Development of A New Ocean Bottom Seismometer (Model IV of Kyoto University)

Abstract

Seventy percent of the surface of the earth is covered by ocean. It is necessary to observe earthquakes in the sea for the study of solid earth. The group of ocean-bottom seismological observation of Kyoto University has developed ocean bottom seismometers (OBS) of three types over the last several years. In the present study, we present our newly developed OBS which deploys free-fall and pop-up methods. This OBS is equipped with a three-component geophone (Mark Product L-22D) and a digital recorder. The recorder digitizes seismic data by a 16 bit analog-to-digital converter and writes the data on a magneto-optical disk with a capacity of 326 megabytes (MB). The electronic circuit boards are all housed in a 17-inch-diameter glass sphere. Thus, we will be able to obtain seismic data of high quality reading the seismographs from this high dynamic range seismometer installed on the sea bottom. The present OBS has an electrical corrosion mechanism permitting the release of the main OBS unit from its anchor at the sea bottom when it is retrieved. Recording tests on land and popping-up tests in the sea were performed, confirming that the present system is reliable enough to record seismic data under long-time submarine deployment.