Geohazard Surveying in the Yellow Sea

ABSTRACT It was expected that the Yellow Sea would have a complex seafloor geology posing possible geohazards to offshore engineering projects. Two cruises were undertaken in Nov. 1983 and July 1984 to test a large suite of geological investigation tools (primarily seismic) and to collect the first high resolution seismic data in the area. The data collected identified a large number of geohazards and indicated which seismic systems were best suited for data collection in the area. INTRODUCTION The Yellow Sea between the People's Republic of China and Korea has experienced alternating periods of intense sedimentation and erosion as its level rose and fell. The prime contributors to both of these processes have been the Huangho (Yellow) and the Chankiang (Yangtze) Rivers which have deposited tons of sediment during high water or carved deep channels when the sea level was low. The result is a seafloor criss-crossed with filled erosional channels, sediment settling faults, methane deposits, etc. As well as being of great interest to the geologist and oceanographer, these conditions can pose severe problems for the offshore engineer. The level of activity in the Yellow Sea has recently increased with the search for oil and gas. This search brings with it the need for reliable seafloor information to enable accurate predictions of safety for jack-up drilling rigs, to predict anchor holding capabilities, and to identify shallow gas or fault zones. If large reserves of oil or gas are discovered, additional information will be required for production platform and pipeline design and positioning. In 1983 and 1984, two cruises were undertaken by the Institute of Ocean logy based in Qing dao, P.R.C. The Institute was assisted during these cruises by scientists from Woods Hole Oceanographic Institute (W.H.O.I.) and engineers from Ferranti O.R.E., Inc. The purpose was to test a large suite of geological sampling equipment as well as to collect physical oceanographic data. These cruises, which were the first of their kind ever undertaken in the Yellow Sea, produced a quantity of high resolution seismic data as well as 'valuable information on sediment transport processes. This paper will deal primarily with the techniques employed and the data collected by the various seismic systems. STATEMENT OF INTENT The intent of the cruise was to collect data which would enable the Institute of Ocean logy and W.H.O.I. scientists to study the forces which were responsible for the present seafloor geology and predict present as well as future geologic trends. To accomplish this, a comprehensive view of the bottom and sub bottom topology and geology was required.