a g i n a | 41
VES interpretation VES17
VES17 Inversion results (SchlumBG modelling)
Possible model concepts
3 or 4 layer model unshifted data
3 layer unshifted
4 layer unshifted
VES17 Results including 90% confidence levels
Depth (m) Rho (Ωm)
(top of the layer)
topsoil (1000) 12 (10 – 15) >>>1000
Conclusion VES17
• High resistive basement at ≈10 – 15 m depth
• No indication of fracture zone (discontinuity)
1 10 100 L/2
10
1e+2
1e+3
1e+4
1e+5
1e+6
Rho
1 10 100 L/2
10
1e+2
1e+3
1e+4
1e+5
1e+6
Rho
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P a g i n a | 42
VES18 Inversion results (SchlumBG modelling)
Possible model concepts
5 layer model unshifted or shifted data
5 layer unshifted
5 layer unshifted leaving out last measurements
5 layer with shifted data
1 10 100 L/2
1
10
1e+2
1e+3
Rho
1 10 100 L/2
1
10
1e+2
1e+3
Rho
1 10 100 L/2
1
10
1e+2
1e+3
Rho
-
P a g i n a | 43
VES18 Results including 90% confidence levels
Depth (m) Rho (Ωm)
(top of the layer)
topsoil (1000) 80 (>30)) >1000
Conclusion VES18
• Shallow clay layer
• Basement at 80 m
• Discontinuity at potential electrode change AB/2 = 63m
Comparing VES17 and VES18
Although the two soundings are just 10 m apart, there is a clear difference:.
• Basement in VES18 is deeper and there is a shallow clay layer.
• Basement resistivity in VES17 is higher.
This indicates a significant difference in lithology within short range. Remaining question is: why doesn’t VES18 show
discontinues measurements at the short range, and only at AB/2 = 63??
VES17 VES18
1 10 100 L/2
10
1e+2
1e+3
1e+4
1e+5
1e+6
Rho
1 10 100 L/2
1
10
1e+2
1e+3
Rho
-
P a g i n a | 44
Kajiado VES /IP sounding nr.19 Technical interpretation report
Harry Rolf for SamSamWater Foundation 26/04/2017
Client : Acacia Water
General
VES sounding for VIA Water ISGEAG project.
Aim: to confirm TDEM sounding at the east end of ERT line2
Location name: Kajiado on the East bank of Kajiado River (end of HEP line 8 and ERT line2)
Date : 26/04/2017
Location
VES sounding information
Location: (WGS84): 360 46’ 11.6” E 010 49’ 06.2” S
Azimuth of sounding: 205 GR SSW
AB reach: 1000 m
Array: Schlumberger
Elevation: 1675 mas
Depth to groundwater (if any)
estimated at: 15 m
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P a g i n a | 45
Measured data Resistivity (Ωm) and IP (mS)
Remarks:
When the measured line showed large discontinuity 2 additional points (in blue) were measured with a larger MN/2 of 100m
instead of 50m (red points).
obviously a major geological change was reached at ≈ AB/2 100 m (or even at 63 m).
DATA table
MN/2 AB/2 VES19 MN/2 100
0.5 1.6 116
2 135
2.5 140
3.2 159
4 167
5 154
6.3 119
8 132
10 125
0.5 13 130
5 13 119
16 127
20 139
25 156
5 32 179
10 32 173
40 207
50 233
10 63 249
25 63 239
80 219
100 207
25 130 230
50 130 216
160 217
200 199 262
250 216 319
50 320 242
100 320 370
400 471
500 652
DATA table IP
MN/2 AB/2 VES19 MN/2 100
0.5 1.6 0.21
2 0.19
2.5 0.2
3.2 0.21
4 0.24
5 0.27
6.3 0.24
8 0.43
10 0.67
0.5 13 0.75
5 13 0.75
16 0.75
20 0.75
25 0.77
5 32 0.9
10 32 1.08
40 1.12
50 1.44
10 63 2.1
25 63 1.89
80 1.97
100 2.89
25 130 1.94
50 130 2.1
160 2.09
200 2.23 2.71
250 2.49 3.68
50 320 2.67
100 320 2.33
400 2.82
500 4.96
VES measurements
100
1000
1 10 100 1000AB/2 (m)
Rh
o (
oh
mm
)
MN/2 100
VES19
IP measurements
0
1
2
3
4
5
6
1 10 100 1000AB/2 (m)
IP (
mS
)
MN/2 100
VES19
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P a g i n a | 46
VES interpretation
(IP interpretation is not discussed here)
Inversion results (SchlumBG modelling)
Possible model concepts
4 layer only using the data up to AB/2 = 63 m
Or
6 layer using the full measurements and leaving out the 3 points with MN/2 50 at 200, 250 and 320 m
4 layer using only data up to AB/2 = 63m
6 layer model
Overall results including 90% confidence levels (between brackets)
Depth (m) Thickness (m) Rho (Ωm)
(top of the layer)
topsoil (10 500 (300 – 800)
Basement (ρ>1000) > 25 >1000
Alternative interpretation:
Low resistive layer 24 (large) 9 13 (large)
Basement 33 (large) >>> 1000
Conclusion
• Measurements show major discontinuity at AB/2 > 63 m indicating lateral lithological changes
• Basement deeper than 25 m, may be at 33m (with large uncertainty) underlaying a low resistivity weathered zone between
24 and 33 m.
