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

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.