The main conclusions and findings of this investigation
may be summarised as follows:-(1) The Geophysical Megger Earth Tester is an admirable
machine for resistivity work. The only trouble experienced
was the entry of water into the generator during very wet
weather.(2) With very thin vertical sheets the longitudinal traverse
gives a more positive and more easily recognised anomalous
resistivity curve.(3) With thin vertical sheets the transverse traverse gives a better idea of the likely direction of the sheet.(4) W- shaped resistivity curve is obtained from longitudinal
traverse over both conductors and insulators, but with the
former, the centre of the anomaly never rises above the country
resistivity.(5) The problem of traversing over vertical sheets may be
solved by the theory of images. The resulting equations,
although complicated looking, may be solved by various short
cut methods.(6) The types of curves obtained from longitudinal traverses
vary greatly according to the thickness of the sheet, there
being a typical curve for widths of sheet between integral
multiples of the electrode interval until the sheet becomes
wider than the whole electrode spread. These curves should
be valuable in the interpretation of vertical bodies with wide
extension along the strike such as dykes, fault planes or zones
and highly dipping strata.(7) Transverse traverses over thin insulating sheets yield a
double peaked apparent resistivity curve. This soon gives way
to a single peak as the sheet width increases. The converse,
i.e. double and single troughs, is found over conductors.(8) The problem of traverses over inclined sheets cannot be
solved by the theory of images, but type curves can be obtained
from laboratory experiments.(9) Buried sheets yield similar but more rounded and less
evident curves as the depth of cover increases.(10) The method of traversing can often prove valuable in the
field in the location and exploration of faults, dykes and
buried channels. Shallow underground fires may also be within
its scope.(11) Two -layer problems are best_-solved by Tagg's method.
Moore's method of cumulative resistivity curves cannot be held
as satisfactory.(12) The effect of lateral variations in resistivity on depth
probes can be investigated by the theory of images, and can
often be recognised in the field curves. Expanding probes
crossing an interposed sheet of different resistivity will
yield resistivity -depth curves with recognisable discontinuities. Depth probes parallel to the strike of such a sheet
yield curves similar to those of two or three horizontal layers
and great care will be required in their interpretation and
recognition
