Page 304 - Introduction to Mineral Exploration

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center. Unfortunately the major geophysical ductivity meant that the method proved to be
surface did not correlate well with the surface very successful in delineating the shallower
of the markfieldite as identified by borehole in- (<20 m deep) bedrock features (Fig. 12.5). Al-
formation. It was assumed that this was be- though borehole information allowed some
cause the seismic method was delineating the correlation of conductivity with depth, the
base of a weathered or fractured layer. In this results remained essentially qualitative and a
particular deposit the depth of weathering is up resistivity survey was carried out to quantify
to 20 m on the higher ground, but significantly the data (Fig. 12.6).
less in the bedrock on the flanks of the The quantification of the conductivity con-
hill where the older rocks were more deeply tours was attempted by taking a series of resis-
buried below the marls of the Triassic Marcia tivity soundings (electrical depth probes) along
Mudstone. Much of the weathered rock has sectors of equal conductivity. This technique
to be quarried conventionally and can be used produces a more accurate depth reading at a
for lower specification purposes. The seismic point, by avoiding significant lateral variations
refraction method in this application had the in overburden thickness. The resistivity sound-
effect of making the bedrock surface appear ing methods used employed the British Geo-
generally deeper than it is and also less variable logical Survey (BGS) multicore offset sounding
at depth. cable (a Werner configuration) with an Aberm
Terrameter. An electrical current is passed
through two electrodes and the potential dif-
Electrical resistivity and electromagnetics
ference is recorded across a further pass of
Planning permission was granted in August electrodes to produce a resistance value in the
1983 by the Leicestershire County Planning standard array. The curves produced are com-
Department. Immediate priority was given to puter processed to give depths to bedrock at the
the detailed investigation of the site for devel- measurement sites.
opment. An accurate overburden volume figure With the highly variable nature of the over-
was required and insufficient boreholes had burden thickness, the electromagnetic results
been drilled. Further geophysical surveys were proved to be very important for accurately
commissioned to obtain additional detail of the positioning the resistivity lines so that they
overburden distribution. did not cross any sharp lateral variations in
Overburden thicknesses were required on a bedrock topography. This greatly increased the
50 m grid across the area investigated in Phase quality of the resistivity data.
1, and it was decided to use a combination Combining the electromagnetic and resist-
of electrical resistivity and electromagnetic ivity results with borehole data produced an
techniques to produce the detail required. overburden isopach map (Fig. 12.4). The most
The area was traversed using the Geonics important features identified from this plan are
EM34 Ground Conductivity Meter (20-m and the two shallow “ridges” (marked A and B on
40-m coil separations). The Mercia Mudstone, Figs 12.5 & 12.6) away from which the overbur-
which composes most of the overburden, is den thickness increases rapidly. Neither of
considerably more conductive than the under- these features had been identified by the earli-
lying markfieldite. Thus an increased thick- est drilling and, because of their narrow nature,
ness of the overburden produced an increased might have remained undetected for some
conductivity reading. The readings, reflecting time. Their presence resulted in a significant
the degree of conductivity of the different ma- reduction in calculated overburden volume.
terials, were then used to produce a conductiv-
ity contour map. Therefore, by traversing the
site it was possible to produce a qualitative Reflection seismics
picture of overburden variation. Although the Recent modification to seismic source, detec-
assumption of a two-layer model of overburden tor, recording equipment, and field techniques
and bedrock oversimplified the situation some- has improved the seismic reflection method of
what, e.g. an additional boulder clay layer, etc. exploration to enable it to be used sucessfully
could be present, the marked contrast in con- for exploration at shallow depths. The system