SEISMIC NOISE IN DEEP BOREHOLES

Abstract

Seismic noise in the frequency range 0.2 to 5.0 cps was observed in three wells approximately 10,000 feet deep. The wells are in the Texas Gulf Coast, the Val Verde Basin of West Texas and the Williston Basin of North Dakota. Observations were made at 1000 foot intervals in each well and simultaneous observations were made in a 500 foot reference hole and at four surface stations approximately 1500 feet apart. These seismometers were vertical component Benioffs. Two surface horizontal components were also recorded. Power spectral ration, coherence and cross correlation phase were computed for most seismometer pairs, particularly the deep well seismometer and the 500 foot reference. The theoretical response of appropriate plane layered elastic models to trapped Rayleigh and Love waves as well as to plane P, SV, and SH waves incident from below was investigated in detail over the applicable frequency range for each of the three sites. Two essentially independent models for the noise have been investigated. One is based on the assumption that at a given frequency the average energy density is the same for each trapped surface mode (Rayleigh and Love) and that the modes are independent; the other assumes that at a given frequency independent plane P, SV, and SH waves with equal energy flux in all directions are incident from the lower half space. The theoretical response parameters for the two noise models applied to a given layered model have many similar features. The observed noises at all three sites vary systematically with depth and exhibit a behavior roughly predicted by an average of the two noise models. Both trapped waves and plane waves are probably present. Similar conclusions apply to the noise observed on the surface array and the surface horizontal components.