The application of remote sensing and geophysics to the detection and monitoring of acid mind drainage is beyond the experimental stage and is being applied in the management of waste from a number of producing and abandoned mines. Experimental studies, mainly in North America and Australia, have shown that non-invasive measurements by satellite, airborne, ground and waterborne platforms can be used effectively in recognising and mapping the movement of acid effluents in and around mine workings. Some methods can only recognise changes in the first meter or so of the ground surface; others are limited to depths of one to five meters; others are capable of detecting plumes at depths of several tens of meters or more. Some methods are qualitative in nature while others can provide quantitative answers within various degrees of accuracy and reliability. Studies, mainly sponsored by government agencies, but supported in many cases by industry, are attempting to establish the effectiveness of a wide variety of methods and techniques, mainly by conducting test surveys and examining available data in the vicinity of abandoned mines. Possibly the most ambitious of these studies has just been carried out in the Sudbury area, Ontario, under the MEND program by INCO Exploration and Technical Services Limited, (King, 1994). This study establishes useful parameters for applications of specific techniques in a specific geological environment. Similar studies are urgently needed to expand the range of methods and applications and extend into other geological and topographic settings.
Important progress has been made in the establishment of guidelines for the cost-effective application of both remote sensing and geophysics for AMD problems. To begin with, the non-geophysical remote sensing techniques are most effective in establishing terrain and thematic mapping parameters required for the proper monitoring of changes in condition over time for both potential and active mine drainage environments.
Mapping of vegetation encroachment, die-off, stress and morbidity, as well as percentage and distribution of ground cover and type, are effective techniques in monitoring for the impacts of AMD seepage, and any remediation of such conditions. While we can confidently recommend LANDSAT and other satellite-borne remote sensing data for preliminary studies, recent activity in airborne multi-spectral techniques would indicate that this is a more effective process, by nature of its improved resolution, both spatial and spectral.
The direct detection and effective mapping of surface moisture from seepage, ponding, and drainage patterns, uses the traditional techniques of air photos for generation of three dimensional terrain or drainage mapping, and satellite and airborne multi-spectral data for mapping of ponds and detection of surface penetrating seepage. Detection of sub-surface moisture uses thermal infrared techniques or the indirect method of monitoring stress and vigor patterns in the vegetation and ground cover. Other indicators may be found in mapping open pit mines, diversion channel silting, dam failure and changing conditions leading to imminent dam failure all of which can be detected using combinations of air photo, multi-spectral and infrared monitoring.
While traditional remote sensing methods, with the possible exception of near-infrared measurements, are generally considered to be limited to the direct detection of very shallow seepage conditions, the use of multi-spectral techniques, both satellite and airborne, provides an ability to detect changes in the vegetation or ground cover that are indicative of sub surface AMD problems. These techniques are quite recent and represent an area that deserves. and will likely see. more active duty in the near future.
The use of spectroradiometers is a relatively new field that is growing rapidly. The collection of reference spectral signatures has become an important component for site characterization for environmental remediation. In addition, geologists are starting to use spectroradiometers for mineral exploration in arid and semi-arid environments and for characterizing mine tailings and waste rock. Image analysis software vendors are beginning to market software that enable analysts to use collected reference signatures or access existing spectral libraries that can be used as an integral part of the analysis of airborne or satellite data.
The real strengths of these methodologies are to be found, not so much in the raw data, but in the ability to analyze and integrate this data with other data sources, homing in on the actual realities contained in a series of inferential data sets. It is the image analysis and GIS (Geographic Information System) systems that make this possible. AMD is an application where this ability to integrate is crucial. As the necessary sources of information are quite diverse, not only do the multiple remote sensing data sets have to be properly analyzed and integrated, but a wide range of geophysical data must also be incorporated into, and analyzed through, this process.
Conventional airborne geophysical survey data, primarily electromagnetic, but also magnetic and gamma-ray spectrometry, can be used as baseline information to establish the natural physical parameters of the ground, and in the detection and ongoing monitoring of changes caused by acid mine effluent. Depending upon the magnitude and composition of the seepage, changes may be detected to tens or possibly hundreds of meters below ground.
Borehole geophysical techniques are being used widely in the waste management industry to map contaminant plumes as well as the structure and stratigraphy of disposal sites. These techniques are commonly followed by traditional drilling and in-situ chemical and physical monitoring procedures. The geophysical surveys act as inexpensive reconnaissance techniques to recognise and establish limits for seepage problems, to direct cost-effective drilling programs and to monitor changes taking place outside the drilled areas.
Borehole geophysical logging is widely used to determine physical parameters that are affected by acid contamination and to identify zones of contamination detected by surface measurements. Borehole-to-borehole and surface-to-borehole measurements are being tested to provide three-dimensional images of zones of anomalous electrical conductivity or acoustic impedance. Borehole methods are outside the scope of the present study but are referred to briefly in the text. References are included in the General Bibliography.
All of these geophysical techniques have, as a second objective, the determination of stratigraphy, bedrock structure, waste-pile architecture etc. that can assist in the design of a testing and remedial follow-up program.
Ground geophysical methods of greatest potential in AMD studies appear to be wideband or multi-frequency EM, ground penetrating radar (GPR), Induced Polarization (IP) and, oddly, Self Potential (SP). All three methods provide direct information on contaminant plumes as well as indirect data on the structure of the ground and potential leakage pathways. The SP method is the only one that can sense the actual movement of acid contaminant.
One message that repeats Itself again and again in the available literature is the importance of integrating methods in order to remove ambiguities and improve identification of acid drainage. Very frequently an anomalous condition might be caused by a change in lithology or the presence of acidic groundwater. The use of a second method can often eliminate one of these possibilities. Another important message that applies to geophysical surveys is that neither raw nor processed data, presented as profiles, contours or pseudo sections of measured parameters is nearly as effective for understanding subsurface conditions as inverted data. Data inversion, whether 2- or 3dimensional, attempts to present a physical property distribution in the ground, which should correspond to the properties encountered in a drill program. Techniques for inverting most types of geophysical data are now available but are only very recently finding their way into the industrial marketplace. The authors believe that this is an area where important advances will be made in the next few years.
This handbook is the result of an m-depth literature search, an information survey of more than 900 organizations, synthesis of catalogues from suppliers of equipment and services, and numerous discussions between the authors and scientists working in the remote sensing and geophysical disciplines. None of these phases is, by itself, complete. For example, the information survey failed to reach a number of organizations active in the remote sensing and geophysical fields. However, our analysis of the state of the art in terms of methods and applications, again through a synthesis of information from the above sources. is believed to be fairly accurate.
With respect to specific techniques and instrumentation, it has not been possible to include full details of all of the systems available for remote sensing and geophysical surveys. Readers are encouraged to contact the customer services departments of the firms listed in Chapters 5 and 12 and to consult professional directories for other suppliers that have been omitted in this compendium.
Finally, it should be recognised that while the technologies described in this manual are, for the most part, relatively mature, their application to environmental problems in general and AMD in particular, is quite new. The authors have attempted to identify additional ways of applying remote sensing and geophysical methods to AMD problems. Readers are encouraged to conduct their own experiments, thereby adding to the existing experience and information base.