THE BREMEN OCEAN BOTTOM TILTMETER (OBT) - A TECHNICAL ARTICLE ON A NEW INSTRUMENT TO MONITOR DEEP SEA FLOOR DEFORMATION AND SEISMICITY LEVEL

Abstract

The Bremen ocean bottom tiltmeter is a new 6000m-depth deep sea instrument for autonomous observation of sea floor tilt with signal periods longer than 7.5s. The instrument also records vertical acceleration in the frequency range from DC to 1Hz. The tiltmeter has an Applied Geomechanics Inc. 756 wide angle biaxial bubble tilt sensor with a resolution of 1.0μ rad (0.2 arc second). A Kistler Corp. MEMS accelerometer of type Servo K-Beam 8330A2.5 with about 10−5m/s2 resolution is used for the acceleration measurements. An Oceanographic Embedded Systems AD24 24bit Sigma-Delta converter, which is controlled by a low-power Persistor Inc. embedded computer system of type CF 2, samples the data. The duration of tiltmeter operation is more than oneyear, which is controlled by the battery life. In our design the tiltmeter does not need active leveling devices, i.e., servo motors or other moving components to adjust sensors or frame. We designed the instrument for deployments by means of a remote operated vehicle. Since May 2005 the Bremen ocean bottom tiltmeter has recorded sea floor deformation and seismicity level in the Logatchev hydrothermal vent field, Mid-Atlantic Ridge. The tiltmeter is a part of the monitoring system of project ‘Logatchev Long-Term Environmental Monitoring,’ called LOLEM, of the German research program with the name ‘Schwerpunktprogramm 1144: Vom Mantel zum Ozean.’