Specifications

University of Pretoria etd – Combrinck, M (2006)

Chapter 2

2 OVERVIEW OF ELECTROMAGNETIC THEORY FOR GROUND, IN-

LOOP TDEM DATA

"There is a theory which states that if ever anybody discovers exactly what the Universe is for and why it is here,

it will instantly disappear and be replaced by something even more bizarre and inexplicable. There is another

theory which states that this has already happened.”

(from “The Hitchhiker’s guide to the Galaxy”, Douglas Adams)

2.1 Introduction

It is the inherent complexity contained in the electromagnetic method that on the one hand

forces us to simplify it almost beyond recognition in order to apply it practically; and on

the other hand compels us to keep on searching for the “truth” (or the most complete set

of information that can be obtained from EM measurements). This continued search is

conducted in many, painstakingly slow, very small steps – sometimes referred to as

research.

All EM methods (except the magneto-telluric method) utilise active sources, or

transmitters, in order to generate measurable fields. This is in contrast to passive methods

such as the potential field methods (e.g. the gravity and magnetic techniques) where the

earth’s natural occurring magnetic field is measured. Passive source method instruments

only consist of receivers and the data acquired by different instruments are all reduced to

the earth’s total field with no or minimal computational effort. Active source instruments,

on the other hand, require the complete specifications of the source and receiver to be

incorporated into the processing and interpretational procedures. Unless stated otherwise,

algorithms derived in this study are only suitable for systems conforming to the same basic

geometrical set-up and operating parameters as described in the following paragraphs.

2.2 System geometry and operating specifications

The theory developed in this study is directly applicable to central-loop ground TDEM

systems with step-current excitation, e.g. Geonics EM37 or EM47 and interpretation