Specifications

University of Pretoria etd – Combrinck, M (2006)

Chapter 1

1 INTRODUCTION

"I believe that we cannot live better than seeking to become still better than we are."

(Socrates)

1.1 General overview

The Time Domain Electromagnetic (TDEM) method, also referred to as Transient EM

(TEM), has been used in mineral exploration since the late 1950’s. The basic theory of

this method is defined in totality by four differential equations, known as Maxwell’s

equations. These equations describe the relationships between electric and magnetic

fields and sources, enabling the geophysicist to derive the electric field (E) in the earth

through measuring the magnetic field (B), or it’s time derivative (dB/dt) on, above or

below the surface. The electric field in the earth is a function of the electromagnetic

(EM) source and of the earth’s conductivity distribution. If the source is known, the

geophysicist should be able to derive the conductivity distribution of the subsurface (and

therefore be able to isolate and interpret zones with contrasting conductivities).

• The success of this procedure is dependant on three factors:

• Choosing a source (transmitter) of appropriate dimensions and magnetic moment

to induce currents in the subsurface volume of interest

• Measuring the induced magnetic field and/or its derivative with the required

accuracy and resolution in time and space (receiver) to resolve the target

• Accurately solving Maxwell’s equations

The first two factors are concerned with the design of the measuring instrument and data

acquisition parameters. Survey costs are the largest limiting influence on these factors

and increases in cost can only be justified by producing increasingly accurate geological

models leading to savings in other stages of exploration expenses, e.g. drilling.

Theoretically, within some engineering constraints, one can choose to measure or

“know” the whole field exactly. This is a scenario utilised very effectively through