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Geophysical Technologies for Detecting Underground Coal Mine Voids:
|Material||Average Density (g/cc)|
Table 1. Average density values of rocks and fill materials (after Telford et al., 1976)
Figure 2 is an example of microgravity anomalies due to a simple model of an air-filled, 20-foot diameter cylinder at three different depths. If the cylinder were filled with water, the anomaly magnitudes would decrease by a factor of 0.6. In reality, a mine may extend out laterally much more than a cylinder, and the associated gravity anomaly would be much larger than shown in this example. The model illustrates some basic concepts:
Figure 2. Simple gravity model of mines at three different depths
Prior to carrying out a microgravity survey, the parameters for the survey should be carefully planned so that adequate data (spatial coverage and orientation) are obtained to detect the feature of interest. A forward model should be developed to calculate the expected magnitude and width of an anomaly (as in Figure 2).
Microgravity data are obtained at stations along a survey line or within a survey grid. Optimally, the survey lines should extend from the area of interest into background conditions. Survey line lengths and station spacing can be planned from the modeling. Depending on the depth of the feature, a station spacing of 10 to 50 feet is normally used for environmental and geotechnical applications. Microgravity data are extremely sensitive to elevation, and the station elevations must be obtained with a precision of 0.01 feet for microGal level surveys.
The raw data are processed to remove instrument drift and external gravitational effects such as tides, elevation, latitude, and nearby topography. The resulting data are known as Bouguer gravity values and are directly related to lateral density variations in the subsurface. The data may be plotted as profile lines or contour maps depending on the applications. While the gravity technique measures the density variations in the subsurface, it is the interpreter who, based on knowledge of the local conditions or other data, must interpret the data. Interpretation can simply be the location of a gravity anomaly or can require extensive modeling to support interpretation of size and depth and calculations of mass deficit.
As with any geophysical technique, there are limitations to the microgravity method. These include:
Case History: London Mine, Isabella, Tennessee
The London Mine is an abandoned copper mine in the hills of southeastern Tennessee. The mine closed in the late 1920's and currently contains surface structures that are heavily decayed by the pyrite-rich mine tailings (Figure 3). Dangerous open shafts and pits are common in this area, which is closed to the public. One large collapse, with subsidence to depths of 100 feet or more, has occurred since the mine closed (Figure 4). Some portions of the collapse were filled with unconsolidated material providing a surface indication of the collapse location, however the exact location of the collapse area, the thickness of the fill and the presence of voids within the fill or surrounding rock were not known.
Figure 3. Surface of the London Mine in 1998
A site characterization was carried out in and around the known collapse area prior to demolition of structures at this location (Technos, 1998; Marrich, 1998). The main objective of the characterization was to determine if subsurface voids were present that would pose a safety risk for demolition crews working on the surface. Microgravity was used as the primary geophysical method to meet the project objectives.
The London Mine is located in the Blue Ridge physiographic province of southeastern Tennessee. The mine is in massive sulfide deposits occurring within highly folded and metamorphosed clastic sediments. The massive sulfide deposits are bounded by interbedded metagraywackes and mica schists with a steep (50 to 80 degree) dip (Slater, 1982). Copper deposits were mined at depths of approximately 100 feet below the surface using both subsurface and open-pit mining techniques.
Microgravity data were acquired along eight survey lines with a station spacing of 10 feet. The survey lines were positioned to provide dense data coverage in and around the approximate location of the collapse area. Access was limited in many locations due to surface structures and topography (Figures 3 and 4). Localized topographic variations, including a large rock outcrop, posed additional challenges for the microgravity survey because of their gravitational effects at nearby gravity stations.
Figure 4. Photo showing surface of collapse area
Data were acquired at a total of 257 stations using a Scintrex CG-3M gravimeter. Twenty-one percent of the stations were measured more than once at different times during the survey to provide a measure of quality control. The multiple readings show an average deviation of ±6 µGals, which represents the noise threshold for the survey. The data were reduced to Bouguer values using standard formulas (Telford et al., 1976) and corrected for the effects of nearby terrain variations.
Areas of low gravity anomalies (100 to 175 µGals) were identified in five of the survey lines. The anomalous areas are centered within the collapse area and are aligned with the strike of the mine. An example of the data along one of the profile lines is shown in Figure 5a.
Figure 5. Microgravity data (a) and model (b) showing anomalous area
Based on the microgravity data alone, it is not possible to provide a unique interpretation of the gravity anomalies. Two possible scenarios were analyzed as possible causes of the anomalies: water-filled voids (mine shafts) and collapse areas filled with unconsolidated material. Both scenarios and combinations of them are realistic possibilities for the site, but pose different levels of safety hazards. Therefore, the locations of these anomalies guided the placement of borings to identify the exact cause of the anomalies.
Vertical and angle borings were drilled in anomalous and non-anomalous areas of the site. The information from the borings was used to constrain the microgravity models to provide an accurate model of the subsurface. Figure 5b shows a model of the gravity data constrained by the results of a boring. The boring was drilled to a depth of 99 feet and encountered unconsolidated fill material through its entire length. The boring verified that the gravity anomaly is due to the density contrast between the fill material and surrounding host rock and not due to large air or water-filled voids.
The microgravity data successfully guided the boring placement in anomalous areas that had the potential of containing voids. These areas were checked with the borings and it was determined that the cause of the anomalies was lower-density fill material. Based on the results of the microgravity data and subsequent boring information, the surveyed area does not contain large voids that would be a safety concern during demolition activities.
This case history demonstrates how microgravity can be used as an effective first step in an abandoned mine characterization. The microgravity data allow a much more thorough characterization of the subsurface than random borings alone could have provided. It is imperative, however, that the microgravity data be integrated with other data sources to develop an accurate subsurface characterization.
Butler, D. K, 1980, Microgravimetric techniques for geotechnical applications, Miscellaneous Paper GL-80-13: U.S. Army Engineer Waterways Experiment Station, Vicksburg, MS, 121 pp.
Kaufmann, R. D., and Doll, W. E., 1998, Gravity meter comparison and circular error, Journal of Environmental and Engineering Geophysics, 2, 165-171.
Marrich, Inc., 1998, Geotechnical Report: Microgravity survey, London Mine, Isabella, Tennessee, Job # 98065, April, 1998.
Seigel, H. O., 1995, High precision gravity survey guide, Scintrex Ltd., 120p.
Slater, R., 1982, Massive sulfide deposits of the Ducktown Mining District, Tennessee, Exploration for Metallic Resources Southeast Conference, Athens, Georgia, Sept. 28-29, 1982.
Technos, Inc., 1998, Final Report: Microgravity survey at the London Mine, Isabella, Tennessee, Technos Project No. 98-118, April 15, 1998.
Telford, W. M., Geldart, L. P., Sheriff, R. E., and Keys, D. A., 1976, Applied Geophysics, Cambridge University Press, 860p.
1Technos, Inc., 10403 NW 31stTerrace, Miami, FL 33172
2MARRICH, Inc. P.O. Box 9179, Knoxville, TN 37940
©Technos, Inc. Used with permission