Gas Bubbles and the Acoustically Impenetrable, or Turbid, Character of Some Estuarine Sediments
Many shallow water, fine-grained sediments apparently contain appreciable quantities of interstitial gas bubbles produced by the biochemical degradation of organic matter. There have been relatively few direct observations of entrapped bubbles, but the presence of extensive gaseous sedimentary zones has frequently been inferred from high resolution sub-bottom profiling records. Frequently, large segments of many records made in estuaries and bays with high-frequency, low-energy profilers are confused and characterized by bands of strong diffuse reflection that mask the underlying features. Investigators have repeatedly attributed this anomalous acoustic behavior to the presence of interstitial gas bubbles, which produce excessive reverberation of sound within the sediment. Until recently, however, this hypothesis for the explanation of the acoustically impenetrable or ‘turbid’ character of sediments had apparently not been tested.
Acoustically ‘turbid sediments’ cover large areas of Chesapeake Bay. Along a cross-section parallel to the Bay Bridge near Annapolis, acoustically turbid zones alternate with zones where good sub-bottom records can be obtained. The sediments are quite similar in grain size, mineralogy, and water content. Determinations of the compressibility of sediment samples from the acoustically turbid zones, and from contiguous zones where good sub-bottom records were obtained, showed that the acoustically turbid sediments are several orders of magnitude more compressible than the acoustically clear sediments. The increased compressibility is a result of the presence of interstitial gas bubbles. X-ray examination of frozen cores confirmed this. Bubbles were scarce or absent in cores from acoustically clear areas. Although the acoustically turbid zones are of considerable horizontal extent, their vertical dimensions may be only 1 to 3 meters.
The cause of the acoustically turbid character of some other zones in Chesapeake Bay is buried shell beds (bars). The compressibility of sediments from these areas is not anomalously high. The two kinds of acoustically turbid zones can normally be distinguished by a careful examination of the sub-bottom profiling records.
High resolution sub-bottom profilers can be used to map the distribution of gas bubble zones, and for the selection of coring stations for gas analysis.
KeywordsSpherical Cavity Estuarine Sediment Core Segment Turbid Zone Continuous Seismic Profile
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