To better detect anomalies it makes sense to measure parallel with alternative MN potential electrode distances
IP measurements show discontinuities at larger range as well. Shallow IP confirms that there are no shallow clay layers
1 10 100 L/2
10
1e+2
1e+3
1 10 100 L/2
10
1e+2
1e+3
1e+4
-
P a g i n a | 47
Kajiado VES nr.20 Technical interpretation report
Harry Rolf for SamSamWater Foundation 27/04/2017
Client : Acacia Water
General
VES sounding for VIA Water ISGEAG project.
Aim: to confirm ERT/TDEM soundings in the centre of ERT line2
Location name: ,Kajiado ,Ildamat, Osiligi , Kajiado River
Sounding date : 27/04/2017
Location
VES sounding information
Location: (WGS84): 360 46’ 04.3” E 010 49’ 03.7” S
Azimuth of sounding: 7 degrees NS
AB reach: 400 m
Array: Schlumberger
Elevation: 1680 mas
Depth to groundwater (if any)
estimated at: 15 m
Measured apparent resistivity data (Ωm)
Remarks:
Discontinuity at AB/2 = 63m. The AB reach is short; the sounding had to be stopped due to planned TDEM measurements at the
same location.
DATA table
VES20
MN/2 AB/2 Rho Rho shfted
0.5 1.6 86 86
2 103 103
2.5 116 116
3.2 119 119
4 125 125
5 121 121
6.3 112 112
8 86 86
10 79 79
0.5 13 72 72
5 13 74 74
16 72 72
20 71 71
25 78 78
5 32 102 102
10 32 106 106
40 127 127
50 150 150
10 63 179
25 63 151 179
80 174 206
100 196 232
25 130 246 292
50 130 234 277
160 289 343
200 338 401
250
50 320
100 320
400
500
VES measurements
10
100
1000
1 10 100 1000AB/2 (m)
Rho (ohm
m)
Rho shfted
Rho
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P a g i n a | 48
VES interpretation
Inversion results (SchlumBG modelling)
Possible model concepts
5 layer model with unshifted or shifted data
5 layer model, basement can be anywhere deeper than 55 m
5 layer model shifted data basement anywhere deeper than 45 m
Overall results including 90% confidence levels (
Depth (m) Thickness (m) Rho (Ωm)
(top of the layer)
topsoil (1000) 90 (> 45) 2500 (>1000)
Conclusion
• Clay layer
• Basement depth uncertain but anywhere deeper than 45 m
1 10 100 L/2
1
10
1e+2
1e+3
Rho
1 10 100 L/2
1
10
1e+2
1e+3
Rho
-
P a g i n a | 49
ERT comparison
The ERT2 profile (ISGEAG research) shows a high resistive ‘hard cap’ at 25 m, overlying a less resistive (250 ohmm) zone and the
actual basement somewhere at 100m.
We forced the interpretation (by fixing layer parameters) to see if the VES data could correspond with such a solution.
The answer is YES it could (see graphs below), but any solution of such reverted low resistivity under a high resistivity is possible.
So this cannot be found without additional knowledge (the ERT info in our case).
