Laserfiche WebLink
June 2Q 2000 Page 7 <br />' (Niitsuma et al. 1997). Re-precipitation of salts due to cooling changes the shape, extents, and <br />volume of the cavern, making the time of a geophysical cavern survey an important consideration. <br />' Certain well hole completions can present problems and result in poor resolution. Also, as an <br />' invasive technique, cross-hole seismic methods have the potential to interfere with mine production. <br />Separate bore holes could be developed specifically for across-hole survey, but this would obviously <br />result in a cost increase. The cost ofcross-hole seismic for this application would be approximately <br />$15,000/day, not including development of any boreholes (Schlumberger-Brian Pluemer). <br />' 4.5 Magnetotellurics <br />t Magnetotellurics is anon-invasive geophysicalrnethod which utilizes natural electromagnetic <br />(EM) waves to detect changes in rock types below the earth's surface. Solar wind from the sun <br />' reacts with the earth's natural EM field to create an EM wave front (also termed a telluric current) <br />which travels vertically into the earth. The depth to which the wave front penetrates is determined <br />' by the wave frequency; low frequency waves travel deepest. The frequency of the wave front <br />remains unchanged until a change in lithologic formation is encountered. At this point, a frequency <br />' shift occurs and a portion ofthe altered wave form is reflected to the surface where it can be recorded <br />(Figure 4). The depth associated witb a recorded reelection wave is determined by calibration It is <br />1 <br />reported that depth error can be as low as 10 ft where suffident calibrations are available (Telluric <br />Technologies, Inc.). It is also reported that aberrations as small as 0.8 ft at depths of 5000 ft can be <br />detected (Digital Magnetotellurics Technologies). Other companies report 5 percent of the depth as <br />the resolution (Chinook Geoconsulting and Geosystems). This contradiction between <br />magnetotellurics vendors and consultants makes evaluation of this technology difficult. <br />Claims of successful applications of magnetotellurics in groundwater and fluid reservoir <br />characterization are sited by several sources (Electromagnetic Instruments, Inc., Digital <br />Magnetotellurics Technologies, Telluric Technologies, Inc., and Geosystems). One case ofparticular <br />interest is the successful use of magnetotellurics technology in determining the depth, shape, and <br />volume of salt caverns (Woods 1999). The volume and geometry of a salt cavern were measured <br />using magnetotellurics technology, as well as by a sonar suwey. Close agreement between the <br />independent measurements gives some credibility to this application of magnetotellurics technology. <br />Agapito Associates, Inc. <br />