The Delaware Basin is a broad asymmetric sedimentary trough in southeastern New Mexico and west Texas. Basin subsidence occurred from the Pennsylvanian into the Triassic. The basin also underwent tilting since the early Cenozoic. Layered evaporite units of Ochoan age in the basin are 1000 m thick. These evaporites are divided into three stratigraphic units (listed in order of increasing age): the Rustler Formation, the Salado Formation, the Castile Formation. These units, especially the Castile, are deformed along portions of the margin of the Delaware Basin and in some areas internal to the basin. In the northern Delaware Basin adjacent to the WIPP site, the term "Disturbed Zone" (DZ) has been applied to an area in which deformation structures are found in boreholes and from which chaotic seismig reflection data were obtained. The origin and timing of this deformation is considered important for the determination of This docunlent is PUBLI&'L%7 RELEASABLE Authorizing Official Date: 81/31 / W O d

A portion of the WIPP site has been extensively surveyed with high-precision gravity. The main survey (in T22S, R31E) covered a rectangular area 2 by 4-1/3 mi encompassing all of WIPP site Zone II and part of the disturbed zone to the north of the site. Stations were at 293-ft intervals along 13 north-south lines 880 ft apart. The data are considered accurate to within a few hundredths of a milligal. Long-wavelength gravity anomalies correlate well with seismic time structures on horizons below the Castile Formation. Both the gravity anomalies and the seismic time structures are interpreted as resulting from related density and velocity variations within the Ochoan Series. Shorter wavelength negative gravity anomalies are interpreted as resulting from bulk density alteration in the vicinity of karst conduits. The WIPP gravity survey was unable to resolve low-amplitude, long-wavelength anomalies that should result from the geologic structures within the disturbed zone. It did indicate the degree and character of karst development within the surveyed area. r NOTSCE PffprmMR nP THIS REPORT ABE lUEfilBlE. It has been reproduced from the best available copy to permit the broadest, possible availability. .,.;.!..« fiSTRIBUTION OF THIS DOCUMENT IS !!NL(MITFD jji 3 Acknowledgment Appreciation is extended to the Sandia National Laboratories Applicon Graphics Personnel, Dept. 9761, for preparing appropriate base maps and to Carmen de Souza for patiently typing and retyping the manuscript. D. J. Borns and C. A. Searls reviewed the report and offered many helpful suggestions. The interpretation is that of the authors and is not necessarily agreed to by the reviewers.

No abstract

Field tests were conducted to determine representative total‐intensity magnetic anomalies due to the presence of underground storage tanks and 55‐gallon steel drums. Three different drums were suspended from a non‐magnetic tripod and the underlying field surveyed with each drum in an upright and a flipped plus rotated orientation. At drum‐to‐sensor separations of 11 feet, the anomalies had peak values of around 50 gammas and half‐widths about equal to the drum‐to‐sensor separation. Remanent and induced magnetizations were comparable; crushing one of the drums significantly reduced both. A profile over a single underground storage tank had a 1000‐gamma anomaly, which was similar to the modeled anomaly due to an infinitely long cylinder horizontally magnetized perpendicular to its axis. A profile over two adjacent tanks had a smooth 350‐gamma single‐peak anomaly even though models of two tanks produced dual‐peaked anomalies. Demagnetization could explain why crushing a drum reduced its induced magnetization and why two adjacent tanks produced a single‐peak anomaly. A 40‐acre abandoned landfill was surveyed on a 50‐ by 100‐foot rectangular grid and along several detailed profiles. The observed field had broad positive and negative anomalies that were similar to modeled anomalies due to thickness variations in a layer of uniformly magnetized material. It was not comparable to the anomalies due to induced magnetization in multiple, randomly located, randomly sized, independent spheres, suggesting that demagnetization may have limited the effective susceptibility of the landfill material. A different 6‐acre site survey conducted on a 10‐ by 10‐foot grid was analyzed to determine the maximum station spacing and line separation that could have been used. Essentially, all of the anomalies at this site would have been resolved by a survey conducted on a 20‐ by 20‐foot grid and the larger anomalies would have been detected by a 50‐ by 50‐foot grid.

Gravity terrain corrections account for the upward pull of topographic features which are higher than a gravity station (hills) and the lack of downward pull from open space which is lower than the station (valleys). In areas of rugged topography or in high precision surveys, the magnitude of the terrain corrections can be comparable to the anomalies being sought and the uncertainties in the terrain corrections can limit the accuracy of the survey. Also, calculating the corrections can require more time and effort than gathering the original field data. Even if terrain corrections are not made, it is necessary to show that their omission does not compromise the integrity of the survey.