1st EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems

An attempt is made in this article to provide a readable summary of the bases for
and problems encountered with the resistivity and induced polarization methods in gencrat
application. The article commences with a discussion of aqueous electrolytic conduction
in rocks including consideration of the effects of temperature, rock texture, rock type,
geological processes, and of the presence of clay minerals. Doth electrode and membrane
polarization in soils and rocks are described in simple terms.
The elementary theory for the rcsistivity and induced polarization methods are next
described in terms of formulas for electrodes on a homogeneous and layered half-space.
The notion of apparent resistivity is introduced. Vertical electric sounding is described
in relation to curve types, inversion, equivalence, anisotropy, and correlation. ProfXug
with resistivity is briefiy described while combined sounding-profiling is treated in more
detail for both the resistivity and induced polarization methods. The parameters used
to describe the induced polarization phenomena are next introduced. A brief discourse
follows on data acquisition and processing including design considerations for transmitters
and receivers, electrodes, and wire logistics.

Refraction and reflection seismic methods are named for a key element of
the geometry of the ray paths in each method, respectively. Both methods are
affected by the refraction of seismic rays at velocity contrast boundaries.
However, the key geometrical element in the reflection method is that the rays
incident on a target boundary are reflected back to the surface. In refraction
surveying, the incident ray is critically refracted along the target
boundary before it returns to the surface. Refraction seismic methods were
employed in the petroleum industry before the reflection methods were develaped,
and the same evolution has been seen in the shallow-target application
of the seismic methods.

The following is a selected bibliography on ground
penetrating radar. It is excerpted from a more comprehensive
bibliography (approximately four times larger) to be
published next year in the second volume of the Society of
Exploration Geophysicist's Electromagnetic Volumes, edited by
Misac Nabighian.
Radar was invented during World War II and first applied to
earth science problems during the 1940's and 1950's for ice
sounding and planetary exploration (Evans, 1963; Thompson,
1979). Geotechnical applications of ground penetrating radar
to rock and soil did not occur until the 1970's (Ulriksen,
1980). This selected bibliography is a comprehensive listing
of post-1980 references (and selected pre-1980 key
references) on radar applications in well logging, tomography
and terrestrial surface sounding (near surface exploration).

A general introduction of the elements to be included in the planning
and costing of geophysical investigations for engineering and
environmental problems. Some of the elements included are
organization, purpose, method, approach, goals, personnel,
instrumentation, logistics, analysis, reporting, client/geophysicist
relationship, and pitfalls and benefits. Additional information
sources will be given by references to literature, publications,
standards, individuals.

A program of ground magnetometer surveys was designed and implemented
at the U.S. Army Rocky Mountain Arsenal, Denver, Colorado, in an attempt to
locate a number of 1940's vintage water wells. These wells were associated
with homesteads and farms that occupied the area prior to the construction
of the arsenal. The surveys were part of the overall program to properly
abandon these farm wells to eliminate potential pathways for the migration
of contaminants.
Total field and vertical gradient magnetic data were collected along 14
survey grids encompassing nearly 30 acres of property at a 10' x 10'
station/line spacing. Each survey grid was constructed approximately 300'
on a side to encompass an area that was reasonably certain to contain the
targeted well based on the apparent accuracy of the well location
information available in the historic records. In most cases, historic
information on the depth extent of well casings, casing diameter or casing
thickness was either absent or contradictory, and as a consequence,
modeling of expected anomalies for each well target was not performed.
However, test surveys were conducted to observe anomalous response
characteristics at several known farm wells. These wells, for which
information on the depth extent, casing diameter and casing thickness is
recorded, all produced sharp, positive anomalies characteristic of
monopolar type sources. Amplitudes ranged between 300 and 4000 gammas with
half-widths of 20'.
The relatively large database obtained at each survey grid was reduced
on a PC and outputted as a contour map which readily permitted an analysis
of the relative shape and intensity of anomalies. The data, in most cases,
revealed the presence of strong positive anomalies that are easily
recognizable from other dipolar features detected at the sites and, based
on the test survey results, are interpreted to represent well casings.
Follow-up excavation sites were selected accordingly. The cost of the
magnetometer/gradiometer surveys at the 10' x 10' spacing, including data
reduction and interpretation, was estimated at $1000/acre.

