an e-w gravity profile across the la bajada fault zone in the rio grande rift, north central new...
TRANSCRIPT
An E-W gravity profile across the La Bajada fault Zone in the Rio Grande Rift, North Central New Mexico
Rajesh GotetiUniversity of Rochester
SAGE 2007
OutlineOutline
IntroductionIntroduction
Gravity SurveyGravity Survey
Data Analysis and InterpretationData Analysis and Interpretation
ConclusionsConclusions
Introduction
La BajadaFault Zone
Profile Line
Lacoste Romberg analog gravity
meterScintrex digital gravity meter
Equipment
Data Reduction Tidal and Drift Correction
Latitude Correction
Free-air Correction
Bouguer correction
Terrain Correction
Drift & Tidal Corrections
Inner Zone Terrain Correction
Complete Bouguer Anomaly
Contouring the Anomaly
Gravity Method Flowchart
La Bajada Fault Zone
Complete Bouguer Anomaly Map on the geologic map
∆ρ = 0.47 g/cc
C. I. = 1 mGal
Steep gradient across the anomaly contours coincideswith the location of mappedfault zone trace
0 24 km
Complete Bouguer Anomaly Map
C. I. = 1 m. Gal
∆ρ = 0.47 gm/cc
High Low
Possible Location of the La Bajada Fault Zone
+ Gravity Stations
SFFLBF
TF
LBF
High Low
Differences Between CBA and RBA
SFF
Regional Aeromagnetic Survey Map
Gravity Profile Line
nT
High
Low
Steep gradient supports the possibilityfor a fault zone
La BajadaFault Zone
Profile Line
Geological Map showing the location of the profile
in the gravity survey
Residual Gravity Anomaly Profile
Easting (m)
RA(mGal)
So what do we have so far ?
Complete and Residual Bouguer Anomalies maps tohelp identify the La Bajada fault surface trace
Aeromagnetic map which predicts a fault trace that agreeswith the fault trace above
Residual Gravity anomaly profile for the line of interest Model Density contrast of 0.47 gm/cc
In addition
Transocean Oil Company seismic lines to (1) constrain the depth to
‘basement’ in both the footwall and hanging wall of the La Bajada fault
and (2) throw on both the fault segments
Velocity estimates from Baldridge et al (1994)
TransoceanSeismic Line78-7 SP 90
TransoceanSeismic Line79-1 SP 70
N
12 km
2-D Forward Model using GM-SYS
TO lines
Model DerivedObserved
Misfit
78/7Sp 90
79/1Sp 70ρ = 2.2 gm/cc
ρ = 2.67 gm/cc“Basement”
3.6 Km
1.4 Km
0.5X
(km)
TO lines
Model DerivedObserved
Misfit
78/7Sp 90
79/1Sp 70ρ = 2.2 gm/cc
ρ = 2.67 gm/cc “Basement”
3.8 Km1.4 Km
0.5X
ρ = 2.4 gm/cc
ρ = 2.67 gm/cc “Basement”
Non- uniqueness
Conclusions
Model DerivedObserved
Misfit
78/7Sp 90
79/1Sp 70ρ = 2.2 gm/cc
ρ = 2.67 gm/cc“Basement”
3.6 Km
1.4 Km
0.5X
Conclusions (contd..)
Gravity surveys can be used to locate faults (e.g. La BajadaFault) and estimate approximate depths of layers based ondensity contrast.
Gravity surveying is a relatively inexpensive, fast techniquefor a first order insight into the subsurface geology.
Subsurface models based on gravity cannot yield uniquesolutions. Other geophysical techniques (e.g., seismics) cancomplement and constrain the gravity model better.
Acknowledgements
-Shawn, George, Darcy
-Scott for the wonderful field trips
-Team members
-SAGE faculty/staff for giving this enrichingexperience
Group Conclusions
1D Forward Model1D Forward Model
101
102
103
Depth(m)
101 102100
Rho(ohm m)
App Rho(ohm m)
Phase
Period(s)
45o
10-4 10-3 10-2
90o
0o
101
102
100
1D Occam Inversion, 8 layers forward modelStation 1 Station 2 Station 3
104
103
102
101
100
101 102
(m)
104
103
102
101
100
101 102
(m)
104
103
102
101
100
101 102
(m)
Gravity modeling can be used to locate faults and othergeophysical techniques can be used to constrain a gravitymodel and generate a reasonable subsurface geologicalinterpretation
Seismic reflection shows no evidence of Tanos fault. Creating a synthetic seismogram for Hawley’s geologicalcross-section may hint about the signatures of Tanos fault
The shallow depths along the survey line modeled withrefraction seismics does not reveal any faults
Thanks!!!!