Corehole seismometer development for low‐noise seismic data in a long‐term seafloor observatory

Abstract

Abstract. Longstanding problems unique to marine seismology (in contrast to land-based studies) include high levels of environmental noise, unpredictable instrument placement, short measurement duration, and limited numbers of instruments. Traditional instruments are deployed and recovered from a surface ship with no capability to verify data acquisition until the end of the experiment. They are often poorly sited and usually poorly coupled to the seafloor. Experiments have typically been short in duration with only a minimum number of instruments. The result of these logistical problems is high background noise, sparse data recovery (compared to continental stations), and low fidelity data. In an effort to solve many of these problems, we have developed and deployed "corehole seismometers"(1-90 Hz) into small boreholes drilled by an underwater drilling system, using a tethered Remotely Operated Vehicle (ROV). The results are seismic data with a significantly lower background noise level, a lower threshold for recognition of small events, well-oriented (and useable) horizontal seismograms, and a much longer deployment capability. We present new data from these instruments and compare them with data from two types of traditional instruments deployed from a surface ship. These contemporaneous deployments were conducted during 1996 and 1997 in Monterey Bay.

Source levels of fin whale 20 Hz pulses measured in the Northeast Pacific Ocean

“…2). Each OBS was positioned using an ROV equipped with an ultra-short baseline (USBL) beacon with an estimated absolute horizontal position error of $10 m. Seven of the seismometers were short-period instruments sampled at 128 Hz with a flat frequency response between 1 and 90 Hz (Stakes et al, 1998). One of the instruments was a broadband seismometer with a flat frequency response from less than 1 Hz to approximately 50 Hz, sampled at either 50 or 100 Hz (Romanowicz et al, 2003).…”

Fin whale tracks recorded by a seismic network on the Juan de Fuca Ridge, Northeast Pacific Ocean

“…The network aperture was 10 km along the axis and 6 km across the axis with a station spacing of 3 km (Wilcock et al, 2009). Seven of the stations were three-component MBARI/GEOSense seismometers (Stakes et al, 1998) with a flat response from 1 to 90 Hz that was sampled at 128 Hz. The eighth station was a three-component MBARI Guralp broadband seismometer (Romanowicz et al, 2006) with a flat response from 2.8 MHz to 50 Hz that was sampled at 50 and 100 Hz.…”

Termination of a 6 year ridge‐spreading event observed using a seafloor seismic network on the Endeavour Segment, Juan de Fuca Ridge

“…From August 2003 to October 2006, an eight‐station, 10 km aperture array was deployed along the central portion of the Endeavour Segment of the Juan de Fuca Ridge (Figure b). The array comprised seven short‐period three‐component corehole ocean‐bottom seismometers (OBS) deployed in horizontal drill holes or concrete “seismonuments” [ Stakes et al ., ] and one buried broadband Guralp CMG‐1T OBS [ Romanowicz et al ., ; Stakes et al ., ]. The short‐period sensor comprised three orthogonal Mark Products L‐28LB geophones that have a flat frequency response from 1 to 90 Hz and were sampled at 128 Hz.…”