A new seafloor electromagnetic station with an Overhauser magnetometer, a magnetotelluric variograph and an acoustic telemetry modem

A new type of Seafloor ElectroMagnetic Station (SFEMS) has been newly devel oped by adding a magnetotelluric (MT) variograph to its prototype built pre viously (Toh and Hamano, 1997). New SFEMS is able to conduct longterm elect romagnetic (EM) observations at the seafloor, which is one of the principal goals of the Ocean Hemisphere Project (OHP). Long-term seafloor EM observa tions enable us to probe into the deep Earth (both the mantle and the core) by improving the spatial coverage of the existing EM observation network. The SFEMS has been tested in three sea experiments to yield 3 components of the geomagnetic field, 2 horizontal components of the geoelectric field an d 2 components of tilts in addition to the absolute geomagnetic total force . The SEEMS is designed for measuring these EM signals at the seafloor cont inuously for as long as 2 yrs. The SFEMS mainly consists of the following three parts: (1) An Overhauser p roton precession magnetometer for the absolute measurements of the geomagne tic total force with a possible bias of less than 10 nT. (2) An MT variogra ph that measures the rest of the EM components and tilt. (3) An Acoustic Te lemetry Modem (ATM) that allows us to control/monitor the seafloor instrume nt as well as data transmission at the maximum rate of 1200 baud. Construction of seafloor EM observatories in regions where significant EM d ata have never been collected is now quite feasible by development of the S FEMS.