Variations in noise and signal levels in a pair of deep boreholes near Amarillo, Texas

To better understand the depth dependence of improvements in signal-to-noise ratio for borehole seismic data, we have collected and analyzed a data set recorded from a pair of high-fidelity, broadband (1 to 80 Hz) seismometers sited in two closely separated deep boreholes near Amarillo, Texas. The total decrease (surface to 1951 m) in minimum noise level is up to 30 dB; the total decrease in maximum noise level is up to 35 dB; and the range of noise values (maximum to minimum) at a given depth decreases from 30 to 10 dB. The majority of the noise reduction occurs within the first few hundred meters below the surface, but the decrease continues with depth and there is no indication that even lower noise levels do not exist below our deepest recording depth (1951 m). Cultural work-day noise (prominent during normal working hours) from 0 to 40 Hz is observed at all depths, suggesting a strong body-wave component. Wind-generated 15 to 60-Hz noise is strongest at the surface, and is observed as deep as 367 m without a properly shielded hole. With the addition of shielding to reduce the coupling between the borehole casing and/or cable with the wind, wind-generated noise above 40 Hz can be eliminated at 367 m. Events with near-vertical propagation paths and signal power above 1 Hz show signal strength decreasing uphole, with a steady winnowing of high frequencies that can be fit by a simple frequency-independent Q of 35.5 ± 8 for the material between 1951 and 367 m.