Subsidence above two active longwall mines located in Utah, Deer Creek and Dugout Canyon, was monitored using differential interferometric synthetic aperture radar. Time-lapsed images acquired over a period of 46, 92, and 132 days by the ALOS PALSAR sensor were used to locate areas of ground movement and to image centimeter-scale displacements associated with mining induced subsidence. Over a period of 132 days DInSAR was used to identify and measure the growth of a subsidence trough over an active longwall panel at Deer Creek. The behavior correlates well with field data collected via a photogrammetric survey. At Dugout Canyon the maximum rate of subsidence at a field monitored point measured using DInSAR was 14 cm per month which compares favorably to the field data rate of 10 cm per month. Measurements made using DInSAR a above a second panel at Dugout Canyon during 2006 and 2007 indicated 18 cm of subsidence, nearly identical to the 20 cm measured via GPS. In both cases DInSAR provided a measurement density of 5000 points per square kilometer (the typical size of a single longwall panel). This case study indicates that DInSAR permits the derivation of high resolution ground surface displacement maps and provides a more rational, statistically significant basis for subsidence monitoring and modeling.
A major consequence of underground mining is the subsidence that takes place in the overburden strata, eventually propagating to the ground surface. Over 1 million hectares (2.5 million acres) of land in the United States are affected by mining-induced subsidence. With increased underground mining, and an increase of coal mining at depths greater than 300 meters (1000 ft), surface subsidence is a major environmental and engineering issue. Subsidence is a complex combination of highly discontinuous rock mass flow surrounded by a zone of minor discontinuous/continuous ground relaxation. Measuring the rate and distribution of this deformation is necessary in order to understand the geological and hydrological factors that influence ground behavior, thereby improving prediction of further movement, and ultimately enhancing the effectiveness of coal mine design and ground control. In addition to characterizing the nature, extent, and magnitude of expected and actual surface displacements, mining-induced subsidence must be monitored in order to identify environmental impacts with high risk levels and/or severe consequences. These impacts can include loss of productive land, damage to buried utilities, destruction and/or decreased stability of surface structures, and dewatering of streams and groundwater supplies. Proper subsidence engineering not only requires accurate prediction of ground movements and the determination of the effects such movements on structures and resources, it is calls for accurate measurement of actual ground movements. This is a difficult task, and one that is presently limited by deficiencies in data collection and evaluation. Subsidence measurements are typically based upon a survey of selected vertical and horizontal displacements that take place on the ground. Various methods and equipment are used to document vertical and horizontal movements associated with subsidence. Practically all predictive models address this aspect of mining-induced ground movement .