A COMPARISON OF THE HIGH-FREQUENCY (GREATER-THAN-1 HZ) SURFACE AND SUBSURFACE NOISE ENVIRONMENT AT 3 SITES IN THE UNITED-STATES

Surface and subsurface high-frequency (>1 Hz) noise data were recorded using nearly identical instrumentation at three widely separated site s in the United States (Amarillo, Texas; Datil, New Mexico; and Pineda le, Wyoming) for extended periods of time under varying wind condition s. While the sites are geologically distinct, the near-surface noise e nvironments have many common features that we believe may be due in la rge part to the presence of a surficial layer of highly attenuative un consolidated material at each site. Noise levels seen at or near the s urface (5 m or less) are much higher (up to 30 dB) and much more varia ble (power range up to 44 dB) than those seen at depth (the smallest r ange was 9 dB for 1951 m at Amarillo). The greatest gains in noise lev el reduction are realized within the first 100 m and probably much sha llower (< similar to 10 m). Regardless of the wind profile or local li thology, all sites show an excellent correlation between increased noi se levels and higher wind speed, even at significant depths (367 m at Amarillo). Wind-generated noise is broadband (at least 15 to 60 Hz) an d apparently nonlinear, increasing dramatically when a wind speed thre shold is exceeded (3 to 4 m/sec within a few meters of the surface; as high as 8 m/sec at a depth of a few hundred meters). It is possible t o be essentially completely shielded from the wind-generated component of seismic noise by deploying instruments at sufficient depth, but we observed this only for the two deepest deployments (1219 and 1951 m, both at Amarillo). Reducing the wind profile at the surface, however, can yield similar reductions for a much smaller cost. Cultural or ''wo rkday'' noise, if present (depending on the remoteness of the site), i s typically much weaker (10 dB or less) than wind noise but may propag ate very effectively to great depths and therefore could be of concern for very deep deployments where wind is not a factor.