Echo Sounder Theory
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.
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.
An echo from a sounder contains two major components – E1 (first echo) and E2 (second echo).
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.
A plot of E1 versus E2 can be used to classify different types of sea floor materials.
(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.)
Ocean Ecology used a JFC-130 single-beam echo sounder to measure depth and bottom hardness (E2 component).
This particular echo sounder had the following characteristics:
- had a dual frequency (50/200 kHz) transducer.
- operated at 1 kW power.
- a beam angle of 9 degrees at 50 kHz or 17 degrees at 200 kHz.
- a bottom discrimination mode that visually showed the E2 signal – a long bottom tail indicates a hard bottom (such as rock) while a short bottom tail indicates a soft bottom (mud and sands).
Using the data from our single-beam sounder, we created detailed bathymtric charts of the ocean floor. Our seafloor mapping system is described here. Sounding data were typically recorded every second and logged on a computer. Surveys were carried out in a grid pattern, with both shore-normal and shore-parallel transects. The survey resolution varied from 10 to 60 m. The recorded bathymetric data were then corrected for towfish depth and tidal height in ArcGIS. Tidal height values were generated with WXTide32, an XTides-based program, using the closest reference station to the survey site. The corrected data were then exported from ArcGIS, and used to generate a depth grid in Surfer (a more specialized gridding and 3D surface mapping program than ArcGIS). The depth grids were then imported back into ArcGIS, where contour plots and 3D maps were generated. An example of 3D bathymetry generated for Lucy Island is shown below. The 3D model can be included in a pdf file as well (download and view in Adobe Acrobat to enable manipulation of the model).
Bottom Hardness and Rugosity
In one of our research projects, Ocean Ecology studyed 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 was 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 was 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 used data from a number of sites to model the relationship between rugosity (a measure of seafloor roughness) and the E2 signal (hardness).