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Exploration, Recovery, and Transportation
Furthermore, the methods can be subdivided into those that focus on gravitational properties, magnetic properties, seismic properties, electrical properties, electromagnetic properties, properties,
and radioactive properties. These geophysical methods can be subdivided into two groups: (1) those
methods without depth control and (2) those methods having depth control.
In the first group, the measurements incorporate spontaneous effects from both local and distant
sources over which the observer has no control. For example, gravity measurements are affected
by the variation in the radius of the Earth with latitude. They are also affected by the elevation of
the site relative to sea level, the thickness of the Earth’s crust, and the configuration and density
of the underlying rocks, as well as by any abnormal mass variation that might be associated with a
mineral deposit. In the last stages of assessment, the interpretation always depends upon the geological knowledge of the interpreter.
In the second group of measurements (those with depth control), seismic or electric energy is
introduced into the ground and variations in transmissibility with distance are observed and interpreted in terms of geological quantities. Thus, depths to geological horizons having marked differences in transmissibility can be computed on a quantitative basis and the physical nature of these
horizons deduced. The accuracy, ease of interpretation, and applicability of all methods falling into
this group are not the same, and there are natural and economic conditions under which the measurements of the first group are preferable for exploration studies despite their inherent limitations.
However, it must be recognized that geophysical exploration techniques cannot be applied indiscriminately. Knowledge of the geological parameters likely to be associated with the mineral or
subsurface condition being studied is essential both in choosing the method to be applied and in
interpreting the results obtained. Furthermore, not all the techniques described here may be suitable
for petroleum exploration. Nevertheless, the techniques that are described here are included since it
is valuable to know their nature and how they might be applied to subsurface exploration.
It should also be noted that such terms as geophysical borehole logging can imply the use of one
or more of the geophysical exploration techniques. This procedure involves drilling a well and using
instruments to log or make measurements at various levels in the hole by such means as gravity
(density), electrical resistivity, or radioactivity. In addition, formation samples (cores) are taken for
physical and chemical tests.
6.2.1 grAvIty metHods
Gravity methods are based upon the measurement of physical quantities related to the gravitational
field, which in turn are affected by differences in the density and the disposition of underlying geological bodies. In oil and gas exploration, in which no direct density control is associated with the
material being sought, exploration is based on the mapping of geological structures to determine
situations that might localize the material being sought. In such cases, the significant density values
are salt 2.1–2.2, igneous rocks 2.5–3.0, and sedimentary rocks 1.6–2.8. The last value increases
with depth owing to consolidation and geological age, and as a result structural deformation associated with faults and folding can be detected. Compaction of sediments over edges or knolls on the
underlying crystalline rock surface also leads to a local increase in mass, as does the development
of calcareous cap rock over the heads of intrusive salt columns.
Thus, the gravimeter detects differences in gravity and gives an indication of the location and
density of underground rock formations. Differences from the normal can be caused by geological
and other influences, and such differences provide an indication of subsurface structural formations.
In the early days of gravity prospecting both the torsion balance and the pendulum apparatus were
extensively employed, but these have been supplanted by spring balance systems (gravimeters). The
latter can be read in a matter of minutes, in contrast to the several hours required in obtaining readings with the earlier instruments.
There is a variety of gravimeters, but those in common use consist essentially of a weighted boom
that pivots about a hinge point. The boom is linked to a spring system so that the unit is essentially
Exploration, Recovery, and Transportation
Furthermore, the methods can be subdivided into those that focus on gravitational properties, magnetic properties, seismic properties, electrical properties, electromagnetic properties, properties,
and radioactive properties. These geophysical methods can be subdivided into two groups: (1) those
methods without depth control and (2) those methods having depth control.
In the first group, the measurements incorporate spontaneous effects from both local and distant
sources over which the observer has no control. For example, gravity measurements are affected
by the variation in the radius of the Earth with latitude. They are also affected by the elevation of
the site relative to sea level, the thickness of the Earth’s crust, and the configuration and density
of the underlying rocks, as well as by any abnormal mass variation that might be associated with a
mineral deposit. In the last stages of assessment, the interpretation always depends upon the geological knowledge of the interpreter.
In the second group of measurements (those with depth control), seismic or electric energy is
introduced into the ground and variations in transmissibility with distance are observed and interpreted in terms of geological quantities. Thus, depths to geological horizons having marked differences in transmissibility can be computed on a quantitative basis and the physical nature of these
horizons deduced. The accuracy, ease of interpretation, and applicability of all methods falling into
this group are not the same, and there are natural and economic conditions under which the measurements of the first group are preferable for exploration studies despite their inherent limitations.
However, it must be recognized that geophysical exploration techniques cannot be applied indiscriminately. Knowledge of the geological parameters likely to be associated with the mineral or
subsurface condition being studied is essential both in choosing the method to be applied and in
interpreting the results obtained. Furthermore, not all the techniques described here may be suitable
for petroleum exploration. Nevertheless, the techniques that are described here are included since it
is valuable to know their nature and how they might be applied to subsurface exploration.
It should also be noted that such terms as geophysical borehole logging can imply the use of one
or more of the geophysical exploration techniques. This procedure involves drilling a well and using
instruments to log or make measurements at various levels in the hole by such means as gravity
(density), electrical resistivity, or radioactivity. In addition, formation samples (cores) are taken for
physical and chemical tests.
6.2.1 grAvIty metHods
Gravity methods are based upon the measurement of physical quantities related to the gravitational
field, which in turn are affected by differences in the density and the disposition of underlying geological bodies. In oil and gas exploration, in which no direct density control is associated with the
material being sought, exploration is based on the mapping of geological structures to determine
situations that might localize the material being sought. In such cases, the significant density values
are salt 2.1–2.2, igneous rocks 2.5–3.0, and sedimentary rocks 1.6–2.8. The last value increases
with depth owing to consolidation and geological age, and as a result structural deformation associated with faults and folding can be detected. Compaction of sediments over edges or knolls on the
underlying crystalline rock surface also leads to a local increase in mass, as does the development
of calcareous cap rock over the heads of intrusive salt columns.
Thus, the gravimeter detects differences in gravity and gives an indication of the location and
density of underground rock formations. Differences from the normal can be caused by geological
and other influences, and such differences provide an indication of subsurface structural formations.
In the early days of gravity prospecting both the torsion balance and the pendulum apparatus were
extensively employed, but these have been supplanted by spring balance systems (gravimeters). The
latter can be read in a matter of minutes, in contrast to the several hours required in obtaining readings with the earlier instruments.
There is a variety of gravimeters, but those in common use consist essentially of a weighted boom
that pivots about a hinge point. The boom is linked to a spring system so that the unit is essentially