Downhole Seismic Noise Measurements in the Beowawe Geothermal Field, Nevada

Abstract

A down hole seismic noise study was conducted at the Geysers area of Chevron's Beowawe geothermal field. Four wells were acoustically monitored with sensors placed simultaneously downhole and at the wellhead. Analyses included the correlation to downhole to surficial noise characteristics, well to well data correlations for noise source location or direction, and testing for the presence of borehole acoustic coupling between downhole and wellhead receivers. Intrawell cross correlations in cased or lined boreholes clearly indicate acoustic coupling between wellhead and downhole receivers. Mean integrated power values calculated over three frequency intervals indicate that the coupled signal is in the frequency interval 30 to 85 Hz and is the dominant component of signal downhole. Surficial variations of noise intensity in the frequency interval 0.5 to 15 Hz show little relation to simulteously monitored downhole noise intensity. Downhole noise measurement appears to be predominantly a function of near borehole phenomena in lined or cased holes. Measurements in an uncased borehole showed good correlations with surficial variations. Interwell correlations of noise could not be founds. Reservoir noise in the Beowawe field indicated by conventional geophysical surveys could not be corroborated.