Reverted low resistivity below a ‘hard cap’ high resistivity (VES20 6 layers, unshifted data)
Reverted low resistivity below a ‘hard cap’ high resistivity (VES20 6 layers, shifted data)
1 10 100 L/2
10
1e+2
1e+3
Rho
1 10 100 L/2
10
1e+2
1e+3
Rho
-
P a g i n a | 50
Kajiado VES /IP sounding nr.21 Technical interpretation report
Harry Rolf for SamSamWater Foundation 30/04/2017
Client : Acacia Water
General
VES sounding for VIA Water ISGEAG project, to verify an existing VES sounding
Aim: to confirm VES4 (clay on top of basement) from survey report (Kariuki for mr. Osman, HARAF
Location name: Kajiado, Ildamat, Osiligi , at HARAF farm West of Kajiado River.
Sounding date : 28/04/2017
Location
VES sounding information
Location: (WGS84): 360 45’ 54.4” E 010 49’ 25.9” S
Azimuth of sounding: 7 degr. NSouth
AB reach: 160 m
Array: Schlumberger
Elevation: 1670 mas
Depth to groundwater (if any)
estimated at: 5 m
Measured data Resistivity (Ωm) and IP (mS)
Remarks:
discontinuity in measurements at AB/2 40m. indicating fracture/lateral change. Measurements > 32 m left out in the inversion.
DATA table VES21
MN/2 AB/2 Rho IP (mS)
0.5 1.6 23 0.23
2 23 0.23
2.5 21 0.2
3.2 19 0.19
4 16 0.23
5 14 0.47
6.3 13 0.46
8 13 1.12
10 14 0.94
0.5 13 18 0.47
5 13 19 0.46
16 24 0.82
20 29 0.85
25 36 1.48
5 32 56 1.1
10 32 52 1
40 56 1.73
50 65 1.93
10 63 82 2.04
25 63 76 1.94
80 99 2.48
100
25 130
50 130
160
200
250
50 320
100 320
400
500
VES measurements
0.1
1
10
100
1 10 100
AB/2 (m)
Rho (ohmm)
Rho
IP (mS)
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P a g i n a | 51
VES interpretation
Inversion results (SchlumBG modelling)
Possible model concepts
3 layer Measurements > 32m left out
Overall results including 90% confidence levels (
Depth (m) Thickness (m) Rho (Ωm)
(top of the layer)
topsoil (1000) 3.4 >>>>1000
Conclusion
• This verification VES clearly shows a shallow clay layer on top of high resistive (basement) rock
• Discontinue measurements (>40m) indicate anomaly (fractured?)
The results of this VES can be compared with the VES4 results in the existing survey report, see
borehole survey for Mr. Osman, HARAF farm near Kajiado River, page @
1 10 100 L/2
1
10
1e+2
1e+3
Rho
-
P a g i n a | 52
Kajiado mini VES Technical interpretation report
Harry Rolf for SamSamWater Foundation 27/09/2017
Client : Acacia Water
General
Mini VES sounding for VIA Water ISGEAG project.
Aim: short range VES to test the resistivity of the clay topsoil and training purposes
Location name: ,Kajiado ,Ildamat, Kajiado River on West-East HEP/ERT profile
Sounding date : 27/04/2017
Location
VES sounding information
Location: (WGS84): 1°49'3.50"S 36°46'8.04"E
Azimuth of sounding: Northeast - Southwest
AB reach: 64 m
Array: Schlumberger
Elevation: 1676 mas
Depth to groundwater (if any)
estimated at: 15 m
Measured apparent resistivity data (Ωm)
Remarks:
The mini VES sounding was conducted at a location where compact clays were found (and confirmed by shallow hand drillings).
The electrode distances are very short, starting with potentials at only 5 cm and current electrodes at only 20 cm from the
centre point (see photo). The electrode distances were increased in small steps, from 20cm, 40cm,… up to AB/2 of 32 m.
The field curve shows apparent resistivity initially going down and consequently going up when the current starts passing the
bottom of the clay layer.
VES measurements
1
10
100
0.1 1 10 100AB/2 (m)
Appare
nt re
sis
tivity
(ohm
m)
DATA table
Mini VES
MN/2 AB/2 Rho
0.05 0.2 14
0.4 9.8
0.8 8.7
1.6 7.8
3 7.9
6
0.5 6 9.6
12 15
20 24
32 38
5 32 39
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P a g i n a | 53
Foto ISGEAG field team conducting mini VES (a ‘veeeeery small ves’)
VES interpretation
Inversion results (SchlumBG modelling)
The SchlumBG software assumes the AB/2 distances are in meters. SchlumBG in this case appears to have difficulties with the
data points within one meter. Since the inversion mathematics is linear with electrode distance, the distances are multiplied by
factor 10 to avoid the software problem. The layer model depth results are therefore in dm (decimetres).
