3rd EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems

The underlying purpose of any geophysical data-acquisition
program is the discerning of some aspect of sub-surface conditions.
The process of drawing inference from accumulated data is that
which we call "interpretation." With some justification and no
deprecation, the step of "making sense" of a set of geophysical
survey results is often viewed as more an art than a science. The
thesis of this presentation is that the interpretive art is most
productively pursued as an intuitive process guided by an
understanding of the controlling physics. The process is generally
assisted by consideration of the results expected from known
models. Accordingly, we shall review the governing physics of
electrical properties of earth materials and of the measurement
techniques commonly employed and then turn to consideration of some
of the computing methods available to assist the interpreter,
including both forward and inverse methods.

Self-potential (SP) surveys detect surface voltage variations
caused by subsurface flows of fluid, heat, and ions. Therefore
the SP method can be useful in applications where such flows are
related to subsurface targets of engineering or environmental
interest. Examples of engineering and environmental applications
of the SP method discussed in this paper include studies of fluid
flow in the vicinity of dams, reservoirs, wells, and faults;
investigations of coal mine fires, steam injection, and nuclear
tests; and surveys for environmental contaminants.
Although the equipment and field procedures used for SP
surveys are relatively simple, considerable care must be taken to
ensure that data are reproducible, that sources of noise are
recognized, and that appropriate data reduction techniques are
used to correct for electrode drift and polarization.
Interpretation of SP data then may be carried out using
qualitative, geometric, or analytic methods. Qualitative
recognition of typical anomaly patterns is helpful for
determining source locations and selection of more quantitative
interpretation methods. Geometric source modeling, similar to
that done for other potential field methods such as gravity and
magnetics, is used to estimate source configuration and depth.
Analytical modeling, based on concepts of irreversible
thermodynamics and coupled flows, can provide information about
the nature, location, and intensity of SP anomaly sources.

A variety of remote sensing techniques are able to detect oil slicks on
water, including side-looking radar, passive microwave radiometers,
ultraviolet-infrared line scanners, laser fluorosensors and various visual
sensors. Each technique keys on some property of the oil which differs
from that of the open ocean (such as emissivity) or on some property of
the ocean which is altered by the presence of oil (such as the surface
roughness). Combinations of one or more of these sensors have been
installed on various aircraft and are in routine use around the world.
They are generally able to detect and map oil slicks reliably under most
weather conditions.
A more difficult problem is measurement of the thickness of an oil slick,
which can range over six orders of magnitude from less than a micrometre
to ten centimetres. The thickness of a spill varies spatially and over
time as a slick spreads. Clearly knowledge of the thickness is required
to estimate oil volumes and thereby plan efficient countermeasures.

With the advent of small and powerful computers, new borehole geophysical logging
systems are being designed and manufactured. Many of these systems lack an
integrated approach of combining digital downhole probes, uphole instrumentation,
and acquisition and analysis software in an interactive, simple to use, and flexible system.
This case study describes the “systems approach” during the design of logging
hardware and software which allows for the equipment and support programs to be
altered and updated for a changing market demand without requiring massive engineering
or conceptual changes. This “systems approach” allows for a fully interactive
logging acquisition, presentation, and analysis system capable of supporting a variety
of geophysical downhole probes, and analysis routines.
The “systems approach” to the design of a fully-digital borehole logging system
incorporates a three-phase plan. The first phase consists of the design of fully digital
downhole instrumentation requiring no additional uphole electronics, other than a
universal probe interface board located in the surface instrumentation. The second
phase consists of a standard computer system used as a surface recording, analysis,
and output device for the log data. The third phase is an integrated software package
that monitors, controls, and allows input from the digital probes and the logging operator.
This case study describes the advantages to this “systems approach” of design in log
accuracy, ease-of-use, and integration of data for post-analysis using log data derived
from an actual field-proven system.

A geophysical investigation consists of six steps:
1. Defining the problem (target) and demonstrating the feasibility of achieving the required goals.
2. Designing the experiment, including method(s) to be used and the survey parameters.
3. Quality control/real time interpretation to insure that reliable data is being acquired and that the premises arising from experiment design are valid.
4. Interpreting the data in terms of the problem (target) which has been defined.
5. Estimating the range of alternative interpretations and their effect on the goal of the experiment.
6. Reinterpretation of the data at a later date when more information becomes available.
Computers and their associated software play an increasingly
important role in the efficient and precise execution of
geophysical experiments. Digital data acquisition and storage is
becoming more and more prevalent. This allows the geophysicist
to collect vast amounts of data with relative ease. The
increased data volume, coupled with the requirement for more
sophisticated data processing and analysis has resulted in the
development of more advanced software packages. Equally
important to the software's capability to handle sophisticated
analysis on large data sets is the human interface which enables
the geoscientist to access these tools in an productive manner.
Modern interpretation software enables the feasibility study
and survey design to be conducted in an efficient and accurate
manner. Real time data interpretation and quality control
requires on site use of such software. Finalizing the results
and evaluating the range of alternative interpretations requires
software if it is to be carried out thoroughly and cost
effectively. Some of the more sophisticated field techniques
cannot be interpreted at all without such software. The addition
of report ready displays and interfaces to powerful word and
drawing processors further improves quality, productivity and
cost effectiveness.

