Specifications

University of Pretoria etd – Combrinck, M (2006)

where mathematical solutions are found subject to geological truths such as dip, strike,

conductivity ranges and limits on dimensions of bodies. However, this information is very

rarely available in the exploration industry before TEM interpretations have to be done.

Consequently, it is very difficult at this point in time to see inversion as a fully automated

procedure for interpretation of TEM data, although it could possibly be involved as a final

stage of processing after initial models have been found through alternative routes.

3.5 Decay curve analysis

Decay curve analysis is an extremely useful tool with its major strength probably being

simplicity. The TEM method is based on current distribution changes with time, and

decay curve analysis is the simplest way of analysing the time-varying fields associated with

this phenomenon. In Chapter 2 the specific equations describing the decay behaviour for

general models were given with specific reference to the late time. A summary of these late

time approximations is given in Table 3.1. Decay curve analysis is a tool that helps the

interpreter to distinguish between the two basic classes and subsequent interpretation

strategies as mentioned in 3.1. Plotting all data, station by station, on both log-log and log-

linear graphs and analysing the slopes of any data forming straight lines in the late time

yields very good information on the geological structure in two dimensions. It allows the

interpreter to immediately divide the survey area up into regions containing the four

models shown in Table 3.1, with the only complication that a conductor in conductive host

rock and with very low contrast may be grouped with half space occurrences at this point.

However, what is more likely to happen from experience in Case History 1, Chapter 5, is to

find stations showing both isolated conductor and half space behaviour but at different

time channels. Similar behaviour was also described at the Elura massive sulphide deposit

which is situated under a conductive overburden, Australia, by Spies (1980). Decay curve

analysis as described here doesn’t give any information on depth of conductors, although

the decay constant (τ) found from inverse slope of model 3 (and sometimes model 4)

graphs is related to the dimensions and conductivity of the causative conductor McNeill

(1980), allowing a further division to be made between targets worth further investigating

or not. In instances where the subsurface geo-electrical structure is too complex to be

approximated by either of the models in Table 3.1, neither of the described characteristics

will be found on the decay curve and stations like this will form a class of their own and

require some special attention in later stages of interpretation. One of these more complex

decay curves involves a sign change in the vertical component of the ∂

B/∂t measurements