Geophysical Methods in Mineral Exploration
geophysical surveys have been applied to mineral and petroleum exploration for
many years. A magnetic compass was used in Sweden in the mid-1600s to find iron
ore deposits. The lateral extent of the Comstock ore body was mapped using
self-potential methods in the 1880s. A very crude type of seismic survey
measured the energy resulting from blasting operations in Ireland in the late
1800s. The idea that energy travels through a material with a certain velocity
came from this survey. During World War I, geophysical techniques were used to
locate artillery pieces. Anti-submarine warfare in World War II led to magnetic
and sonar surveys.
main emphasis of geophysical surveys in the formative years was petroleum
exploration. Technology developed for oil and gas surveys led to the use of
geophysical surveys in many important facets of geotechnical investigations.
Geophysical surveys have been applied to civil engineering investigations since
the late 1920s, when seismic and electrical resistivity surveys were used for
dam siting studies. A seismic survey was performed in the 1950s in St. Peter’s
Basilica to locate buried catacombs prior to a renovation project. From the late
1950s until the present time, geophysical techniques have had an increasing role
in both groundwater exploration and in geotechnical investigations. Geophysical
surveys are now used routinely as part of geological investigations and to
provide information on site parameters (i.e., in place dynamic properties,
cathodic protection, depth to bedrock) that in some instances are not obtainable
by other methods. Values derived from seismic geophysical surveys are obtained
at strain levels different from some site parameters obtained by other means.
geophysical techniques are based on the detection of contrasts in different
physical properties of materials. If contrasts do not exist, geophysical methods
will not work. Reflection and refraction seismic methods contrast compressional
or shear wave velocities of different materials.
Electrical methods depend on the contrasts in electrical resistivities.
Contrasts in the densities of different materials permit gravity surveys to be
used in certain types of investigations. Contrasts in magnetic susceptibilities
of materials permit magnetic surveying to be used in some investigations.
Contrasts in the magnitude of the naturally existing electric current within the
earth can be detected by self-potential (SP) surveys.
refraction surveys are used to map the depth to bedrock and to provide
information on the compressional and shear wave velocities of the various units
overlying bedrock. Velocity information also can be used to calculate in place
small-strain dynamic properties of these units. Electrical resistivity surveys
are used to provide information on the depth to bedrock and information on the
electrical properties of bedrock and the overlying units. Resistivity surveys
have proven very useful in delineating areas of contamination within soils and
rock and also in aquifer delineation. Gravity and magnetic surveys are not used
to the extent of seismic and resistivity surveys in geotechnical investigations,
but these surveys have been used to locate buried utilities. Self-potential
surveys have been used to map leakage from dams and reservoirs.
surveys provide indirect information. The objective of these surveys is to
determine characteristics of subsurface materials without seeing them directly.
Each type of geophysical survey has capabilities and limitations and
these must be understood and considered when designing a geophysical
interpretations should be correlated with real “ground-truth”data such as
drill hole logs. It is very
important that the results of geophysical surveys be integrated with the results
of other geologic investigations so that accurate interpretation of the
geophysical surveys can be made.
Airborne versus Ground Surveys
In general, airborne geophysical methods are used in reconnaissance and ground geophysical methods are used in more detailed investigations. There are, however, many instances in which either airborne or ground methods could be used. In an extended exploration program, combinations and sequences of methods may be appropriate, and there is often a need to weigh their individual advantages.
Using Helicopters or Fixed-wing Aircraft:
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