Possible model concepts
3layer model (note: electrode distance in decimetres!
Overall results including 90% confidence levels (
Depth (m) Thickness (m) Rho (Ωm)
(top of the layer)
topsoil (1000 ohmm basement)
• According to the ‘rule of thumb’ the Depth of Investigation (1/6 of AB) in this case is like 10 m (AB = 64m)
1 10 100 L/2
1
10
1e+2
Rho
layer depth in dm!!
-
P a g i n a | 54
Existing survey reports The following section contains the re-interpretation of VES soundings from existing survey reports in the area of Kajiado
Town
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P a g i n a | 55
MTTI Masai Technical Training institute survey re-interpretation of VES soundings report
Harry Rolf for SamSamWater Foundation 26/02/2017
Client : Acacia Water
General
Existing survey report: Hydrogeological survey for Masai Technical Training Institute, Kajiado. Report nr. AWS/1572/2008;
S.N.Njoroge. Aquawell Services Nairobi, February 2008
Aim: verification/re-interpretation of existing VES sounding data
Location
VES sounding information
Location: (WGS82): unknown (general location MTTI compound = 36°46'55.3"E 1°51'16.1"S)
Direction of sounding: ?
AB reach: 400 m
Array: Schlumberger
Elevation: 1715 - 1730mas
Depth to groundwater (if any)
Estimated at: ??
MTTI VES Measured data (for numbers, see report)
Remarks
VES4 was advised for drilling, the data may have been shifted to get smooth curves
VES Graphs MTTI (survey report)
10
100
1000
1 10 100 1000
AB/2 (m)
Resis
tivity
MTTI VES1MTTI VES3MTTI VES2MTTI VES4
Dry boreholes issue The MTTI survey comprised 2 HEP lines and 4 VES soundings. Unfortunately the precise locations are not given in the report. (in the
report the VES soundings all have the same coordinates)
Striking fact is that two borehole were initially drilled at the MTTI parcel, both were found dry.
According to a visit paid on 15 Feb 2017 (info Mr. Stephen Musyimi, (0725326932) the situation became more clear. The first dry
borehole (1) (): 36°46'58.83"E 1°51'9.42"S) presumably was drilled at location of VES4 (as advised in the survey report). The second
dry drilling, at 36°46'56.52"E 1°51'30.75"S most likely was at VES1 location. This drilling had a water strike at 70mbgl, but the water
‘disappeared’ when drilling continued to 90 mbgl)
Later (unknown date) a third, good yielding borehole was drilled at 36°47'2.71"E 1°51'9.63"S, just 120 m East of the dry borehole
(1). This borehole is presently known as the Masai Tech borehole. Unfortunately no technical borehole documents could be
made available.
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P a g i n a | 56
MTTI VES1 interpretation
Possible model concepts
Survey report MODEL VES1
(re-interpreted) 6 layer model
There may as well (giving the same fit) be basement (1000ohmm) at 80 m:
8 layer model with basement at 80m
Results VES1 (90% confidence levels)
Depth (m) Rho (Ωm )
topsoil (1000) anywhere > 80m
Conclusion
• Due to the undulations in the field curve, many solutions are possible
• Unknown to what extend the measured data are corrected and whether these undulations are realistic
• Up to 20 m (‘overburden’) a variety of low and mediate resistivities
• Basement may be anywhere below 80 m
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P a g i n a | 57
MTTI VES 2 interpretation
Inversion results (SchlumBG modelling)
Possible model concepts
VES2 MODEL acc. to the Survey report
5 layer model
There may as well (the same fit) be basement (1000ohmm) at 80 m:
6 layer model with basement at 80m
Results MTT VES2 (90% confidence levels)
Depth (m) Rho (Ωm )
topsoil (1000) anywhere > 80m
Conclusion
• Below (low resistive) topsoil there is an overburden zone of 70 -80 ohmm
• Resistivity of the shallow aquifer is depending on the actual depth of basement
• Basement may be anywhere below 80 m
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P a g i n a | 58
MTTI VES 3 interpretation
Inversion results (SchlumBG modelling)
Possible model concepts
Interpreted model from the SURVEY REPORT
5 layer model THE LAST 4 measurements are shifted (indicating lateral change)
There may as well (same fit) be basement (1000ohmm) at 130 m:
6 layer model with basement at 130m
(Even when the last measurements are corrected for the shifts, basement is deeper than 130m)
Results (90% confidence levels)
Depth (m) Rho (Ωm )
topsoil (1000) anywhere > 130m
Conclusion
• Below (low resistive) topsoil there is an overburden zone of 70 -100 ohmm
• Resistivity of the shallow aquifer is depending on the actual depth of basement
• Basement may be anywhere below 130 m
1 10 100 L/2
10
1e+2
1e+3 acc to survey report
-
P a g i n a | 59
MTTI VES 4 interpretation (location advised for drilling)
Possible model concepts
Survey report MODEL VES4
4 layer model
If the 3 (declining) last measurements were correct (!!), basement would be deep (as 200m or more)
5 layer model leaving out last (3) measurements (basement could even be at 50m) giving the same, good fit
Results (90% confidence levels)
Depth (m) Rho (Ωm )
topsoil (1000) can be deep but could as well be at 50m (depending on the last 3 measurements being y/n correct)
Conclusion
• Below topsoil there is an thin overburden zone of 30 ohmm
• Resistivity of the shallow aquifer is depending on the actual depth of basement
• Basement may be at 50m if the last declining measurements are false, if not basement is deep.
