TM 5-818-1 / AFM 88-3, Chap. 7
Table 4-4. Surface Geophysical Methods
Applicability
Name of Method
Procedure or Principle Utilized
Seismic methods:
Based on time required for seismic waves to
Utilized to determine depth to rock or other
travel from source of blast to point on ground
lower stratum substantially different in wave
surface, as measured by geophones spaced at
velocity than the overlying material. Used only
intervals on a line at the surface. Refraction of
where wave velocity in successive layers
seismic waves at the interface between
becomes greater with depth. Used to determine
different strata gives a pattern of arrival times
rock type, rock and soil stratification, depth of
vs distance at a line of geophones.
weathered zone, etc.
The travel time of transverse or shear waves
Velocity of wave travel end natural period of
generated by a mechanical vibrator consisting
vibration gives some indication of soil type.
Continuous vibration
of a pair of eccentrically weighted disks is
Travel time plotted as a function of distance
recorded by seismic detectors placed at
indicates depths or thicknesses of surface
specific distances from the vibrator.
strata. Useful in determining dynamic modulus
of subgrade reaction and obtaining information
on the natural period of vibration for design of
foundations of vibrating structures.
Electrical methods:
Used to determine horizontal extent and depth
Based on the difference in electrical
of subsurface strata. Principal applications for
conductivity or resistivity of strata. Resistivity of
Resistivity
investigating foundations of dams and other
subsoils at various depths is determined by
large structures, particularly in exploring
measuring the potential drop and current
granular river channel deposits or bedrock
flowing between two current and two potential
surfaces, sources of construction material,
electrodes from a battery source. Rerirstivity
potential infiltration and seepage zones, and in
irs correlated to material type.
cavity detection.
Similar to resistivity methods but gives sharper
Based on the determination of the ratio of
indication of vertical or steeply inclined
potential drops between three potential
and
more
accurate
depth
electrodes as a function of the current imposed
Drop in potential
determinations.
More susceptible than
on two current electrodes.
resistivity method to surface interference and
minor irregularities in surface soils.
Currently
used
in
shallow
underwater
The time of travel of sound waves reflected
exploration to determine position of mud line
from the mud line beneath a body of water and
Acoustic method
and depth to hard stratum underlying mud.
a lower rock surface is computed by
Method has been used in water depths greater
predetermining the velocity of sound in the
than 100 feet with penetrations of 850 feet to
various media.
bedrock.
Excellent display of subsurface
stratification. Used most efficiently in water
depths up to 50 feet with penetrations of
additional 350 feet to bedrock.
Note:
Instrumentation for methods listed above currently available at WES. May be furnished as a service to Districts
and Divisions on request. Table adapted from "Design Manual, Soil Mechanics, Foundations, and Earth
Structures, " Department of the Navy. Bureau of Yards and Docks.
4-11
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