15th EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems

The Wabar meteorite impact site in central Saudi Arabia, first reported by Henry St. John
(Abdullah) Philby in 1933, is unique in several ways. Despite a > 6,000 year fission-track date, the
impact must be very recent according to certain field relationships, a conclusion supported by
thermoluminescence dates of less than 450 years. Along with several other small meteoroid impact
events that have come to light in the past few decades, the new age-date suggests a reconsideration of
the frequency of these types of "city-buster" impacts. The Wabar impact event also took place entirely
in sand, permitting a much clearer and simpler reconstruction of the physical processes of a
hypervelocity impact. The event sequence, which took place in just a few seconds, generated a unique
array of impactite that includes a black glass (90 percent local sand and 10 percent meteorite) and a
shock-generated, coarsely laminar bleached-white sandstone. The event took place in one of the most
inhospitable areas on Earth, the middle of the Rub' Al-Khali, the largest contiguous sand desert in the
world. Here temperatures during one magnetic profile reached 61o C. (142o F.).

To answer one of the most frequently asked questions in humanitarian demining, "Which is the
best detector for my country to use?,” the International Pilot Project for Technology Cooperation
(IPPTC) sought to demonstrate the mine detection capabilities of commercial-off-the-shelf (COTS)
metal detectors. Based on controlled laboratory work to determine key detector parameters, the IPPTC
also used actual in-country field-testing to acquire real world data on these detectors. These field tests
were performed in mine-afflicted areas of Croatia and Cambodia. Later, an additional follow-on field
test was performed in Nicaragua drew upon the IPPTC results and used some of the same detectors.
Taking the detectors in-country for testing is one way to provide useful information about the
effects on detection of indigenous soil conditions. It also creates significant issues in trying to perform
reliably repeatable tests while exposed to the widely variable and often rigorous conditions under
deminers must work. This presentation describes the standard test procedures that we have followed and
the often ad hoc additions that were made to account for local conditions and needs in these countries.
The ability to plan and conduct field efforts flexibly allows such testing to be performed even in
uncontrolled environments. The results of the testing itself will also be presented and analyzed for the
relative effects of local soil conditions on rates of detection.

Bay Geophysical acquired a grid of shear wave reflection seismic data in Des Moines, IA at a proposed building renovation site. The purpose of the survey was to map the extent of a coal seam beneath the site and, if possible, identify the abandoned mine workings within the coal. A movie theater is located in the northern part of the center of the grid. A seismic profile was acquired down the center aisle of the theater as a means of extending the centerline of the survey grid. This paper addresses the entire seismic survey with respect to the channel identified: the coal seam is incised by a paleochannel immediately south of the theater. The center profile of the grid was located almost entirely within the channel and was thus uninterpretable without the extension into the movie theater.

Extensive numerical simulation is used to examine the sensitivity of an Electrical Resistance
Tomography (ERT) array to a subsurface inhomogeneity. A sensitivity criterion is developed based on
the cumulative response of an array to perturbations in the subsurface. This criterion is applied to a wide
variety of array types (i.e. Wenner, Schlumberger, and double dipole, at different electrode spacings).
The results are in good agreement with existing analyses in showing that the absolute response of dense
arrays (i.e. Wenner with an electrode separation of one) is high to near surface perturbations, giving high
sensitivity to near surface inhomogeneities. However, these arrays have poor cumulative sensitivity,
leading to poor sensitivity to deeper inhomogeneities. Similarly, wide arrays (i.e. a Schlumberger or
double dipole array with a large current-potential separation and a small potential-potential separation)
are less sensitive to near surface conditions, giving a better measure of the overall subsurface conditions.
However, this numerical approach allows for more precise examination of the sensitivity of ERT arrays
than is possible with geometrical analyses. Specifically, our results suggest that arrays with low current
density in the vicinity of the potential electrode (e.g., Schlumberger or double dipole) are generally
superior to arrays with higher current densities (e.g., Wenner). Finally, we show that the numerically
determined sensitivity criterion can be used to optimize the total time required for an ERT survey,
leading to improved characterization of dynamic processes through improved inversion of the
subsurface resistivity structure using a smaller number of higher quality arrays.

An iterative, Occam’s-style, regularized inverse approach was used to invert a finite-difference based forward model. The new algorithm was tested on existing data from several sites. The use of the anisotropic inversion algorithm produced images that are less prone to artifacts. In turn, this makes the images less prone to noise and more robust with respect to initial starting models. One challenge has been to incorporate smoothness constraints that include anisotropic effects. Three different smoothness constraints were tested in the inversion routine. Although we have conducted tests on data from several sites we feel that more testing is necessary to determine the optimal regularization method. Despite the increased complexity of the new algorithm, it is faster than older algorithms. This new algorithm converges in the same amount of or less iterations and requires significantly less (30 to 50% less) time per iteration than existing ERT algorithms.

Advanced Technology for Mapping Subsurface Water Conductivity is a threeyear
research program which was funded by the Department of Energy in October 2000.
Its objective is to combine Magnetometric Resistivity (MMR) surface magnetic field
measurements with Electrical Resistivity Tomography (ERT) borehole potential
measurements to calculate subsurface conductivity. The data sets measured by ERT and
MMR will be input to a combined ERT/MMR inversion code for the purpose of
calculating the subsurface conductivity distribution with increased resolution. This paper
presents the results of the work performed during the first year of this project.
MMR instrumentation was developed to spatially resolve the surface magnetic
field associated with an induced subsurface alternating current low between borehole
ERT electrode pairs. It consists of a variable frequency alternating current source and a
synchronously detected, spatially resolved vector B-field measurement system. MMR
data preprocessing algorithms are complete and include the removal of magnetic fields
associated with the surface conductors delivering current to the subsurface structure. A
three-dimensional Finite Difference Time Domain forward model that calculates
electrical and magnetic fields based on current flow through a medium characterized by
conductivity, permeability, and permittivity was also developed. Initial tests have been
conducted in the Mud Lake, Idaho sediment beds.

