The normally-incident reflectivity of the sea floor at 12 kc and its correlation with physical and geological properties of naturally-occurring sediments

Abstract

The objective of this investigation was to measure bottom loss in normal incident reflection of pulses of twelve kcps sound and to study its geological significance. To this end a semi-automatic instrument system was developed which is capable of making continuous measurements of the peak pressure and the time integral of the square of the pressure of the sea floor echo, from a vessel underway. Observations were taken in both deep and shallow water areas in the Western North Atlantic. The early cruises were conducted in deep water to investigate the range and variability of bottom loss values. Geological control consisted mainly of a precise bathymetric record. The later cruises were conducted in shall ow water, in areas where the geology has been well studied previously by investigators using techniques of classical geology. In these latter cruises the acoustic measurements were correlated with a schedule of sediment dredging and underwater photography. Thirty-one thousand acoustic measurements were made. Median bottom loss values and standard deviations were computed and the results summarized in eleven hundred sets, each set corresponding to a location at sea. Seventy-seven sediment stations were occupied. A complete particle size analysis and a water content analysis were performed on these sediments to determine their size and mass characteristics . The size characteristics included the median grain size, the sorting coefficient, and the percentages of gravel, sand, silt, and clay. A sediment class name was determined from the gravel, sand, silt, and clay percentages according to the Shepard system of classification. The mass characteristics included porosity, bulk density, sound velocity, acoustic impedance, Rayleigh reflection coefficient, and theoretical bottom loss. The combined results show a good correlation between measurements of bottom loss and both mass and size characteristics of the sediment. The measured bottom loss increases as the porosity increases. The measured bottom loss also increases as the silt-clay percentage increases since the porosity of sediments generally increases as this fraction increases. It seems that the Rayleigh reflection coefficient can be used to predict acoustic bottom loss at normal incidence. Conversely, normally-incident bottom loss can be used under the assumption of a Rayleigh reflection process to determine the nature of the bottom sediment. The acoustical and geological results have been made available in tabulations, scatter diagrams, and as geographical plots. Except for the initial measurements, all operations, including the final displays, were accomplished through automatic digital processing machines.