Description
For many remediation technologies, a good understanding of the subsurface geology must be obtained to understand contaminant transport and to best devise the proper remediation scheme. Much of this geologic input is presently derived from well log data, which may be scarce, especially in contaminated areas where drilling must be kept to a minimum. Seismic imaging provides a means to image the geology between boreholes nonintrusively.
Crosshole seismic imaging involves the fielding of a downhole source and a downhole receiver in two boreholes, one on each side of the region to be imaged. Seismic travel times are measured between a great number (>300) of source and receiver locations in the two boreholes. These travel times are then inverted into a map of the two-dimensional velocity structure through a method known as tomography. Some of this imaging can be done with surface seismics. However, placing both the source and receiver downhole results in shorter travel paths that preserve higher seismic frequencies and result in better resolution.
For remediation processes where the properties of the subsurface are changed, comparing seismic velocity images before and during the process can provide needed information on where the technology is being effective and to what degree changes are being implemented in the subsurface. The seismic sources used generate primarily shear or primarily compressional waves, depending on the source used. The shear wave source is a controlled vibrator, while the compressional wave source is an impulse source. Comparing the velocity structures for both the compressional and shear waves provides additional information about rock properties and fluid content.
The system is reliable and functions well with only general maintenance. Operation of sources requires training of personnel in both source operation and operation of winches for fielding tools. The seismic sources are pneumatic and require compressed gas, usually air, argon, or nitrogen, that is supplied from tanks at the surface. The only physical output will be the vented gas. Effects of failure are restricted to high-pressure hazards and are easily controlled. Components must be periodically pressure tested for continued integrity. Electrical power for running direct current motors and computer control and recording equipment is also required. Some additional site power may be required for winch operation and for appropriately sizing and casing the boreholes that are needed.
Technical Performance
Field Demonstration. Field demonstrations have shown good correlation of imaged velocities with geology interpreted from well logs. Changes in saturation caused by injected air during an air sparging experiment have also been modeled. Resolution of the present system is approximately 1m in size and saturation changes of about 5%.
The seismic method will not provide the spot resolution that well logging can. Also, some a priori knowledge of the geology is needed to interpret the velocity models in terms of geology. The expense of crosshole seismics is greater than surface seismics, although this may change as crosshole seismics become more routine.
Cost. Initial one-time expenditures for equipment needed to field the system include costs for sources and receiver, winches, tripods, PCs for source control, and the seismic recording system. Estimated cost for this full system is $400K. Much of this may be available for rent or lease at a much lower cost. Field operations have been running on the order of 1 to 3 weeks, depending on survey size, for a three-man crew. Processing and interpretation presently take on the order of 1 month, but will probably decrease significantly as software is streamlined. Life-cycle costs should not exceed start-up and operations costs except when additional wells need to be drilled. Data collection takes about 1 to 3 weeks, depending on survey requirements. The interpretation of the data takes about 1 to 2 months.
Projected Performance
The major technical challenges are increasing the frequency and power output of the sources to increase resolution, improving imaging and inversion codes to handle such things as anisotropy, and decreasing the survey time through development of more rapid fielding sources and multistation receiver strings. Times for fielding and interpretation should decrease significantly as the method develops further.
Waste Applicability
Since this technology is specifically used for characterization and monitoring of the ground media at problem sites, the technology is waste independent.
Status
Although all sources are not commercially available at this time, steps are being taken to transfer the technology. There are several commercially available compressional-wave sources, such as the airgun, that are used primarily for oil and gas exploration, but can be readily modified for use in the environmental arena. No shear-wave sources are commercially available, though there is a poorer quality shear wave generated by the compressional wave sources. The complete fielding system and interpretation software are currently available, though not commercially.
Regulatory Considerations
Compliance with the Occupational Safety and Health Administration regulations is required for hazardous waste operations and protection of occupational workers from electrical power, pressurized gas, and mechanical hazards associated with operating winches and working under overhead tripods. Personnel should be trained in operating pressurized systems. In addition, permits may be required for drilling at hazardous waste sites.
Potential Commercial Applications
This technology can be of use at a wide variety of sites and in conjunction with several different remediation processes. Seismic imaging can be used at any site where information on geology between boreholes is necessary. This is especially useful when the number of boreholes that can be drilled is restricted. It can also be used for monitoring any remediation technology that significantly changes the seismic properties of the subsurface such as air sparging, steam flooding, and water flooding.
Baseline Technology
Although several companies are currently working on downhole seismic sources for oil and gas exploration, no one else is applying these techniques to environmental remediation sites.
For characterization, well logging and surface seismics offer alternatives. Drilling and logging require a large number of wells to be drilled to obtain a continuous picture of the subsurface. This makes this technology relatively more expensive and provides only isolated point information. Surface seismics are relatively limited in resolution. For monitoring imaging, only crosshole electromagnetic techniques being developed are a viable alternative.
Intellectual Property Rights
The patent owners are DOE, Sandia National Laboratories, and Richard Hills. Patent Number: 504,317,171 ``Advanced Downhole Periodic Seismic Generator.''
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References
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