CORE HOLE SEISMOMETER DEVELOPMENT FOR LOW-NOISE SEISMIC DATA IN A LONG-TERM SEA-FLOOR OBSERVATORY

Longstanding problems unique to marine seismology (in contrast to land -based studies) include high levels of environmental noise, unpredicta ble instrument placement, short measurement duration, and limited numb ers of instruments. Traditional instruments are deployed and recovered from a surface ship with no capability to verify data acquisition unt il 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 t hese logistical problems is high background noise, sparse data recover y (compared to continental stations), and low fidelity data. In an eff ort to solve many of these problems, we have developed and deployed '' corehole seismometers'' (1-90 Hz) into small boreholes drilled by an u nderwater drilling system, using a tethered Remotely Operated Vehicle (ROV). The results are seismic data with a significantly lower backgro und noise level, a lower threshold for recognition of small events, we ll-oriented (and useable) horizontal seismograms, and a much longer de ployment capability. We present new data from these instruments and co mpare them with data from two types of traditional instruments deploye d from a surface ship. These contemporaneous deployments were conducte d during 1996 and 1997 in Monterey Bay.