Page 164 - Introduction to Mineral Exploration

7: GEOPHYSICAL METHODS 147

10

D S
20 B

30

M
40

50

60

70

FIG. 7.16 Seismic reflection profile across a buried submarine channel. Vertical scale is two-way reflection
time (TWT), in milliseconds. Ten milliseconds TWT is approximately equivalent to 7.5 m of water or 10 m
of soft sediments. The reflector indicated by “S” is the sea floor and “B” is a strongly reflecting “basement”
surface. “M” is a multiple of the seafloor, created by wavefronts that have made two trips between the
reflector and the sea surface. “D” is created by the wave that has traveled directly through the water from
the source to the receiver. (Reproduced with the permission of Dr D.E. Searle and the Geological Survey of
Queensland.)

far enough from the survey area for plane-wave lack coherent layering. One obvious applica-
approximations to apply, since complex correc- tion is in the search for offshore placers. Sub-
tions are needed where this is not the case. bottom profiling using a sparker or boomer
Unfortunately, because distances in the AMT source and perhaps only a single hydrophone
equations are not absolute but are expressed in detector can allow bedrock depressions, and
terms of wavelengths, near-field corrections hence areas of possible heavy mineral accumu-
have to be applied to at least the low frequency, lation, to be identified. Subsea resources of
long wavelength data in almost all surveys. bulk minerals such as sands and gravels may
For an example of CSAMT use see Fig. 16.3. be similarly evaluated. The images produced
can be very striking (Fig. 7.16) but, being scaled
vertically in two-way reflection time rather
7.12 SEISMIC METHODS than depth, need some form of velocity control
before they can be used quantitatively.
Seismic methods dominate oil industry geo- Where ores occur in sedimentary rocks that
physics but are comparatively little used in have been only gently folded or faulted, seismic
mineral exploration, partly because of their surveys can be useful. However, reflection
high cost but more especially because most work onshore is slow and expensive because
orebodies in igneous and metamorphic rocks detectors (geophones) have to be positioned