INTERMEDIATE PERIOD RESPONSE OF WATER LEVELS IN WELLS TO CRUSTAL STRAIN: SENSITIVITY AND NOISE LEVEL

Abstract

The response of water levels in wells to earth tides indicates that wells can be used to detect small crustal strain. Vertical groundwater flow between the well intake and the water table can significantly attenuate this sensitivity. The attenuation of strain sensitivity as a function of frequency can be inferred from the response of water wells to atmospheric loading. For the wells examined in this study, significant attenuation due to water table drainage can occur when strain accumulates gradually over periods of days to weeks. Despite the presence of attenuation the wells are still sensitive strain meters over this range in period. At a frequency of 2.5 cycles/day, the noise level of the water level records examined is −130 dB relative to 1 strain²/Hz. This noise level is about the same as that reported for dilatometers and wire strain meters, but is at least 10 dB higher than that reported for laser strain meters. In the frequency band of 0.025 to 2.5 cycles/day, the noise level of the water level records examined increases roughly 25 dB‐per‐decade decrease in frequency. Some of this noise is due to the influence of atmospheric loading. When the effects of atmospheric loading are removed from the record, the noise level is reduced to roughly 20 dB‐per‐decade for frequencies above 0.08 cycles/day, a rate typical of high‐quality strain meters. For periods slightly less than a month, the wells have a lower noise level in terms of areal strain than that of the best geodetic distance measurements.