Specifications

University of Pretoria etd – Combrinck, M (2006)

Chapter 3

3 CONVENTIONAL INTERPRETATION TECHNIQUES

"The definition of insanity is doing the same thing over and over and expecting different results.”

(Benjamin Franklin, 1706-1790)

3.1 Introduction

The ultimate goal of doing any geophysical survey is to deliver a map or model indicating

the subsurface distribution of physical properties, i.e. conductivity in the case of TEM and

to interpret this data in terms of geology. When geological information are added to the

geophysical model it is possible to link lithology or structures to distinct geophysical units

of different conductivities and this serves as a very useful tool in constructing a final

geological model.

This study is concerned mainly with the conversion of TEM data to a reliable subsurface

conductivity distribution. Traditionally there are two separate classes of exploration targets

for TEM surveys and based on these classes different survey geometries and

interpretational procedures are followed. In modern day geophysics these two classes of

surveys increasingly overlap but it is still important to understand the different approaches

that were followed in the past as most of the interpretation techniques being used today

were developed for very specific target conditions.

The first type of target is considered to be a confined conductor(s) in a resistive host rock.

The theoretical assumptions made in this case are basically that the host rock has no

influence on the electromagnetic propagation of fields and conductors are considered to be

suspended in free space for all mathematical purposes. TEM surveys to detect this type of

target are designed to emphasize lateral variations in conductivity and are called profiling

surveys. The geophysical model derived from interpreting a survey like this would contain

the location, conductivity and geometrical parameters of one or more finite conductors

(two- or three-dimensional). These conductors would be simplified geometrical shapes

such as spheres, ellipsoids, plates or prisms approximating true geological units. Typical

geological targets are massive sulphides or linear structures like weathered faults or shear

zones.