Because seismic shear waves (S waves) propagate in earth materials differently than do
compressional waves (P waves), the addition of a shear-wave capability to the engineering
geophysicist’s shallow-depth seismic system markedly broadens its usefulness. In general, S waves
respond only to lithologic changes, whereas P wave are affected by both the lithology and the
fluids contained. In one area studied, both the saturated overburden and the bedrock had nearly
the same P-wave velocity, and therefore the boundary between them was undetectable by P-wave
methods; however, it was detectable by S-wave methods. Since the water table could be seen with
P-wave methods, the volume of the unconfined aquifer could be estimated--P waves for its top; S waves for its base.
The amplitude, frequency content, and arrival time of shear waves all change significantly
across a near-vertical boundary. I have used these S-wave characteristics to map earth fissures
and to locate the edge of a concealed high wall of a reclaimed surface mine.
Reflections of horizontal shear (SH) body waves are commonly masked by SH surface
waves, but these surface waves do not occur where the surface layer’s shear-wave velocity is close
to or higher than that of the lower layers. Because of this lack of surface-wave noise, and
because of the inherent simplicity of SH-wave propagation (for example, no wave-type conversion
at interfaces), studies have obtained usable SH reflections from a tar sand underlying a
limestone sequence, possible SH reflections from within an ignimbrite section, and excellent
shear-wave reflections interpreted as having come from lenticular clay bodies at depths of 12 and
32 m beneath the surface of an alluvial fan.

The seismic refraction method has been widely used in Japan for engineering purposes
such as investigations for tunnel and dam construction. The following features of the
seismic refraction survey are considered as reasons why it has been widely used.
(1) It is feasible to determine subsurface structure (in seismic velocity) with enough
resolution and accuracy even though at a site with complex topography and
subsurface structure.
(2) It is feasible to detect faults and fracture zones where careful design will be required.
(3) A lot of data relating with mechanical properties of rock, including seismic velocity,
have been gathered and are utilized as database. By using the database, mechanical
properties of rock can be evaluated with seismic velocity, so that results of the
seismic refraction survey bring useful information for designing and execution
management of construction.
This paper reviews practical technique of the seismic refraction method that is commonly
carried out in Japan, including survey planning, data acquisition technique and analysis
technique, necessary for getting reliable results. Also, a case study of the seismic refraction
survey with survey line 6.5 km long, carried out to investigate tunnel construction site is presented.

The antennas are a significant component of the underground radar
tomography system developed for in-seam hazard detection. Several antenna
configurations were tested above ground to select those which might be best
for underground use. The relative forward gain of each antenna was determined
in a series of experiments. The antennas were tested both alone and with
reflection shields spaced from fractional to several wavelengths behind the
antennas. Antennas included triangular (bow-tie), linear dipoles and
monopoles and a series of horn antennas. The results indicate few significant
differences in agreement with laboratory modeling tests and place the emphases
on operational features i.e. the ease of use as a chief factor in antenna
selection.

A combination of ground penetrating radar (GPR) and seismic
transmission delay was used to determine the thickness of stone
blocks making up the abutments of a bridge crossing trolley track
of the Massachusetts Bay Transit Authority's (MBTA) Red Line in
Dorchester, Massachusetts. The work was performed for Barnes and
Jarnis, Inc. and the MBTA to assess the adequacy of the bearing
strength of the abutments for a planned new bridge deck.
GPR data were obtained on both abutments with a 500 MHz system
on horizontal traverses at 3 to 5 foot intervals, on vertical
traverses at 5 foot intervals, and on single blocks. The GPR
survey encountered three types of complications: (1)
interpreting multiple reflections of the radar signal from the
backs of irregular blocks; (2) differentiating signals reflected
from the sides of blocks from signals reflected from the backs of
blocks; and (3) limited penetration of the radar signal in the
central portions of the abutments due to infiltration of salty
water from the roadway above.
These difficulties were resolved with a second, independent
survey method. A 12-channel seismograph was used to obtain
seismic data for one of the abutments. The geophones were
attached to the face of the abutment and seismic signals were
generated by a source 30 to 80 feet behind the wall. Two seismic
profiles were obtained with geophones attached in horizontal
lines and four profiles with geophones attached in vertical
lines. Differences in transmission times were used to estimate
total thickness of the stone blocks.
Using the combination of GPR and seismic transmission delay
techniques, we found the thickness of the stones near the top of
each abutment to be approximately 5 to 6 feet, increasing toward
the bottom to a thickness of approximately 10 to 12 feet.

Underground multicomponent DC-resistivity can be used to locate
conductive zones in the vicinity of underground excavations.
The method was used at the Waste Isolation Pilot Plant in
southeast New Mexico to locate fluid saturated fracture zones
within the Salado evaporite formation. Two deep cased drill
holes north of the facility were used as a bipole current source.
A two-meter dipole was used to observe the three orthogonal
components of the potential difference along profiles in the
underground excavations. Fracture zones were located in several
areas and later verified by hydrological tests and subsequent drilling.
The method can.be readily adapted to other applications such as
road- and water-tunnel testing, and underground mining.