A Nuclear Gage for In-Place Measurement of Sediment Density
Vincent C. Rose, James R. Ronsy · Offshore Technology Conference · 1971
ABSTRACT A nuclear gage for measuring sediment density in place is being developed under an Atomic Energy Commission contract. This gage will be added to the Deep Ocean Sediment Probe (DOSP), a multi-sensor system. The gage uses a gamma radiation isotope source in a concurrent transmission and backscatter arrangement. Direct correlation with undisturbed cores and acoustic data from the DOSP is possible. An electrical receptivity sensor is being added and will provide additional in place data for correlation. Results of testing both laboratory and prototype design configuration are presented. Transmission gages are usually superior to backscatter gages but require a two probe fork arrangement which is more difficult to force into sediments than a of paper. single probe backscatter type arrangement. In the design presented, the backscatter gage arrangement has been optimized to approach transmission results by special design of radiation shields around the isotope source and radiation detector and by the design of energy level discriminating circuits in the. instrumentation. This design limits the zone of material sensed to a well defined volume and avoids variations in readings resulting from changes in chemical composition of the sediment. INTRODUCTION The Deep Ocean Sediment Probe (DOSP) was designed and built by the University of Rhode Island under contract for the U.S. Navy Underwater Sound Laboratory in 1968. An electric motor drive system can insert four probe legs incrementally into sediment to a distance of five feet. The four legs are mounted at the corners of the base structure at a separation distance of four feet. A sparker source in one probe tip provides a signal which ts received by a hydrophone on one adjacent leg and by another hydrophone diagonally oppodiyr. Acoustic velocity and a measure of acoustic attenuation can be obtained. A thermistor is located on one probe tip. Another drive mechanism at the center of the DOSP slowly drives a thin walled corer. The DOSP has a design operating depth of 5000 ft. The principal motivation for the DOSP is to make in-situ sediment measurements. The core permits laboratory verification of in-situ measurements. It was desired to expand the number of sediment parameters measured to include the density and porosity of the sediment. After a review of the literature a nuclear gage was selected as the best method to obtain in-situ sediment density and moisture. This paper will describe the research and development being conducted on a. gamma ray probe for density measurements. Density of marine sediments isa basic property needed for the physical description of a sediment, and is a basic parameter in. equations used in submarine soil mechanics and underwater acoustics. Present methods of measurement require extracting a sample, physically transporting it to a laboratory and weighing it. The process necessarily alters the physical properties of the sediment to some extent and also presents a limited quantity of sample information for the time and expense involved in ocean coring operations. Acoustic velocity and electrical conductivity measurement probes for insertion into marine sediments have been developed and are in use.