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Exploration, Recovery, and Transportation
6.2.3 seIsmIC metHods
Seismic methods are based on determinations of the time interval that elapses between the initiation of a sound wave from detonation of a dynamite charge or other artificial shock and the arrival
of the vibration impulses at a series of seismic detectors (geophones). The arrivals are amplified
and recorded along with time marks (0.01 s intervals) to give the seismogram. The method depends
upon (1) the velocity within each of the layers penetrated at depth is greater than that in the layers
above; (2) the layers are bounded by plane surfaces; and (3) the material within each layer is essentially homogeneous.
The seismograph measures the shock waves from explosions initiated by triggering small controlled charges of explosives in the bottom of shallow holes in the ground. The formation depth is
determined by the time elapsed between the explosion and detection of the reflected wave at the
surface.
The depths and media reached by seismic waves depend on the distance between shot point and
receiving points. The first impulses or breaks in a seismogram are caused by waves that have traveled quickly between the shot point and any receiving point. At short distances this is usually also
the shortest path, but beyond a certain distance it is quicker for a refracted pulse to travel via a longer
path involving underlying layers with a higher velocity. From a plot of travel time as a function of
surface distance, data are obtained for determining both the velocity of the material and number of
layers present. From the distances at which changes in velocity are indicated the depth of each layer
can be computed.
In general the deeper, older formations as a result of higher compression have a higher density
and also a higher seismic velocity than the overlying material. Observed differences in velocity not
only define the direction of slope of the rock surfaces but also provide information for computing
the degree of slope present. For what might be termed normal conditions (increase in velocity with
depth) the error determined in depths is usually less than 10% with this method.
Seismic geophysical work is also carried out on the water, greatly aiding the search for oil on
the continental shelves and other areas covered by water. A marine seismic project moves continually, with detectors towed behind the boat at a constant speed and a fairly constant depth. Explosive
charges are detonated at a position and time determined by the speed of the boat, so that a continuous survey of the reflecting horizons can be obtained.
6.2.4 eleCtrICAl metHods
Electrical prospecting methods depend upon differences in electrical conductivity between the geological bodies under study and the surrounding rocks. In general, metallic minerals, particularly the
sulfides, range in resistivity from 1.0 to several Ω-cm, whereas consolidated sediments of low water
content average about 10 4 Ω-cm, igneous rocks range from 10 4 to 10 6 Ω-cm, and saturated unconsolidated sediments from 10 2 to 10 4 Ω-cm. The resistivity of the last depends largely on the amount
and electrolytic nature (salinity) of the included water.
On the other hand, the self-potential method makes use of the fact that most metallic sulfide minerals are easily oxidized by downward-percolating groundwater. As a result of this surface oxidation, the elements of a simple chemical battery are established and an electrical current flows down
through the ore body and back to the surface through the surrounding water-saturated ground, which
acts as the electrolyte. It is possible to locate these localized electrical fields and, hence, ore bodies
by mapping points of equal electrical potential at the surface using nonpolarizing electrodes and a
sensitive ammeter, or a milliammeter. Alternatively, measuring the potential differences between
successive profile stakes forming a grid over an area using a potentiometer can also be employed.
A special application of electrical methods is in the study of subsurface stratigraphy by measuring the potential differences between the surface and an electrode lowered in a borehole and by also
measuring variations in electrical resistivity with depth (electrical logging). This method produces
Exploration, Recovery, and Transportation
6.2.3 seIsmIC metHods
Seismic methods are based on determinations of the time interval that elapses between the initiation of a sound wave from detonation of a dynamite charge or other artificial shock and the arrival
of the vibration impulses at a series of seismic detectors (geophones). The arrivals are amplified
and recorded along with time marks (0.01 s intervals) to give the seismogram. The method depends
upon (1) the velocity within each of the layers penetrated at depth is greater than that in the layers
above; (2) the layers are bounded by plane surfaces; and (3) the material within each layer is essentially homogeneous.
The seismograph measures the shock waves from explosions initiated by triggering small controlled charges of explosives in the bottom of shallow holes in the ground. The formation depth is
determined by the time elapsed between the explosion and detection of the reflected wave at the
surface.
The depths and media reached by seismic waves depend on the distance between shot point and
receiving points. The first impulses or breaks in a seismogram are caused by waves that have traveled quickly between the shot point and any receiving point. At short distances this is usually also
the shortest path, but beyond a certain distance it is quicker for a refracted pulse to travel via a longer
path involving underlying layers with a higher velocity. From a plot of travel time as a function of
surface distance, data are obtained for determining both the velocity of the material and number of
layers present. From the distances at which changes in velocity are indicated the depth of each layer
can be computed.
In general the deeper, older formations as a result of higher compression have a higher density
and also a higher seismic velocity than the overlying material. Observed differences in velocity not
only define the direction of slope of the rock surfaces but also provide information for computing
the degree of slope present. For what might be termed normal conditions (increase in velocity with
depth) the error determined in depths is usually less than 10% with this method.
Seismic geophysical work is also carried out on the water, greatly aiding the search for oil on
the continental shelves and other areas covered by water. A marine seismic project moves continually, with detectors towed behind the boat at a constant speed and a fairly constant depth. Explosive
charges are detonated at a position and time determined by the speed of the boat, so that a continuous survey of the reflecting horizons can be obtained.
6.2.4 eleCtrICAl metHods
Electrical prospecting methods depend upon differences in electrical conductivity between the geological bodies under study and the surrounding rocks. In general, metallic minerals, particularly the
sulfides, range in resistivity from 1.0 to several Ω-cm, whereas consolidated sediments of low water
content average about 10 4 Ω-cm, igneous rocks range from 10 4 to 10 6 Ω-cm, and saturated unconsolidated sediments from 10 2 to 10 4 Ω-cm. The resistivity of the last depends largely on the amount
and electrolytic nature (salinity) of the included water.
On the other hand, the self-potential method makes use of the fact that most metallic sulfide minerals are easily oxidized by downward-percolating groundwater. As a result of this surface oxidation, the elements of a simple chemical battery are established and an electrical current flows down
through the ore body and back to the surface through the surrounding water-saturated ground, which
acts as the electrolyte. It is possible to locate these localized electrical fields and, hence, ore bodies
by mapping points of equal electrical potential at the surface using nonpolarizing electrodes and a
sensitive ammeter, or a milliammeter. Alternatively, measuring the potential differences between
successive profile stakes forming a grid over an area using a potentiometer can also be employed.
A special application of electrical methods is in the study of subsurface stratigraphy by measuring the potential differences between the surface and an electrode lowered in a borehole and by also
measuring variations in electrical resistivity with depth (electrical logging). This method produces