68 PETROLEUM TECHNOLOGY, ECONOMICS, AND POLITICS
Once an area has been selected for further investigation, more
detailed methods are brought into play.
The principles used are basically magnetism (magnetometer),
gravity (gravimeter), and sound waves (seismograph). These techniques are based on the physical properties of materials that can be
utilized for measurements and include those that are responsive
to the methods of applied geophysics. 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
Once an area has been selected for further investigation, more
detailed methods are brought into play.
The principles used are basically magnetism (magnetometer),
gravity (gravimeter), and sound waves (seismograph). These techniques are based on the physical properties of materials that can be
utilized for measurements and include those that are responsive
to the methods of applied geophysics. 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