2nd EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems

The development in the early 1980's of digital enhancement
engineering seismographs with high-pass filtering capabilities, and the
wide availability of increasingly powerful microcomputers, have allowed
the application of seismic reflection methods to groundwater and
engineering problems. The simple "optimum offset" shallow reflection
technique can be implemented with a minimum investment in equipment and
computing capabilities and, therefore, can be cost-effective when
compared to drilling, especially where detailed information on the
bedrock or overburden structure is desired. The data are recorded on a
12-channel engineering seismograph, using a single high-frequency
geophone per channel, and a hammer or in-hole shotgun as the seismic
source. Processing and display of the "optimum offset" records require a
small microcomputer with a dot-matrix printer. A preliminary section can
be produced within hours of collecting the data.
The user must be aware of the limitations on the use of the
"optimum offset" technique imposed by site conditions and other factors
affecting resolution. The technique is most suited to mapping the
overburden-bedrock interface because this often produces a largeamplitude
reflection. The best results are obtained in areas that are
most favorable for the transmission of high frequency energy; that is,
where the surface materials are fine-grained and water saturated.
Under favorable site conditions, the "optimum offset" shallow
seismic reflection technique can be an effective tool for mapping bedrock
topography beneath a minimum of 15-20 m of overburden, and for
delineating structure within the overburden. Thus, it can be a useful
technique for engineering, environmental or groundwater studies, or for
Quaternary mapping projects.
A case history is presented from Dryden, Ontario, where buried
valleys were successfully delineated in support of Geological Survey of
Canada overburden drilling for stratigraphic studies.

A hidden layer is one that lies below a velocity inversion boundary or is
a layer that is too thin to be manifested in first arrivals on a refraction
record. Since the earliest use of the refraction seismic method, the hidden
layer problem has been said to be unsolvable with the refraction data alone.
The hidden layer problem actually has two aspects, only one of which is unsolvable.
The first aspect, the mapping of the top of the hidden layer, is the unsolvable aspect. The second aspect is the mapping of boundaries below the
hidden layer.

In western Ohio there are many industrial cities located along the
Miami River and its major tributary, the Mad River. These rivers flow on
extensive valley train deposits which are the primary source of water for
domestic and industrial purposes. In the late 1940s through the 1960s a
number of studies were conducted to better define the ground water
storage, recharge and transmissivity of these outwash materials.
This area is uniquely valuable for industrial development because of
its geologic history. Pleistocene glaciation covered major river
valleys. These major ancestral valleys defined by Cummings (1959) are the
Hamilton and the Teays (figure 1). The Mad and Miami Valley train
deposits are at least partly superimposed on the buried valleys. These
deposits are exceptional because two ice lobes (the Miami and Scioto)
produced a funnel for meltwater to flow during the Wisconsin glacial
retreat (figure 2). This meltwater deposited the outwash on which the Mad
and Miami Rivers now flow.
The cities of Urbana and Springfield are located in the Upper Mad
River valley. Dayton is located in the lower Mad River and the Miami
River valleys. A series of industrial cities are located in the Miami
River valley between Dayton and Cincinnati. All of these cities get their
water supply from the valley train deposits and discharge their effluents
into the river.

The monitoring of microseismic activity/acoustic emissions in
engineering structures allows the detection and location of preliminary
instabilities that always precede a structural failure. The primary
advantage of this approach is that it can be used on operational structures
and provide early warning of impending failure. Through appropriate
selection of sensors and monitoring frequencies, cultural noise such as
traffic, wind, or vibrations generated by construction equipment can be
reduced while enhancing instability-generated microseismic signals. The
events which generate these signals fall into two major categories: the
events associated with failures of weaker material caused by stress buildup
usually against the contact with the stronger members of the structure, and
the events generated when the stronger members begin to fail. These
conditions and the knowledge of the structure's design/construction
determine the optimum arrangement of a microseismic array on the structure.
The appropriate array can provide sufficient data for the real-time location
of individual failures in the structure along with their character and
magnitude. A brief discussion is provided to illustrate the features of data
recorded during microseismic monitoring of engineering structures such as a
cofferdam, an unstable slope, and both open pit and underground mines.

