“Frequency domain” EM “Time domain” EM

“Time domain” EM

Measurement of signal decay – sample the decaying amplitudes in a number of time windows, or “channels”

As the receiver is moved over the target, the response of each channel is recorded separately

“Time domain” EM example

• graphitic black shale is very conductive

• response persists to very late channels (good conductor)

• asymmetry changes

• early channels have a maximum to the right

• late channels have a maximum to the left

EM methods: General comments

Major advantages

• Quick and efficient (relative to resistivity/IP)• Good depth penetration (up to 1000 m for time domain

EM, but perhaps 50 m for airborne systems)• Tunable for specific targets (coil orientation, separation,

frequencies, etc)• Interpretation charts can yield the conductivity-thickness

product)• Detailed modelling and interpretation possible

EM methods: General comments

Disadvantages

• Requires connected conductive targets• Conductive overburden drastically reduces penetration• Some conductor geometries are invisible – may require

more than one coil orientation/configuration to detect

EM case studies in exploration and engineering

VLF systems

Primary field, P is horizontal

• where a conductor is present this changes (“tilts”) the total field (P + S)

• tilt angle survey will “crossover” over a conductor

Facing the transmitterSchematic view

VLF from Atlantic Nickel, New Brunswicka) Profiles

VLF from Atlantic Nickel, New Brunswickb) Contours

C23= (t3+t4) . – (t1+t2)

(designed to transform crossovers into positive peaks, and smooth noise)

HLEM profiles from Woburn, Quebec

Note absence of a signature from disseminated sulfides

Characteristic curves used to extract the following information:

Dip ~ 60o

Use of characteristic

curves

Student exercise: Given the profile on the right, and the characteristic curves above, estimate: i) The depth to the conductor edge, and ii) The conductivity-thickness product for the conductor.

Survey parameters:Source-receiver spacing 61 mFrequency 800 HzMagnetic permeability 4 x 10-7 N.A-2

(Units should cancel, but perhaps you need to check …)

Quadrature AEM in the Canadian Shield

Note the low separation between in-phase and out-of-phase response, indicating an intermediate conductor, response parameter ~ 1.5

Quadrature AEM in the Canadian Shield

Note rather poor agreement between various methods – the sulphide deposits appear on the mag survey, but the large negative peak remains unexplained.

Airborne EM, northern SwedenContour map of real component anomalies, Skellefta orefield

Notes:

• Mean ground clearance 30 m, operating frequency 3.5 kHz

•Contours are in ppm of primary field

• Anomaly belts to the SW correspond to graphitic shales

• Belt just to the north of these is due to sulphide ore, but partly obscured by power cable

• Northern part of the area contains 3 distinct anomaly centers, all correspond to strong sulphide mineralization

HLEM and resistivity, aquifer,

western Africa

Conductive weathered layer obscures fractured quartzite aquifer

Only one of the three anomalies is visible on DC resistivity

Anomaly C was drilled, aquifer encountered at 30 m depth

HLEM, environmental survey

The extent of the waste dump is clearly indicated by the minimum on all curves

The high conductivity clays shift the curves at different frequencies

Time domain EM, Brazil

Thickness, conductivity of the weathered layer is influenced by rock type

Mafic volcanics are visible in all six channels

Greywacke is only apparent on channels 1-4

Ground conductivity over a contaminant plume, Portugal

Groundwater contamination is mapped using a vertical loop Geonics EM-34

Intercoil spacing was 20 m, contamination plume is observed extending to the west – conductivity is more than 300% of background values

Mapping soil salinityVertical loop, 3.7 m coil spacing

16 lines, spaced at 50 m intervals, stations every 5.5 m

Low apparent conductivity indicates unsalinized soils, but conductivity highs indicate extreme salinity.

Mapping clay and glacial scour

Images from Ford, Keating and Thomas (2007) – see course web pages

Small aircraft fitted with magnetometer in rear stinger (Sander Geophysics)

de Haveilland Dash 7 fitted with MEGATEM EM system (Fugro)

Images from Ford, Keating and Thomas (2007) – see course web pages

Schematic of time-domain airborne EM system

Images from Ford, Keating and Thomas (2007) – see course web pages

Images from Ford, Keating and Thomas (2007) – see course web

pages

Images from Ford, Keating and Thomas (2007) – see course web

pages