Broadband Seismometer

Advances Over Conventional Instruments

The seismic component of the Keck experiment uses broadband and short-period seismometers that have recently been developed by the Monterey Bay Aquarium Research Institute (MBARI) for deployment just below the seabed with a remotely operated vehicle (ROV) such as JASON 2, which is being used on the VISIONS ’05 expedition. These instruments represent an advance over conventional freefall instruments known as Ocean Bottom Seismometers because they improve coupling with the seafloor and decrease the noise generated by ocean currents.

Capturing Signals from Large Earthquakes

Although the short-period corehole seismometers will record local earthquakes, regional earthquakes, and other high-frequency signals such as whale songs, they will not detect the frequencies below 1 Hz that are typical of teleseismic arrivals, signals from large earthquakes around the world. In order to capture these low-frequency arrivals we use broadband seismometers (LINK to photos) that are sensitive to frequencies between 0.03 and 50 Hz.

Deploying the Broadband Seismometers

The MBARI broadband ocean bottom seismometer utilizes a sophisticated Guralp CMG-1T sensor that can accurately record ground motion at periods ranging from 360 s to 0.02 s. The sensor unit is housed in a titanium sphere that was specially designed for deep-water deployments. To insert the sensor below the seafloor, a two-foot length of 24-inch diameter plastic sewer pipe is first pushed into the soft sediments on the seafloor and a suction pump is then used to vacuum the sediments from inside. The sensor is placed carefully inside and small glass beads are then poured in to stabilize and bury the sensor. The sensor requires a lot of power and is attached by a 20-m cable to a large battery pack and data logger, which is replaced each year while the sensor is left in place.