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12th EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems
- Conference date: 14 Mar 1999 - 18 Mar 1999
- Location: Oakland, California, USA
- Published: 14 March 1999
101 - 108 of 108 results
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Orthophoto Site Map
By Ralph SouleAn ORTHOPHOTO SITE MAP is a new type of MAP which is constructed from low level aerial
photography taken from a blimp and has a wide range of applications for a variety of projects
including geophysical, environmental, engineering and wetlands studies. An aerial photo is taken,
digitized and overlain with a Cartesian grid. The MAP is scaled to surface features measured at the
SITE. Surface features are labeled on the MAP and the MAP is then plotted at the desired scale.
The ORTHOPHOTO SITE MAP typically provides higher resolution current information then
standard airphotos which are usually taken at a higher altitude and are outdated.
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A Finite-Difference Frequency-Domain Code For Electromagnetic Induction Tomography
We are developing a new 3D code for application to electromagnetic induction tomography and to
environmental imaging problems. We have used the finite-difference frequency-domain formulation
of Beilenhoff et al. (1992) and the anisotropic PML (perfectly matched layer) approach (Berenger,
1994) to specify boundary conditions following Wu et al. (1997). PML deals with the fact that
the computations must be done in a finite domain even though the real problem is effectively of
infinite extent. The resulting formulas for the forward solver reduce to a problem of the form Ax
= y, where A is a non-Hermitian matrix with real values off the diagonal and complex values along
its diagonal. The matrix A may be either symmetric or nonsymmetric depending on details of the
boundary conditions chosen (i.e., the particular PML used in the application). The basic equation
must be solved for the vector x (which represents field quantities such as electric and magnetic fields)
with the vector y determined by the boundary conditions and transmitter location. Of the many
forward solvers that could be used for this system, relatively few have been thoroughly tested for the
type of matrix encountered in our problem. Our studies of the stability characteristics of the Bi-CG
algorithm raised questions about its reliability and uniform accuracy for this application. We have
found the stability characteristics of Bi-CGSTAB [ an alternative developed by van der Vorst (1992)
for such problems] to be entirely adequate for our application, whereas the standard Bi-CG was
quite inadequate. We have also done extensive validation of our code using semianalytical results as
well as other codes. The new code is written in Fortran 90 and is designed to be easily parallelized,
but we have not yet tested this feature of the code. An adjoint method is being developed for
solving the inverse problem for conductivity imaging (for mapping underground plumes), and this
approach, when ready, will make repeated use of the current forward modeling code.
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Three-Dimensional Inversion Of Ip Data From The Inel Cold Test Pit
Authors William Frangos, Andreas Weller and Matthias SeichterThe Cold Test Pit (CTP) has been carefully constructed to simulate stored hazardous
waste occurrences at the Idaho National Laboratory (INEL). An induced polarization (IP)
survey of the CTP shows a very strong polarization anomaly and a modest resistivity
response associated with the simulated waste.
The data have been interpreted using a three-dimensional (3-D) inversion algorithm
based on the simultaneous iterative reconstruction technique (SIRT) and finite difference
forward modeling to form a subsurface model of complex resistivity.
The edges of the waste zone are well resolved. Limited depth extent is recognized,
though the bottom of the waste appears too deep. The intrinsic polarizability of the waste
material has been estimated by means of a forward modeling experiment.
The IP method holds promise as a technique to distinguish buried waste from pockets of
conductive soil material, such as are present at the CTP.
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A Study Of A Two-Dimensional Resistivity And Ip Array
Authors Kanglin Lu, Jim Macnae and Jock BuselliAdvances in inexpensive data acquisition technology allow us to collect up to 256 channels
of data simultaneously at a total sampling rate of 100 kHz. With so many channels
available, it is possible to set up a two-dimensional (2D) receiver array for resistivity,
induced polarization (IP) and self-potential (SP) surveys. A 2D array would be best suited
for environmental and engineering applications, where the requirement for detail suggests
that measurements be taken at relatively dense station spacings in a small area, and the set
up procedure is thus relatively straightforward. One of the main questions for a 2D array is
the optimum location of the transmitter electrodes. This paper investigates this problem
with the use of 3D numerical modeling and with two field experiments. According to our
modeling and field experiment results, a gradient 2D array is the most efficient array for
resistivity and IP mapping. One set of transmitter electrodes could be enough to determine
the horizontal position of a target and the strike direction of a geological structure. In
addition, the 2D array also gives excellent SP data because it eliminates time-varying noise
from the measurements.
