3rd SAGA Biennial Conference and Exhibition

The basic principles of the ground penetrating impulse radar
(GPR) sub-surface imaging technique are summarised.
Various applications are described where GPR has shown
significant potential in assisting geologists and rock engineers
on mines to delineate ore and detect hazardous mining
conditions. A new mine-worthy GPR system currently under
development is outlined.

This paper documents the application of a seismic
stratigraphic approach to the interpretation of a high
resolution, continuous- reflection shallow-seismic dataset,
collected over the middle shelf off southern Namaqualand.
This technique is more commonly used to interpret low
resolution, deep-penetration seismics and its application
has received little documentation within the context of
high-resolution, shallow-seismic studies.

The primary purpose of this study was to identify geophysical
methods that were cheap, quick, readily available for use which
would yield an indicative response over a known kimberlite. The
following methods were selected for assessment: ground magnetics,
self potential, radiometrics, horizontal loop electromagnetic (HLEM),
and very low frequency (VLF) EM. Gravity results from a previous
survey conducted in the 1970’s by O'Okiep Copper Company were
sufficient to indicate that the gravity method is successful. An
opportunity to evaluate ground penetrating radar became available
and this method was tested and proved unsuccessful. The results of
this survey indicate that the magnetic, gravity, radiometric, and
HLEM methods all yield indicative responses over the Goedgevonden
kimberlite,

The Western Cape is an area of low to
intermediate levels of anomalous seismicity, and as
a result defined as a relatively low risk seismic
hazard area. This assumption has traditionally been
based on relatively few macroseismic observations,
and the apparent lack of geological evidence
supporting neotectonic ground rupture. In this
paper we show that geological methods of seismic
hazard assessment are unsatisfactory for defining
risk assessment in intraplate anorogenic zones, and
report on preliminary findings from an ongoing
microseismic investigation into the South Western Cape.

In this paper the geophysical properties and signatures of the crust
of the Kaapvaal craton are discussed. Particular attention is paid
to observed increased levels of magnetisation, continuity thereof
and correlation with other geophysical signatures. It is proposed
that the higher levels of magnetisation, comparable or even greater
than those reported for lower crustal sections exposed elsewhere,
arise as a result of the recrystallisation of iron to magnetite in a
granite-gneissic basement. The controls for this phenomenon
include the presence of fluids and elevated oxygen fugacity ac
depth. The higher magnetisation manifests itself as regional
magnetic anomalies, providing an excellent means for mapping
crustal structure.

A concise description of the seismic tomography technique is
given and the basic philosophy of the interpretation procedure is
discussed. Results from two recently completed case studies are
described; one application to coal exploration and the other to
determine in situ physical properties of a rock mass in dam
foundation studies.

Positional information derived from the Global
Positioning System is rapidly becoming the
standard for spatial control in geophysical surveys.
Applications vary from general navigation where
100m accuracy is sufficient to sub-centimetre
accuracy for gravity networks. However, the
problem facing the fïeld geophysicist is to obtain
high accuracy data in a very short time.

Geoid anomalies covering both continental and oceanic
domains are computed by subtracting long-wavelength
components from the total gravitational potential field,
using spherical harmonic analysis. These residuals
reflect variations In underlying mass distribution which
may or may not be in equilibrium with surface
topography. The ratio of geoid residuals to
topography, referred to as geoid impedance, gives a
measure of the response of the earth’s surface to
forces responsible for the observed mass distribution,.
Using data from the African plate, we show that
imaging the geoid impedance can aid detect
geotectonic processes. We found that Continental
plateaus and Precambrian shields are correlated with
intermediate-to-high impedance signatures whereas
sedimentary basins are, in general, characterised by
higher magnitudes, Atypical basins in the impedance
map such as the Karoo basin are associated with
background intermediate-amplitude signal.
Geodynamic processes which are congruent with the
impedance observations include lithospheric thinning
and crustal thickening, upper-mantle upwelling,
accumulation of mafic/ultramafic rocks within the
continental lithosphere, amagmatic extension and
volcanic intrusion in the crust,

The deep structure of the East African Rift System and adjoining regions are investigated using recent gravity data from the Western Rift area and in addition, those of the whole East
Africa.