Two distinct coherent noise problems are inherently associated
with the deployment of sources and receivers in fluidfilled
boreholes. The first problem concerns the familiar tube
waves that are generated in the receiver borehole by the conversion
of incident seismic energy, and by interactions of the generated
tube waves with material discontinuities within the
borehole. The second problem is one that is perhaps less
familiar, and concerns P and S radiated fields that are generated
by tube wave conversions at material discontinuities in the
source borehole.
Through the illustrations of field data it is shown that the
converted P and S radiated fields generated by tube wave conversions
in the source borehole present a far greater coherent noise
problem than that associated with tube waves in the receiver
borehole. It is shown that data beyond the first break may be
severely contaminated by the converted wave fields, leaving
little hope for the extraction of viable data without its removal.
A method that combines field techniques and f-k velocity
filtering will be presented that adequately removes both classes
of coherent noise from the data. This method is illustrated with
processed field data. It is shown that field records can be
rendered viable through appropriate data acquisition and reduction
processes.

Borehole logging methods as applied to the non-petroleum
sector have enjoyed a resurgence of growth in recent years.
Engineers and hydrologists are beginning to appreciate the added
value of accurate borehole geophysical data as an aid in solving
engineering and environmental problems. Better understanding of
,traditional techniques and application of new methods have led to
growth in the application of borehole logging. Proper use of
standard logging measurements can serve both the geotechnical and
groundwater scientists if limitations are known. New
developments in borehole geophysics promise to increase the
importance of this data to engineers and earth scientists.

Typically, environmental investigations initially assume simple homogenous subsurface geologic
conditions. However, heterogeneous conditions predominate. Borehole geophysics provides
valuable information for environmental and engineering investigations including data for geologic
control and in situ parameter analysis. The amount and benefit of this information is
determined by the logging suite, borehole conditions, geologic parameters, interpreter
experience, and knowledge of present technology.
Considerations in selecting a suite of logs and the parameters affecting the design of a
logging program include:
1) Goals of the logging program,
2) Geophysical information desired,
3) Minimum and maximum depth to be logged,
4) Economically and commercially available instrumentation,
5) Drilling and completion methods,
6) Fluid level, and
7) Subsurface and surface geological and geophysical control.
Typical costs for drilling and geophysical logging associated with different types of
environmental investigations vary considerably. These costs are a function of the types and
quantity of the desired data, whether the geophysical logging and analysis will be performed in
house or by an outside consultant and the operational field environment.

This study integrated geophysical and geologic techniques to
understand the nature and location of waste material and the local
subsurface geologic setting beneath three inoperative waste
disposal lagoons that rest on top of highly fractured quartzite.
These techniques were used in the early phases of a Remedial
Investigation at the site. Data from the combined
geophysical/geologic surveys were used to efficiently develop a
soil boring and ground water monitoring well program designed to
characterize the extent of contamination resulting from residual
wastes in the lagoon.
A shallow electromagnetic conductivity (EM) survey defined the
horizontal extent of conductive contaminated soil and delineated
areas containing buried drums. Ground-penetrating radar (GPR)
profiles at the site mapped the soil bedrock interface and
determined the vertical extent of disturbed soil. A fracture
trace analysis and survey of outcrops of the bedrock established
the regional foliation within the bedrock and identified several
sets of prominent local geologic features (joints, faults and
bedding) with strong preferred orientation.
In interpreting the GPR sections, several interesting features
were noted. High angle linear reflectors within the bedrock
observed in several of the profiles were interpreted to be
fractures lined with clay, consistent with geologic interpretation
at the site. A depressed area in the bedrock near the lagoons and
other prominent joints may be the expression of structural
features related to a larger fault identified in the study area.
A series of soil borings confirm that the area1 extent and
thickness of waste material within the former lagoons coincides
with the approximate boundaries of affected soil identified
geophysically. Monitoring wells, placed along several of the more
prominent stuctural features identified in this study, indicate
that contaminated ground water flows preferentially along joints,
faults, and other areas of weakness beneath the site.

Thorough geologic and seismotectonic investigations were
completed as part of a comprehensive analysis of the Metropolitan
Denver water supply system. Encompassed in those geotechnical
investigations, which were fed into the Environmental Impact
Statement released by the Army Corps of Engineers, were numerous
geophysical-related studies. Some of these included a
historical seismicity compilation, a microearthquake
monitoring program, seismic velocity analysis, ground motion
attenuation, gravity feasibility, seismic reflection
feasibility, and a probabilistic seismic hazard evaluation.