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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />~~, <br /> <br />-> ~/ <br /> <br />Beaver Reservoir Company <br />Attention: Mr. Neil Thompson <br />February 4, 1993 <br />Page 6 <br /> <br />the highest SP values (greater than 0) are seen where the embankment materials are <br />thickest. Lower SP values (less than 0) are observed where the thickness of embankment <br />materials is relatively thin (approximately 20 feet or less). There are a few anomalous <br />areas that do warrant further discussion, and are described below. <br /> <br />1. Negative (amplitude of -8 mv) SP anomaly near l40E, .sON - this area is <br />along left abutment near dam crest. Area had extremely moist soil relative <br />to other dam crest stations. The anomaly amplitude is not sufficient to <br />indicate substantial fluid flow, although some minor fluid migration could <br />be taking place in a horizontal .or descending direction. <br /> <br />2. Positive (amplitude of 10+ mv)along.200N transect - the areas along the <br />left and right ends of Line 200N were near' zones of observed minor seepage <br />during datacollection:and'apparently had displayed significant seepage prior <br />to reservoir drawdown. -, . <br /> <br />3. Negative (amplitude of -10 to -14 mv) SP anomaly near 435E, 150N - area <br />at base of access road. This anomaly, being very limited in extent, is likely <br />due to varying localized geology (more boulders and less sand) rather than <br />due to seepage. <br /> <br />4. Positive.(amplitude of +6 to + 12 mv) SP anomaly along easterly ends of <br />Lines ON, 50N, and lOON. This area, which is near the right abutment and <br />.spillway ,likely has a minor amount of fluid migration. <br /> <br />REGIONAL GEOLOGY <br /> <br />Beaver. Reservoir is located in the West Elk Mountains on the southeast margin of the <br />Piceance'Basin, and lies in the transition zone between the southern Rocky Mountains on <br />the east and the Colorado Plateau on the west. Bedrock underlying the major portion of <br />the region generally consists of interbedded sandstone and shale with minor amounts of coal <br />belonging to the Upper Cretaceous Age Mesaverde Formation. The formation has been <br />subdivided into four members; the Upper Ohio Creek Member, the Barren Member, the <br />Coal Member, and the Lower Rollins Sandstone Member. The formation has a total <br />thickness of approximately 2,500 feet. The Mesaverde Formation is generally underlain <br />by the Mancos Shale. This formation with a maximum thickness on the order of 5,000 feet <br />consists primarily of shale with minor amounts of sandstone and siltstone (Ellis et al. <br />1987). <br /> <br />23067/LEIO 02-04-93(4:24pm)/RPT <br />