Specifications
University of Pretoria etd – Combrinck, M (2006)
slope values that need to be tested for and any regression fit within the specified R
2
limit is
accepted as a possible conductor. The channels exhibiting the exponential decay are stored
as well as the decay constant which is calculated as the inverse of the “best fit” slope.
4.3.3 IP effect or severe lateral variation in subsurface conductivity
The test for sign change of the ∂B
z
/∂t component is done because with the central loop
sounding configuration it implies a deviation from a layered, quasi-static subsurface. Sign
changes can be ascribed to an IP (induced polarization) effect or significant lateral
variations in the subsurface conductivity distribution, including two- or three-dimensional
geological units. Having negative values of ∂B
z
/∂t in a data set does not necessarily imply a
sign change and it is important to distinguish between a well sampled, repeatable sign
change and (normally late time) erratic noise. This was done by requiring at least two
points of a specific sign to be followed by four points of opposite sign before a sign change
is flagged in the data.
4.3.4 Results, output and presentation
The process of decay curve analysis as described here is extremely quick and takes less than
a second for 1600 stations on a Pentium IV PC. The parameters determined through these
functions are written to the output ASCII file and can be presented in GEOSOFT as
symbols on plan or pseudo-section format. A very useful presentation for first anomaly
isolation was also found through contouring the calculated decay constants (see Chapter 5).
This immediately allowed areas of anomalous conductivities to be isolated in plan view. It
is important to remember that all these procedures will indicate positions of anomalous
data points and not yet positions of conductors themselves.
4.4 Numerical calculation of the S-layer differential transform
4.4.1 Defining the S-layer transform
The S-layer differential transform was discussed briefly in paragraph 3.4.3. It was chosen
as the preferred imaging algorithm to include in the Sounding Interpretation program for
the following reasons,
33