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Reciprocity Formulas For Electromagnetic Induction Systems
Authors Stephen J. Norton, William A. San Filipo and I.J. WonA general formula is developed from reciprocity principles relating electromagnetic
induction measurements to an arbitrary distribution of subsurface conductivity
and permeability. This relation expresses the measurement (the induced emf in the
receiving coil) explicitly in terms of the subsurface parameters while bypassing the
intermediate step of integrating the magnetic flux through the coil or the electric field
around the coil. The reciprocity formula can also be used to compute analytic derivatives
of the measurements with respect to the subsurface model parameters. Such
derivatives are needed in performing sensitivity analyses and in iterative inversion
algorithms.
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Progress In Broadband Electromagnetic Sensors
Authors I.J. Won, Dean Keiswetter, Tom Hall and Bill SanFilipoWe have developed a suite of new broadband electromagnetic (EM) sensors for rapid, ground
geophysical surveys at environmental sites. These sensors (GEM-2, GEM-3, and GEM-5
series) operate simultaneously at multiple frequencies in a bandwidth of 30 Hz to 24,000 Hz,
soon to be increased to 50,000 Hz. These hand-held, digital, multi-frequency sensors, all
weighing about 9 to 12 pounds, can transmit an arbitrary waveform containing multiple
frequencies. We also have developed an airborne EM sensor (GEM-2A) designed using the
same principles as the hand held GEM-2, but scaled up in size and built into a helicopter
towed bird. Owing to the nature of its broadcast waveform and high-speed digitization, the
sensor can operate either in a frequency-domain mode or in a time-domain mode. Its built-in
operating software allows a surveyor to cover about one acre per hour at a line spacing of five
feet. Along a survey line, the data rate is about one sweep per half-foot, resulting in about
10,000 data points or more per acre per hour. Such portability, survey speed, and high data
density are important requirements for geophysical surveys at environmental sites. All of
these sensors share the critical feature that they incorporate a loop configuration in which the
primary field at the receiver coil (i.e., field from the transmitter if no earth or scattering body
were present) is suppressed. This is accomplished either by use of a secondary measuring coil
that measures the same primary (but not scattered) field which is subtracted from the main
receiver coil signal, or by use of a secondary transmitting coil that creates a localized magnetic
cavity around the receiver coil, Attention to detail in the antenna design and front-end
electronics determines the stability of the sensor and the quality of the data, and the use of
state-of-the-art digital electronics with a high level processor provides maximum flexibility.
These aspects continue to evolve from the original prototypes, so that each generation
improves from the previous. Multi-frequency data can be acquired in a continuous mode,
with built-in sine/cosine correlation and instrument response compensation, and recorded on
the fly within the non-volatile memory.
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Comparison Between Stochastic And Occam’S Inversion Of 3-D Ert Data
Authors Xianjin Yang and Douglas J. LaBrecqueA comparison between stochastic inversion and Occam’s inversion was made using both 3-D
synthetic and field ERT data. While occam’s inversion finds the smoothest possible model
whose response best-fits the data to an a priori Chi-squared statistic, stochastic inversion looks
for the solution that best-fits the data and has the smallest deviation from the a priori model.
Both Occam’s inversion and stochastic inversion algorithms end up with a symmetric positivelydefinite
matrix system which can be solved using the conjugate gradient method. Unfortunately
stochastic inversion involves inversion of a large model covariance matrix, which is
computationally prohibitive on a PC. This problem was alleviated by using a variant of
biconjugate gradient methods to solve a nonsymmetric system. This made stochastic inversion
slower than occam’s inversion. Stochastic inversion is also a priori model dependent. The a
priori model was obtained by kriging the near-well apparent resistivities. Synthetic and field data
tests showed that Occam’s inversion with dipole-dipole data has better resolution than that with
pole-pole data. For stochastic inversion, pole-pole data are better than dipole-dipole data since it
gives a better estimate of the apriori model.
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Borehole Geophysical Characteristics Of The Lady Violet Nickel Deposit, Sudbury, Ontario
Authors B.E. Elliott, C.J. Mwenifumbo and G. McDowellMulti-sensor borehole geophysical measurements were acquired at the Lady Violet nickel
deposit to determine the geophysical characteristics and physical properties of the
mineralization and the host rocks. The host rocks for the nickel ore consist of the Sudbury
Igneous Complex (Norite and Sub-Layer Norites) and the Footwall Metasediments. The
natural gamma-ray spectral logs are useful in delineating rock units. Other geophysical
parameters show distinct and anomalous responses within the mineralization and
correlate well with the distribution of the nickel sulphides in the boreholes. The nickel
assays correlate well with the heavy element indicator of the spectral gamma-gamma data
and attempts have been made to derive the in-situ assay from this logging parameter. The
in-situ physical property data, including density, velocity, conductivity, and magnetic
susceptibility, provide fundamental information for interpreting ground and airborne
geophysical surveys. The poster shows examples of the different geophysical log
responses to illustrate the characteristics of this Sudbury-type nickel deposit.
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