New Mexico Geological Society Annual Spring Meeting — Abstracts

Earth fissures occur in alluvial basins throughout the southwestern United States, particularly in areas of large-scale groundwater overdraft. These cavernous cracks can intercept irrigation water, initiate gullying, serve as dumping places for domestic and agricultural waste, and pose a hazard to structures in the vicinity. To better understand the kinematics and mechanics of fissuring, we began an investigation to determine if sensitive borehole tiltmeters could be used to document surficial deformation near a large fissure. The fissure selected for this study is located approximately 12 miles south and 2 miles west of Deming, New Mexico, in the south-central portion of the Mimbres Basin. Water levels in this part of the basin have dropped approximately 70 since the turn of the century, and previously published geophysical profiles suggest the presence of both buried normal faults and stratigraphic irregularities beneath the fissures.

Instrumentation at the site includes four biaxial borehole tiltmeters with a ±2000 µrad, a sensitivity of 0.1 µrad, and built-in temperature sensors. The tiltimeters were sanded in place within 8 inch diameter steel casing, which was driven into rotary-drilled pilot holes. The four stations are located along an east-west line, perpendicular to the trace of the fissure. On each side of the fissure, an inner tiltmeter was installed in a 20 foot borehole and an outer tiltmeter was installed in a 10 foot deep borehole. These locations are based upon an elastic model of surficial drape folding, which predicts that very near surface deformation should persist at greater distances from the fissure than should deeper deformation. A piezometer was also installed on each side of the fissure, and fitted with a 0-15 psi pressure transducer. The use of two wells allows for measurement of any water level might occur across the fissure. Data are collected by a programmable digital data logger, which queries each instrument at one-hour intervals, and the entire system is powered by two 12 volt batteries and a solar recharging system.

Harmonic analysis of early data from the site shows that tiltmeter daily, monthly (28 day), and annual periodicities, and that daily tiltmeter maxima correspond to water level minima. Monitoring at this site is expected to continue for one to two years.

This project is supported in part by USGS grant 14-08-0001-2108 and NM WRRI grant 01423954.