Ocean Ecology |
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A single-beam sounder calculates the depth below the ship using the time it takes a sound pulse to travel to the seafloor, reflect, and then return back to the transducer.  An echogram. The backscatter of the sound pulse can also be analyzed to provide information regarding the nature of the seafloor (e.g., roughness, hardness). Regardless of the size of the footprint of the acoustic beam (which is a function of depth and transducer beam angle), only a single depth value is obtained for each acoustic pulse (or 'ping'). In the case of backscatter analysis, often the returns from a number of ‘pings’ are averaged to derive a value for the ‘roughness’ and ‘hardness’ of the seabed. The values obtained represent a single position below the ship, and thus single-beam surveys consist of a line of discrete points along the ship's track for which a number of parameters such as depth, hardness and roughness have been derived. The spacing of these points is a function of the depth, ship's speed, ping frequency, and number of pings averaged to generate a value.  Click image to enlarge. An echo from a sounder contains two major components - E1 (first echo) and E2 (second echo).  Click image to enlarge. Each echo is generated by a different type of interaction with the seafloor, and thus each carries different information about the seafloor. E1 is produced by scattering, and thus gives seafloor roughness, whereas E2 is produced by reflection, and provides information on seafloor hardness.  Click image to enlarge. A plot of E1 versus E2 can be used to classify different types of sea floor materials.  Click image to enlarge. (Information derived from Acoustic Techniques for Seabed Classification (2005) by J D Penrose, P J W Siwabessy, A Gavrilov, I Parnum, L J Hamilton, A Bickers, B Brooke, D A Ryan and P Kennedy - see link below.) Ocean Ecology uses a JFC-130 echo sounder to measure depth and bottom hardness (E2 component).  Graph from sounder showing bottom descrimination mode. Specifications
 Dual frequency display with bottom discrimination mode Ocean Ecology has been studying the relationship between bottom hardness and rugosity (a measure of how rugged the bottom is; this value is derived from the bathymetry of the site). An intriguing pattern of bottom hardness values has been observed. Marine ridge crests often had low bottom hardness values (suggestive of soft sediments), whereas marine valleys had high bottom hardness values (suggestive of hard sediments). This is reversed from the common pattern (e.g., ridge crests normally consist of bare rock, which should have high hardness values, whereas valleys accumulate sediments, and should have low hardness values). The explanation for this phenomenon is as follows. Bottom hardness is measured using the second, or E2, echo returning to the sounder. The first, or E1, echo is a direct reflection from the seabed, whereas the second echo has a transducer/bottom/sea surface/bottom/transducer path (i.e., it has interacted once with the sea surface and twice with the bottom). The double bottom interaction of the second echo causes it to be strongly affected by the acoustic bottom hardness; however, seafloor roughness has a secondary, and not always negligible, effect. Thus, the E2 signal is often referred to as “hardness”, implying a measure of mechanical hardness, but in reality, it is a measure of acoustic reflectivity with some unknown relationship to seabed conditions. Since the reflection of the E2 signal from the seafloor can be affected by both the acoustic hardness and the acoustic roughness of the seafloor, a hard rough surface can scatter so much energy that it appears acoustically softer than expected. In deep sea applications "Reflection from a very rough rocky bottom may appear to be less than that from a muddy sediment" (Brekhovskikh, L. and Lysanov, Y. 1982. Fundamentals of ocean acoustics. Ed. L. Felsen. Springer Series in Electrophysics Volume 8. Springer-Verlag, Berlin). We are currently using data from a number of sites to model the relationship between rugosity (a measure of seafloor roughness) and the E2 signal (hardness). |
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