Description
Broadband Electromagnetic Induction (BBEM) is an advanced geophysical technique applied for three dimensional characterization of a buried waste site. BBEM uses measurements of a secondary magnetic field that is induced in the subsurface following the linear decay of a source magnetic field. Time domain measurements are taken of the secondary magnetic field transient response at various time intervals over a broadband frequency range to locate, identify, and characterize buried waste areas, buried waste forms, buried waste contaminants, underground utilities, and subsurface geology. This non-intrusive technique is a geophysical approach targeted at electrically conductive objects and media (such as metals, saline solutions, and subsurface geology). BBEM development has initially focused on the adaptation of existing hardware and software for shallow, high-resolution problems. Performance assessment of BBEM will continue as part of the Buried Waste Integrated Demonstration (BWID).
Another name for this measurement technique is Time Domain Electromagnetic Induction (TDEM). A source signal is generated using an asymmetric two-coil induction system, which has a large 5-m-diameter transmitting loop. The BBEM system generates the equivalent of a three-decade frequency spectrum (40 Hz to 40 kHz). When the source current flow is interrupted, the current is allowed to decay at a linear ramp. Afterward, a secondary, transient magnetic field may be observed in the subsurface, and broadband (over a wide frequency range) measurements of frequency, amplitude, and time domain signals can be detected. These measurements indicate the depth, the geologic features, and the apparent conductivity of buried conductors in the medium under excitation. The time domain measurements are useful to delineate between the conductivity and penetration depth of a received signal. By using the system in various configurations (transverse, profile, or vertical electric expander), the BBEM system can be used to describe three dimensional subsurface geometries. Collected numerical data is used to generate three-dimensional graphical descriptions of subsurface materials and buried waste.
Technical Performance
Field Demonstration. Performed in 1992 as part of the BWID indicates the utility of this technique. BBEM has located vertical buried waste pit boundaries within 1 m precision and pit depth extent within 2 m. BBEM will be useful to describe any waste environment where conductive targets are sought. A survey requiring detail coverage over 1/2 acre can be performed in one working day. Only two people are required to take readings, to move coils, and to perform initial quality control readings in the field. It is roughly estimated that 5 h of maintenance is required to operate BBEM 50 h/wk. The electrical energy usage is minimal, less than 100 w. The presently available candidate system may be purchased for $75K from Geonics, Ltd. of Mississauga, Ontario, Canada. Other costs are associated with operator and geophysical analyst salaries.
Projected Performance
The major technical challenges to be addressed to improve the BBEM measurement system include (1) selecting the best available hardware and software system for waste characterization; (2) developing field procedures to improve target definition; and (3) developing methods of analysis that optimize the interpretation capabilities of the operator/data interpreter. Another desired improvement is to develop data analysis methods to increase the definition of small conductors in a conductive host medium (i.e., component delineation and improved resolution).
Waste Applicability
BBEM will be useful to describe any buried waste environment where conductive targets are sought. Applicable wastes include electrically conductive objects, materials, geologic structures, and brine liquid wastes. Examples include ferromagnetic buried material, metal objects, drums, and other containers with conductive contents (saline solutions and other high-ionic content liquids or solids) that may be buried in subsurface soils.
Status
The initial evaluation procedure is scheduled for completion on September 30, 1993. The components of the BBEM measurement system are currently available through commercial vendors, but 2 yrs will be required to complete the evaluation of the candidate hardware and software systems.
Regulatory Considerations
The regulatory considerations are minimal. As BBEM represents a nonintrusive technique, the only input involves the momentary input of a transient electromagnetic field beneath the property surveyed. Nonintrusive BBEM surveys are totally benign and should be completely acceptable to the public.
Potential Commercial Applications
Commercial applications include geotechnical mining applications and underground geological survey studies.
Baseline Technology
BBEM is a superior method to measure earth conductivity because of its utilization and analysis of the full transient decay curve over a wide frequency range with one source and receiver. This provides, in effect, the equivalent of three frequency decades of information and, through a single station, scans a section of ground into the third dimension. An interpretation curve is firmly founded on 20 to 30 data points rather than the usual single point for each combination of frequency and coil separation associated with conventional, narrow-band, frequency-domain electromagnetic systems. Although BBEM is more sophisticated and gives a fuller, more accurate description of the subsurface, it also requires a greater investment for instrumentation and operator training.
Intellectual Property Rights
Development of the TDEM method for applications in mineral exploration started in 1978, by Newmont Mining and CSIRO in Australia. It is not clear who owns the intellectual property rights.
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References
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