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17th EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems
- Conference date: 22 Feb 2004 - 26 Feb 2004
- Location: Colorado Springs, Colorado, USA
- Published: 22 February 2004
161 - 165 of 165 results
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Development Of An Electromagnetic Induction Sensor For Landmine Discrimination
Authors James B. Kingdon, Nagi Khadr, Thomas H. Bell and Lloyd S. RiggsWe describe a time domain electromagnetic induction sensor, specifically designed for landmine
detection and discrimination. The detector utilizes a “figure 8” dual receiver configuration, which
significantly reduces direct coupling with the transmitter, thus allowing for measurements to be taken
more quickly after the pulse decays. Objects are modeled as simple dipoles, and the eigenvalues of an
effective polarizability tensor are used to distinguish mines from clutter. Early field results suggest that
the sensor is capable of differentiating landmines from clutter, and may be useful for identifying
individual landmine types.
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On Inversion Of Gradient Magnetic Data For Detection Of Multiple Buried Metallic Objectives
Authors Dr. Ruizhong Jia and Dr. R.W. GroomIn our previous work (R.W. Groom, R. Jia And C. Alvarez 2003), we have developed algorithms
to implement an Euler depth estimator as well as an inversion algorithm for detecting a simple dipole,
which is often a suitable model for UXO applications. These algorithms worked independently with a
single anomaly. We have combined these algorithms to determine the locations as well as the internal
magnetization vectors of buried objects in a practical field survey. We start with the Euler deconvolution
depth estimator that gives the locations of buried objects using the measured total field and its
measured/calculated gradients. Based on these initial results, a subset of measured data is selected and a
local search grid is set for each individual body. Then the magnetization inversion algorithm is utilized
to find the locations and internal magnetization vectors of the buried bodies. The inversion process
involves performing an automatic iterative grid volume modification according to a prescribed volume
range of the buried objectives. Consequently we can determine the location and internal magnetization
vector of each individual body by applying the inversion algorithm in an automatic way. In general, the
locations identified in this way are more accurate due to the fact that only the measured total field data is
used in the later stage. It is shown that Magnetization Vector Inversion is relatively insensitive to data
density and thus works more stably. Furthermore, starting with good Euler solutions is essential to
guarantee an appropriate selection of dataset that incorporates substantial variation of the field and field
gradients of each object.
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Improving Uxo Detection With Four-Dimensional Matched Filters
Authors Scott MacInnes, Scott Urquhart and Kenneth ZongeA newly developed three-component, transient-electromagnetic (TEM) system for UXO
detection and characterization continuously records full transients 64 times per second. Acquisition at a
walking speed of 1 m/sec generates three transients, each with 31 time windows, every 1.5 cm along
line. Such high-density over sampling provides an opportunity to improve signal-to-noise ratios with
post-acquisition filtering.
Tapered stacking and decimation by eight reduces background noise levels by a factor of three,
while producing a sampling density sufficient for UXO detection and characterization. For detection, a
matched filter derived from UXO modeling projects data onto a scalar profile suitable for target picking.
The filter’s spatial shape is based on the response of a compact spherical body given the TEM system’s
transmitter and receiver loop geometry. The filter’s temporal properties are matched to an expected
polarizability transient shape based on UXO characterization modeling. Using a matched filter tailored
to the expected spatial and temporal TEM response character over UXO has proved to be effective at
increasing UXO detectability relative to methods using only vertical-component amplitudes.
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Effects Of Magnetic Soils On Magnetometry In Uxo Discrimination Problems
Authors David Sinex and Yaoguo LiMagnetometry has emerged as an effective tool for UXO discrimination. The current approach
is based upon the magnetic dipole moment recovered from the inversion of surface total-field magnetic
anomalies. However, the reliability of inversion will be strongly influenced by the noise in the data such
as that produced by magnetic soils in the background geology. We examine this problem to understand
under what conditions the magnetic dipole inversion will be severely affected and, when such conditions
occur, how to alleviate the effect. We achieve this by evaluating the errors of the dipole inversion due to
different soil susceptibilities. The magnetic susceptibility of soils is modeled as a correlated random
process whose spectral property is estimated using the TEM data from Kaho’olawe, Hawaii. The UXO
response is modeled as that of a dipole embedded in the soil. Inverting the total-field response using a
large number of soil realizations yields a reliable estimate of the errors. To remove the soil effect, we
propose to preprocess the data before applying inversion. We show that a Wiener optimal filter can be
used as a preprocessing tool to alleviate the effect of the soil response and reduce the errors of inverted
dipole parameters, thus providing better confidence in discrimination.
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Development Of A Combined Emi/Magnetometer Sensor For Uxo Detection
Authors David J. Wright and Jim KingdonElectromagnetic induction (EMI) and total magnetic field surveys are the two primary
geophysical technologies used for UXO detection. Handheld EMI sensors perform better against
shallow UXO items, and can detect non-ferrous sub-munitions. Cesium vapor magnetometers are
effective against large, deep ordnance items that handheld EMI sensors cannot detect, however they do
not respond to non-ferrous objects. On sites requiring the use of both technologies, the cost of collecting
both data sets can be significantly reduced if they are collected simultaneously in a single survey. An
additional advantage is that the precise relative positioning of these two data sets facilitates joint or
cooperative analyses to provide enhanced discrimination capability.
Cesium vapor magnetometers track oscillations of the magnetic field occurring at frequencies
<200 Hz. For magnetic field oscillations >>200 Hz they simply measure the average effect of these
oscillations. Thus the large low frequency components of a time domain EM (TDEM) field distort the
measured geomagnetic field, but a constant wave frequency domain EM (FDEM) sensor only induces an
offset in the magnetic data. We discuss issues involved in the development of a combined
FDEM/magnetometer instrument, and present the results of bench and field tests.
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