We have applied a 3-D viscoelastic finite-difference modeling method to the simulation of
strong ground motion from earthquakes. Our approach uses a staggered-grid finite-difference algorithm
to model the first-order elastodynamic equations of motion expressed in terms of velocity and stress. We
employed the finite-difference method based on the second-order accurate in time, forth-order accurate
in space, staggered grid scheme. The inclusion of viscoelastic attenuation and the calculation of large
velocity contrast are particularly important for the realistic modeling of seismic wave propagation from
earthquakes. Our viscoelastic modeling is based on the Standard Linear Solid. Memory variables are
introduced to eliminate the convolution in the viscoelastic constitutive relation. In order to reduce
computation time and memory requirement, nonuniform-grid method and parallel computing using
PC-cluster are employed. A seismic source is introduced as a body force term in the equation of motion.
Numerical test was performed and the finite-difference solution agreed with the waveforms calculated
by the discrete wave-number integral method. The method enables us to calculate large-scale
simulations for large magnitude earthquakes at periods down to two seconds using inexpensive
computers. The strong ground motion for the expected large earthquake has been calculated. Model size
is 370km by 240km by 73km with minimum grid spacing of 200m. The number of grids is 1850 by
1400 by 93. Two hundreds seconds of ground motion could be calculated in 7 days.

For estimation of long period earthquake strong motion, it is important to understand S-wave
velocity structure from surface to bed lock. Recently, the microtremors array measurements came
into prominence as a survey method for deep sedimentary basin structures.
We examined the microtremors array measurements at OYO Tsukuba Technical Research and
Development Center to estimate the applicability of the method to detect S-wave velocity structure.
PS-logging had been carried out in the deep borehole at our test site. Therefore, detail P-wave and
S-wave velocity structures were grasped from surface to 700m depth. We compare the result of the
microtremors array measurements with PS-logging results. It is clear that these results agree with
each other very much.

A phased geophysical ordnance detection and discrimination study (ODDS) specific to the Fort Ord, California environment has been performed. This study evaluated existing state-of-the-art OE detection/discrimination systems. Fundamental criteria for instrument selection were proven (demonstrated, documented) success at Government OE detection test sites and/or in OE removal actions. Digital data acquisition capability, ease of deployment and cost effectiveness and system sensitivity, accuracy, and resolution were also considered. Tested systems were the Geonics EM61 and EM61-HH, Geometrics G-858, Schonstedts GA-52/Cx, GA-72/Cv, and GA-52/C, Geophex GEM-3, and both BlackHawk and NRL MTADS, vehicular-towed array systems. The analog Schonstedt instruments were included as part of an evaluation of previous work at Fort Ord. Data analysis included developing Receiver Operating Characteristic (ROC) curves for results obtained over seeded grids and representative field sites at Fort Ord. These ROC curves have been constructed using False Alarm Rates (FARs) defined and calculated three ways: 1) per Acre, 2) per Item Found, and 3) as the percent of target picks that are False. The ROC curves have been used to rank the effectiveness of the tools involved in the study as to their effectiveness on sites with varying OE types, terrain, vegetation, and culture. Additionally, ROC curves have been used to study the effectiveness of variable survey traverse separations and, using a ROC curve developed for data acquired in one grid, refine and predict the anomaly selection process in neighboring grids. Finally, a simple way of using the ROC curves to make comparisons between instruments is to calculate an “Efficiency Rating” which is defined as the ratio of the Probability of Detection to the FAR per Item Detected. An instrument with a high rating will produce fewer false alarms per successful excavation than an instrument with a lower rating.

The Ordnance Detection and Discrimination Study (ODDS) was a phased geophysical study
specific to the former Fort Ord, California. This study evaluated OE detection instruments and
systems and ability to discriminate unexploded ordnance (UXO)/ ordnance and explosives (OE)
at Fort Ord. Study phases included (I) static (free-air) testing of instruments over OE at varying
depths and orientations; (II) seeded plot testing to determine effectiveness of instruments in
locating OE items in the ground; (III) field trial site testing to evaluate instruments at
uninvestigated OE sites; and (IV) evaluation and comparison of results. Geophysical data
collected in instrument evaluation phases by the Geonics EM61 & EM61-HH (hand-held),
Geophex GEM-3 and Geometrics G-858 were compared to evaluate strengths and weaknesses of
individual instruments and methodologies. Data over targets in seeded plots and field trial sites
were plotted in profile and compared with static test profiles over similar targets to analyze
changes in signal to noise ratios (SNR), signal amplitudes and profile signatures. Noise levels
were found to increase by a factor of approximately 3 to 35 times from the static to seeded tests.
The SNR decreased from the Static Test to the Seeded Test to the Field Trial Sites. The EM61-
HH and GEM-3 were determined to have better defined profiles and higher detection ability on
smaller OE items and those located closer to the surface and the EM61 and G-858 were better at
detecting larger, deeper items. The character and nature of these profiles are currently being used
in the analysis and interpretation phases of UXO investigations to gain a better understanding of
what is being measured, how and what should be measured, and how to best use commercially
acquired geophysical data.