Three-dimensional velocity tomograms were generated on a daily basis to image mining-induced changes to the overburden above a longwall mine. The hypothesis was that a coherent redistribution of seismic velocity, due to the development of high-stress zones, could be imaged at the mine scale. Seam depth was 360 m and source location depth varied from 100 to 1000 m. Sixteen geophones were distributed over a 600 by 600 m square area on the surface above the mine. More than 12,500 events were recorded over an 18 day period. The recorded seismicity provided input for the local-earthquake tomography code, SIMULPS. Eighteen tomograms were generated and high-velocity regions correlated well with high abutment stresses. Additionally, the high-velocity regions were observed to redistribute as the longwall face retreated. These results indicate that velocity tomography can be used to provide a better understanding of temporal changes within a rock mass, and can potentially be used to produce a better understanding of the mechanisms that lead to unanticipated ground failures.
The ability to forecast failure of rock is an ultimate goal of the geo-engineering field. Failure within a rock mass is often a poorly understood phenomenon which can have severe consequences, even where best-design practices are employed. Failure within a rock mass is currently predicted by comparing values of stress and strength based on estimated material properties. Frequently, numerical models are used to represent the rock structure by dividing it into elements and summing the behavior of the elements. Each element relates applied stresses to strains through a constitutive equation. Different material properties can be assigned to various portions of the model. However, the correct estimate of the necessary material properties is a key problem with numerical modeling of rock structures; without accurate estimates of the material properties, the changes induced by loading of a rock mass are inaccurate. A potential remedy to this problem is to perform high-resolution imaging to monitor the true alteration of a rock mass as it is loaded. Because rock properties change under increased loads, areas of high stress can be identified using elastic waves [1,2,3]. Elastic waves include ultrasonic waves, typically used in the laboratory , and seismic waves, common in field studies [5,6,7]. These waves can be used to nondestructively image the interior of rock masses and determine changes to loading of the rock mass . One method used to perform this imaging is tomography, which is a rapidly advancing method for imaging the interior of bodies, including rock masses [9,10,11]. Tomography has been used at the field scale as long ago as 20 years in the mining industry to image geologic features as well as stress-related features [12,13,14]. It was not until the last 15 years, however, that it has had ready acceptance within the geosciences for petroleum reservoir characterization and for geotechnical applications. More recently, the method has been adapted to image stress concentrations ahead of a longwall face by a unique application of the longwall mining equipment as the seismic source [15,16].