Date of Award

2022-12-01

Degree Name

Master of Science

Department

Geological Sciences

Advisor(s)

Diane I. Doser

Second Advisor

Marianne Karplus

Abstract

The Mesilla Basin is part of the Rio Grande Basin System. Its northern boundary is definedby the Doña Ana Mountains in New Mexico and to the south by Sierra San Blas, Sierra Almirez, and Sierra La Candelaria in Mexico, where the basin changes its name to Conejos-Médanos. It supplies water for irrigation and public use to the cities of Las Cruces, NM, El Paso, TX, and Ciudad Juárez, Mexico. The southern part of the basin in the U.S. territory, known as the Lower Mesilla Basin, is bounded to the east by the Franklin Mountains, to the west by the Potrillo Mountains, and to the south by the international border. In 1973 the U.S. Geological Survey (USGS) made a series of resistivity soundings to estimate and delineate the portion of the basin containing fresh groundwater. They used automated software to obtain results, but no interpretation was made. In 1996, the soundings were re-interpreted as part of a different research project that produced profiles that fit the observed data; however, these profiles do not fit the known geological features of the area. The cross-sections presented have low resistivity values (3 to 10 ohm-m), at depths within the bedrock that would require bedrock porosities of over 30%. These results are concerning since other studies use this data as part of an analysis of how fast fresh groundwater is being depleted within the basin. This thesis reprocesses the 1973 resistivity sounding data using available water well logs, geologic cross-sections, and gravity modeling studies to help develop subsurface resistivity models consistent with geologic information. This study was able to identify some of the issues present in previous interpretations: (1) the attempt to match soundings at depths beyond resolution of electrode spacing; (2) in soundings located close to rock bodies, extremely high resistivities caused the electrical current to flow sideways rather than deeper into the ground as sounding spacing was increased; (3) there are several topographic changes and fault systems affecting some of the soundings that can cause lateral (2-D) anomalies. These issues were found to explain why low resistivity values were present in some sounding interpretations at depths where low values are not likely. Correcting them gives us a more confident interpretation.

Language

en

Provenance

Received from ProQuest

File Size

139 p.

File Format

application/pdf

Rights Holder

Leslie Bernal

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