Parasound echosounding: Comparison of analogue and digital echosounder records and physical properties of sediments from the Equatorial South Atlantic

Abstract

The new Parasound echosounding system (Krupp Atlas Elektronik GmbH, KAE) recently installed on the German research vessel Meteor proved to be a very useful tool in high-frequency sediment echography during two recent cruises in the Equatorial Atlantic. Conventional echosounder records provide only a qualitative picture of the structure of sedimentary sequences. In contrast, the main advantages of the Parasound system are its ability to change frequency and signal length of the incident signal, and its access to seismogram data via an interface for digital data acquisition. First, we present some examples of typical echosounder records from the African continental margin off Angola which illustrate the capabilities of the new instrument. These records facilitate lateral correlation of even small-scale sediment structures. Secondly, we compare synthetic seismograms, calculated from measured physical properties of sediment cores, with analogue echosounder records. This shows that strong changes in lithology due to turbidites or sand layers, as well as climatically controlled variations in carbonate content, can be correlated directly both with whole-core physical property logs and with seismic reflections in the Parasound echograms. Finally, we focus on digitized echosounder records obtained at coring stations, and compare these data with synthetic seismograms derived from the core logs. This procedure enables us to evaluate the relative importance of interference phenomena vis-a-vis lithological variations in the generation of reflection horizons in deep-sea sediment sequences. Depending on the wavelength of the source signal, small layers, either with sharp or gradational changes in lithology and physical properties, may be undetectable by seismic methods owing to destructive interference. In contrast, often even very weak reflections in echosounder records can be related to small variations in lithology, and thus to physical properties.