The main ground water occurrences in crystalline rock terrains are
generally limited to fracture zones and alluvial pockets. This is particularly
true-for glaciated areas where weathering layers have been removed. However,
the importance of fracture and shear zones in controlling ground water yield and
quality is not confined to crystalline rock. In, for example, the Permian Basin
in West Texas, brine may move along fractures and influence water quality in
tertiary aquifers. In the limestone aquifers of the southeast, karstification
generally is controlled'by shear zones which often are important aquifers.

Many cathodic protection systems are designed in part from data obtained
from surface resistivity measurements. The methodology usually employed
to make these measurements utilizes four (4) metal electrodes which are
arranged equal distance from each other along a straight line. This
electrode array is commonly referred to as the Wenner array. When the
data is collected for the various spacings of the electrodes it is assumed
that the spacing between the electrodes is roughly equal to the depth of
exploration. The apparent resistivity values from the various depths
(i.e., electrode spacings) obtained from these measurements are then used
in the flesign of cathodic protection system.
This paper will discuss that in many geologic situations the use of
surface resistivity employing the Wenner electrode array, the electrode
spacing is not even roughly equal to the depth of exploration. By
employing electromagnetic induction techniques a better representation of
subsurface resistivities can be obtained in certain geologic and
logistical situations. The case history will show that the surface
resistivity s&vey performed at the site yielded incorrect assumptions of
subsurface resistivities. While an electromagnetic (EM) induction survey
over the same area saved the client over $200,000 in anode replacement
costs when compared to the surface resistivity designed cathodic
protection system.

The delineation of existing or potential geologic hazards related to the subsidence,
relaxation or weakening of rock above old mine workings is important
to public safety and the mitigation of property damage. The detection of areas
of destressed rock by conventional exploration methods can, however, be extremely
difficult and expensive.
A number of sites in coal mining: areas of the western and northwestern United
States have been explored by the seismic refraction method. It has been demonstrated
that low velocity zones in bedrock can be diagnostic of destressed
zones in bedrock above old mine workings. In areas where no detailed and/or
reliable mine maps are available, low velocity zones in bedrock interpreted
from highly detailed seismic velocity profiles can provide a basis for establishing
targets for more direct methods of exploration.

A portion of a former SAC Air Force base, purchased by a privately owned utility
company, was converted into a natural-gas-fueled power plant, the Stony Brook Energy
Center, near Springfield, Massachusetts, in the Connecticut River Valley. The site is
underlain by a rifted segment of the northern Appalachian fold belt that is now blanketed
with coarse-grained, sandy, lacustrine, and fluvial glacial deposits. Upper units consist
of eolian deposits and dune sands that overlie varved lake beds and glacial till. Bedrock
consists of late-stage Triassic rift sediments. Fifty-five-gallon steel drums, now empty
but at one time containing waste of unknown origins, and fuel tanks were discarded or
simply abandoned and buried at the site during its occupation by the Air Force. A
geophysical study was designed to determine the location and depth of the waste and also
to determine if contaminants were released into the groundwater. The hydrogeologic
setting, the activities and structural make-up at the former base, the present use of the
site, and information gleaned from the present site engineer were considered in
recommending and conducting a geophysical program.

A geophysical study to map the water table and an impervious till layer was undertaken
for the purpose of delineating the geological controls on several toxic waste plumes.
The data obtained from the geophysical program served to guide the exploration drilling
program for the site. Previous drilling efforts had proven unsuccessful in delineating the
contaminant plumes, because of a myriad of suspect contaminant sources over the 10
square mile area and because of the nature of the previously unmapped complex sub-surface glacial
outwash channeling