Specifications

University of Pretoria etd – Combrinck, M (2006)

3.6 Transforms (Depth imaging)

Spies and Frischknecht (1991) describe depth imaging techniques as low cost (i.e. fast and

automated) alternatives to modelling and inversion to provide an approximate image of the

resistivity section directly from observed data. Originally developed as a “processing” step

they now appear very useful for interpretation. Today there are a number of different

imaging schemes or transforms (Macnae and Lamontagne, 1987; Nekut, 1987; James, 1988;

Eaton and Hohmann, 1989; Smith et. al., 1994; Wolfgram et al., 1998; Tartaras et al., 2000),

all based on simplifying some of the governing equations to such an extent that a direct

solution for the depth and conductivity parameters can be found either analytically or

through curve matching. Although not as accurate as full modelling or inversion the

advantages of these schemes are computational speed and no need for initial models or

other interactive user input. These methods can therefore be considered as completely

automated.

3.6.1 Conductivity Depth Images (CDI’s)

This technique has been developed by Macnae and Lamontagne (1987) for step-response

sounding data and is routinely used in processing airborne TEM data. At each delay time

the variation of the step-response as a function of geometry is transformed to an equivalent

reference depth h, which can be related to the depth of electromagnetic field diffusion.

The behaviour of h as a function of delay time is nearly independent of the source-receiver

geometry. The slowness ∂t/∂h divided by the magnetic permeability is almost exactly

proportional to the cumulative conductance measured from the surface down to a depth h.

Thus an apparent conductivity (termed the “imaged conductivity”) can be estimated at

depth by ∂

t/µ

∂h

. The result is a conductivity-depth section that can be generated

automatically with no prior input needed from the operator. This method is based on the

receding image theory of a plane (or S-layer).

3.6.2 Stationary current images (SCI) (SCI - trademark of Geoterrex-Dighem Pty Limited)

This method highlights the edges of conductors, gives an indication of dip and allows a

qualitative estimate of the conductance of localized conductors (Wolfgram et al., 1998).

The SCI emphasizes structural features because it is optimised for lateral contrasts in

electrical conductivity. It is an empirically developed method only applied to GEOTEM