VES4 was advised for drilling and turned to be a dry borehole (drilling showed “just dust”). I tend to believe that the last 3
(declining) measurements were wrong.
1 10 100 L/2
10
1e+2
1e+3 SURVEY REPORT
MODEL
1 10 100 L/2
10
1e+2
1e+3
-
P a g i n a | 60
Kajiado Masai Stores survey Re-interpretation VES existing survey report
Harry Rolf for SamSamWater Foundation 26/02/2017
Client : Acacia Water
General
Existing survey report:
Hydrogeological/Geophysical survey report for one production borehole in LR.NO; 9923/262 forMasai Store Ltd Kajiado. Report
by E.Njaramba, NYIKA Associates Ltd Groundwater Consultants
Aim: verification/re-interpretation of existing VES sounding data
Location
VES sounding information
VES
Location: (WGS82): 1°50'52.87"S 36°47'24.75"E
Direction of sounding: ?
AB reach: 400 m
Array: Schlumberger
Elevation: 1730 mas
Depth to groundwater (if any)
Estimated at: ??
For measured data, see survey report
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P a g i n a | 61
VES interpretation
Inversion results (SchlumBG modelling)
Possible model concepts
Model and measurements from the survey report and apparent resistivity acc. to SchlumBG
REMARKABLE DIFFERENCES ON THE SAME DATA ???
Above: Model (fixed parameters) and measurements according to the survey report, entered in SchlumBG software does not
give a good fit.
Below: In the report, the same model on the same data shows a good fit,
WHAT COULD EXPLAIN THE GOOD FIT as given in the report (below)??? Can it be difference in the software used or…….??
1 10 100 L/2
10
1e+2
1e+3
Model from SURVEY
REPORT
-
P a g i n a | 62
Re-interpretation:
2 possible model concepts (comparable good fit)
4 layer model and 5 layer model
4 layer model
5 layer model , having an in-between lower resistive layer
Results (confidence levels unknown)
Depth top (m) Rho (Ωm )
topsoil (1000) 50 -130 >>> 1000
Conclusion
• Overburden in the order of 50 ohmm
• Shallow aquifer resistivity uncertain
• Top of Basement in between 50 and 130 m
In the survey report the last layer (> 64m) is interpreted as 390 Ωm. According to the re-interpretation the resistivity is higher
(> 1000 Ωm)
What is the explanation for the bad fit we are getting in SchlumBG for the interpreted model of the survey report?
1 10 100 L/2
10
1e+2
1e+3
Rho
1 10 100 L/2
10
1e+2
1e+3
Rho
-
P a g i n a | 63
Kajiado survey for Mr Osman in Town Re-interpretation existing VES
Harry Rolf for SamSamWater Foundation 27/04/2017
Client : Acacia Water
General
Survey report for Mr. Osman (HARAF):
Hydrogeological report survey for Mr. Hamud Mohammed Osman, carried out along Great North Road. S.N.Njoroge, Aqua Well
Services, Nairobi, report nr. AWS/2673A/2011, December 2011.
