A thick salt bed, the Salado Formation, is present in the northwestern part of the Delaware basin. In an area about 30 mi east of Carlsbad, Eddy County, New Mexico, this salt bed is being investigated as a possible site for a pilot plant for radioactive-waste isolation. Geophysical investigations have been carried out at the proposed

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site since 1976. This work was performed under the direction of Sandia Laboratories, Albuquerque, under contract to the Department of Energy. The purpose of these geophysical investigations was to study the geologic conditions of the bedded salt deposit, including the evaporite section and overlying and underlying formations as part of site evaluation and characterization.

Both nonseismic and seismic geophysical techniques were employed. Seismic investigations were used to map from the top of the Salado Formation downward to Precambrian basement in order to locate and delineate (1) salt breccia pipes or collapse features that penetrate into and possibly through the Salado Formation, (2) an igneous dike that cuts the Salado Formation, (3) salt-dissolution fronts within or above the evaporite section, and (4) slump, faulting, or other structural disturbances above, below, or within the evaporite section.

The seismic site-evaluation studies included two Vibroseis programs plus a review of all the petroleum industry data available in the area of interest. The 1977 seismic program was for the semidetailed analysis of anomalies located during all earlier data studies. Any indications of slumping, faulting, and dissolution within the evaporite zones or any other structural departure from a "stable" condition in the post-Delaware deposits on the first review were subjected to additional seismic study. Forty-eight miles of extended frequency Vibroseis data were gathered over the anomalies in 1977. Sweeps to 110 Hz were utilized.

The results of the 1977 program indicated that in some areas the Castile and lower Salado Formations are deformed, a fact supported by a follow-up series of drill holes. Examples of the early interpreted anomalies with the normal petroleum industry field-recording parameters clearly show the need to modify the parameters to obtain shallow information. The latest Vibroseis data indicate the enhancement obtained by shortening the geophone and source intervals and by raising the sweep range to 100 Hz. The earlier observed anomalies are compared with later examples and, in all cases, the detailed studies give excellent definition as to salt deformation, timing of movement, and extent of structural effect on the salt beds.

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