Short Period 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.

Recording Movements of the Crust
The main kind of seismometer used to monitor earthquakes on the seafloor is called a short-period seismometer (LINK to photo). A typical short-period seismometer can record frequencies from 1Hz to 50Hz. This type of seismometer uses a suspended mass and a magnet surrounded by a coil. An earthquake jiggles the spring-like coil around the magnet generating electricity that is translated and recorded. It serves the purpose of recording the various discrete types of waves (primary, secondary, and surface) generated by earth movement in the crust.

A Miniature Geophone
The MBARI short-period corehole seismometer uses a miniature geophone that can record ground shaking at frequencies as low as 1 Hz. The geophones are mounted in an aluminum cylinder that is housed in a titanium pressure vessel. To deploy the sensor in bare rock environments, an ROV drills a horizontal borehole in the basement rock and the seismic sensor is inserted snuggly into the hole. In sedimented environments, the sensor is mounted in a concrete block, which is buried several centimeters in soft sediments. The sensor connects to a compact logger and battery pack.