Ocean Bottom Seismology: History and Current Status

Abstract

Ocean bottom seismographs (OBS) have been in use for almost 50 years. The earliest instruments were used for short range explosion refraction experiments but were soon abandoned when techniques involving near surface sources and receivers were introduced. Development of OBS technology revived during the late 1950’s inspired by interest in the detection and identification of clandestine nuclear explosions and supported largely by the VELA-UNIFORM program of the United States government. Since that time, OBS research has expanded to include over 25 organizations in several countries. Generally each group has developed its own instrumentation and experimental procedures. Instruments fall into four classes: (1) self-contained, free-fall, pop-up; (2) acoustically telemetering to near surface for retransmission or recording; (3) electrically telemetering via cable to the surface; and (4) permanently linked to shore by cable. Most fall in class (1). Sensor configurations range from a single vertical component (or hydrophone) to three- components plus hydrophone; seismometer free period is generally one second or less; frequency response is generally limited to 100 Hz or less. Instrument masses range from 60 to 600 kg; recording is analog (AM or FM) continuous or digital (time windowed and/or triggered); dynamic range, data storage capacity and deployment time all cover a wide range of values. Most applications are for determination of velocity structure and/or local seismicity. There has been lesser interest in teleseismic, engineering or strong-motion applications. Major problem areas or areas where improvements are needed include bottom-coupling (signal distortion) and noise; data storage; system reliability, especially over long deployment times; ease of handling during launch and retrieval; and ease of shipboard checkout before and after deployment. OBS have recently been supplemented by ocean subbottom seismometers emplaced beneath the sediment in a bore-hole.