GG710 Remote Sensing in Submarine Environments Sidescan Sonar

Transcription

1 GG710 Remote Sensing in Submarine Environments Sidescan Sonar Harold Edgerton, a professor of electrical engineering at the Massachusetts Institute of Technology, developed sidescan sonar technology for use in the civilian community in the early 1960 s. Prior to developing sidescan, Edgerton was known for his work with photographic systems and stopping time via pictures. He applied a similar approach to mapping the seafloor using flashes of sound to create a narrow image of the underlying terrain and then stacking these images in a continuous long picture as the sonar was towed along by a ship. Many of the names that we now associate with underwater exploration and mapping are people who trained with Edgerton during the early days of sidescan development; Marty Klein and Ed Curley were students of Edgerton who would later go on to found Klein Associates Inc. and EPC where you can purchase sidescan sonars and other ocean instrumentation. Sam Raymond founded Benthos, where most of us purchase our transponders and deep-ocean camera systems, and was also an Edgerton student. Sidescan sonars are among the simplest seafloor mapping tools that you can use. In their towed configuration they consist of a subsurface unit called a towfish, a cable for towing and transmitting information, and a topside system that is used to process data and transmit commands. Autonomous systems obviously don t need the cable commands are pre-programmed into the AUV s onboard computer guidance system however, a topside computer is still useful for downloading data and evaluating the performance of the sonar while at sea. The sidescan sonar towfish typically includes the sonar transducers, electronics and their pressure housings and flotation. Over the decades as the technology for creating flotation and electronics has improved and become more compact, sidescan towfish have evolved from cumbersome systems (Figure 1) into lightweight vehicles that can be launched from small platforms with limited handling capabilities (Figure 2). Figure 1 - Sidescan sonar being deployed from the NOAA ship Surveyor in Note the Danger: High Voltage sign. See

2 Figure 2 - Klein sonar system available for purchase on their website: Sidescan sonars work by projecting a narrow beam of sound to either side of a towfish and recording the strength and time of the returned echo. The strength of the echo depends on a number of parameters including the operating frequency of the sonar and the distance to the seafloor, the angle of incidence of the sound, the slope of the seafloor being ensonified (or insonified), and the type of substrate. Figure 3 - Schematic diagram of a sidescan sonar towed instrument ensonifying the seafloor (top) and the sidescan data record created (bottom). The intensity of sound reflected back from rocks, sediment and other features provides information on the distribution and characteristics of the seafloor morphology. Strong reflections (high backscatter) from boulders, gravel and positive topographic features facing the instrument are white and weak reflections (low backscatter) from finer sediments and shadows are in black. This figure is borrowed directly from NOAA s Ocean Exploration website: lark01/background/seafloormapping/media/sidesca n_schematic.html To produce the fan-shaped beam of sound shown in yellow in Figure 3, sidescan arrays are long in the alongtrack direction and narrow in the across-track direction (see Figure 2). In general, the longer the array, the narrower the along-track angular width of the sonar fan-shaped beam. Array length also varies as a function of operational frequency; lower frequency systems need to have longer arrays than higher frequency systems. Terminology The terms sidescan, backscatter and reflectivity are often used interchangeably, and incorrectly, when describing data collected by sidescan sonar. For

3 the record, these are the definitions of these terms: Sidescan is the correct term for the instrument that is used to collect the data. