A Sensitive Three-Axis Accelerometer System For Offshore Sediment Motion Studies
Authors: Niper Ernest D. 
Published: October 2008

A three-axis accelerometer system has been developed to measure low amplitude offshore sediment motion and long-term sediment shifts resulting in tilt of the implanted sensor. A significant property of this design allows extensive signal conditioning on the surface to provide for changes in sensitivity and DC shifts resulting from initial installation angles. Sensitivities tested have provided for sediment motions less than 2.5 mg. Tilt measurements on the order of 0.25 degrees are provided, and when correlated with the initial sensor alignment, the direction of the motions are resolved readily. The sensor package has been designed to operate continuously from low-voltage battery supplies and requires less than 5 w of power including the surface signal conditioning circuits.

Applications of the sensor package include evaluation of sea-floor stability, the study of marine seismic signatures and the interaction of wave motions with under consolidated sediments. An accelerometer package of this design was installed in the Gulf of Mexico in an area of unstable bottom sediment to correlate wave motion with sediment motion. The accelerometer system supplied continuous data for a period of several months in connection with a master data logging system. Data will be presented to show the correlation with a previous accelerometer design.

The design of this accelerometer system offers several distinct advantages over prior accelerometers. The new design operates entirely from battery power sources, supplies tilt information, provides sensitivity and scale factor adjustments from the surface, and offers outstanding long-term stability.

INTRODUCTION

In 1973, the U.S. Geological Survey initiated a project to study and map marine sediment instabilities in the Mississippi Delta. This project rapidly grew and encompassed a much wider area of the Gulf of Mexico by the end of its first year. A multidisciplinary data gathering effort stemming from the Mississippi Delta project1 became known as the Shallow Experiment to Assess Storm Waves Affecting the Bottom, or simply SEASWAB. The SEASWAB experiment demonstrated the feasibility of implanting sophisticated sensors in bottom sediment for extended periods. A continuation of this experiment, known as SEASWAB II,2 was initiated in the fall of 1976 to provide improved data from a variety of sediment imbedded sensors. The three-axis accelerometer is a direct outgrowth of this experiment.

During the planning phase of SEASWAB II, it was decided that, wherever possible, the instrumentation would be operated from float-charged batteries. This technique allowed maximum independence from the platform primary power system and provided continuous data during minor platform power outages. As SEASWAB II planning progressed, it became apparent that only a primary powered three-axis accelerometer would be available by the scheduled transducer installation date. Sandia Laboratories agreed to design and build a single-axis battery-powered accelerometer to provide partial backup for the three-axis system. This single-axis package was designed to provide vertical motion information to full-scale levels of ±50 mg. The early data collected in the SEASWAB II program indicated excellent agreement between the battery-powered accelerometer and the three-axis unit.