Aim: to verify and re-interpret existing survey report soundings
Location name: ,Kajiado Township block 1/229
Date : December 2011
Location
VES sounding information
Location: (WGS84): approximately 360 47’ 25” E 010 50’ 45” S (coordinates in report are probably in ARC1960, not in
WGS84, coordinates estimated from map)
Azimuth of sounding: N-S
AB reach: 500 m
Array: Schlumberger
Elevation: 1733 mas
Depth to groundwater (if any)
estimated at: ? m
VES interpretation
Measured data Resistivity (Ωm) SEE SURVEY REPORT
Remarks on the data:
Remarkable 1 point anomaly at AB/2 = 32 and Only one potential change (at 32m) ??
Inversion results (SchlumBG modelling)
Model from the Survey Report 10 layers!! Basement (688 Ωm) at 200 m !
1 10 100 L/2
10
1e+2
1e+3
SURVEY
REPORT MODEL
-
P a g i n a | 64
Possible model concepts: 4 layer model
4.layer model basement at 134 m
4.layer model basement at 100 m, having he same GOOD fit
Overall results including 90% confidence levels (
Depth (m) Thickness (m) Rho (Ωm)
(top of the layer)
topsoil ( 90 105 (93 – 130)
Basement (ρ>1000) > 90
Conclusion
Instead of the 10 layer model in the survey report, a 4 layer model has the same good fit.
Basement depth uncertain, but deeper than 90 m
1 10 100 L/2
10
1e+2
1e+3
Rho
1 10 100 L/2
10
1e+2
1e+3
Rho
-
P a g i n a | 65
Borehole survey for Mr. Osman, HARAF farm near Kajiado River
Re-interpretation of soundings form existing survey report
Harry Rolf for SamSamWater Foundation 22/04/2017
General
Survey report:
Geophysical and hydrogeological investigation J.W.KARIUKI, Groundwater Max Ltd, report nr. GML- 029/07, Feb 2007.
Aim: re-interpretation of geophysical soundings, to understand high yielding HARAF Trades borehole., as part of ISGEAG
geophysical methods research project near Kajiado (Kajiado River)
Location name: Ildamat , Osiligi, 3 km NW of Kajiado Town
Sounding Date : Feb 2007
Location VES4 has been conducted close to ‘HARAF BH2’ close to the Kajiado River
LOCATIONS ARE NOT CLEAR. The location of VES4 (advised for drilling) is estimated from the hand-drawn map in the survey
report.
Note: the coordinates for the selected site are given in the report as 1 49 15 – 36 46 08. These could be in ARC 1960 grid system,
but, even when transferred to WGS84 (01 49’ 24.2” 36 46 10.9) the coordinates do not point at the location as given on the
hand-drawn map.
5 VES soundings were conducted in the survey, along 1 HEP profile line: VES4 was advised for drilling.
The re-interpretation results of only VES4 is given below. The other soundings are not (yet) re-interpreted.
VESnr.4 sounding information
Location: (WGS84): not given in report presumably: 1°49'25.90"S 36°45'54.40"E
Direction of sounding: not given
AB reach: 500 m
Array: Schlumberger
Elevation: 1670 mas
Depth to groundwater (if any)
Estimated at: 5 m
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P a g i n a | 66
Measured data
Remarks:
the south end of the line must have been close to the powerline at AB/2=250 m
The field curve already indicates a clay layer and shallow basement
VES re-interpretation
Inversion results (SchlumBG modelling)
Possible model concepts
4 or 5 layer model
4 layer model , basement at 23 m
5 layer model, basement at 21 m
DATA table HARAF VES4
MN/2 AB/2 Rho Rho shifted
0.5 1.6
2 10.6
2.5 9.2
3.2 7.4
4 5.9
5 5.7
6.3 6.3
8 7.2
10 9.7
0.5 13 12.2
5 13
16 12.8
20 15.6
25 20.3
5 32 25.7
10 32
40 28
50 36.2
10 63 46.2
25 63
80 63
100 73.6
25 130 92.8
50 130
160 117.7
200 155.6
250
50 320
100 320
400
500
VES measurements
1
10
100
1000
1 10 100 1000AB/2 (m)
Rho (ohm
m)
Rho
Rho shifted
1 10 100 L/2
1
10
1e+2
1e+3
Rho
1 10 100 L/2
1
10
1e+2
1e+3
Rho
-
P a g i n a | 67
Re-interpreted results
Depth (m) Rho (Ωm)