Soil Coupling Of a Strong Motion, Ocean Bottom Seismometer (1979)

ABSTRACT A relatively low-cost seismic station for measuring strong earthquake-generated ground motion on the ocean bottom has recently been developed and tested. The dynamic soil-instrument interaction is a vital consideration in the development of a reliable, strong motion measurement system that can be placed directly on the seafloor without the use of elaborate and expensive installation techniques. Experimental and analytical studies were conducted to establish the influence of soil-coupling on the response of this sensor system. Forced vibration experiments were conducted to determine the dynamic characteristics of the system and to provide a basis for calibrating and verifying, an analytical model. The dynamic tests were analytically simulated and reasonable agreement with the experimental results was obtained. The analytical model was then used to further define the limits of applicability of this system and to assess its sensitivity to parameter variations. The results indicate that this sensor system will provide acceptable ground coupling in the frequency range of interest (0.1 to 10.0 Hz) and for peak acceleration levels up to 1.0 g in most cohesive-type soil conditions. INTRODUCTION The design of oil and gas production facilities in seismically active offshore areas such as the Outer Continental Shelf of Alaska creates a need for understanding site specific (ocean bottom) ground motions generated by severe earthquakes. Direct measurement of seafloor ground motion would provide a means for assessing the physical (amplitude, frequency content, etc.) and statistical seismic characteristics of an area however, no strong motion recordings have been obtained on the seafloor to date. These characteristics subsequently could impact the design practices for offshore platforms, pipelines, and production terminals. Such measurements will help identify earthquake sources and activity and help appraise and improve local attenuation equations. In addition, measurements will help resolve certain generic offshore earthquake concerns such as the effect of soil modulation and water overburden. This paper describes the recent development of a relatively low-cost system for measuring strong ground motion on the ocean bottom. The primary design objectives were to limit the system's cost, size, and weight to a level which makes feasible the deployment and recovery of large numbers of stations using relatively small ships. The system (pictured. in Fig. 1) consists of a retrievable electronic package housed in a glass sphere supported on-bottom by an expendable steel frame footing. A major developmental task was to demonstrate that the signal recorded by the sensor system is an accurate representation of the free-field ground motion within specified frequency and amplitude limits. An understanding of the soil instrument dynamic interaction is of critical importance in evaluating the instrument's performance. Considerable experimental and analytical effort has been devoted to assessing the soil coupling characteristics of the system and to developing the design of an instrument footing that will provide acceptable .ground coupling for the frequency range and acceleration levels of interest.