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III IIIIIIIIIIIII III` 999 DEPARTMENT OF NATURAL RESOURCES Davitl H. Getches, Enecutive Director MINED LAND RECLAMATION DIVISION DAVID C.SHELTO N, Director Richartl D. Lamm Governor MEMORANDUM ry 1986 T0: Susan Mowry FROM: Jim Pendleton P,E: Modificatio Monitor' De J .~-_ ,_ ~. unnison's Aerial Photogrametric Subsidence tion Project - Revision (Permit C-80-007) West Elk Commended its original permit to include an experimental aerial photogrammetric subsidence monitoring demonstration project above its first retreated panel. This panel is to be used to verify permit application projections of the angle of draw and maximum subsidence magnitude. Traditional survey techniques are being used to monitor these physical parameters of subsidence. The company also received permission to use aerial photogrammetric methods to monitor subsidence above the first panel. It is the company's desire to receive approval to use aerial photogrammetric techniques to monitor subsidence of future panels. The Division, in approving the demonstration project, stipulated success criteria to determine whether the aerial photogrammetric methods proved accurate enough to replace traditional subsidence monitoring techniques for future panels. The aerial photogrammetric demonstration project was addressed in Stipulations 16 through 19 of revised Permit C-80-007. As addressed in Dennis Conn's letter to me, dated February 21, 1986, West Elk Coal Company's mining is ahead of schedule. The required traditional pre-mining surveys have been performed. Unfortunately, the pre-mining aerial photographic flights completed last fall failed to correlate within required accuracy. Both the company and the aerial contractor believe this failure is due to a design flaw in the photgrammetric targets. It was their intention to redesign the targets and complete rephotographing the panel prior to undermining. However, because mining is ahead of the anticipated schedule and heavy snow cover may prevent rephotographing the panel until late spring, a portion of the survey monuments (the southern half) may be undermined prior to completion of the required pre-mining aerial rephotography. The company believes that the northern half of the survey grid should suffice to prove correlation of the aerial photogrammetric subsidence monitoring technique. The company has requested the Division's concurrence. I believe the company's proposal to be acceptable, for the following reasons. The company's traditional survey subsidence monitoring program will satisfy the regulatory requirements. The company unilaterally suffers the exposure of failing to satisfy the success criteria for the aerial photogrammetric demonstration project. In my opinion, the approximately sixty (60) monuments which comprise the northern half of the monitoring grid cover a representative portion of the terrain and will suffice to test the accuracy of the photogrammetric monitoring techniques. cc: Bob Liddle Fred Banta . 423 Centennial Building, 1313 Sherman Street Denver, Colorado 80203 Tel. (303) 866-3567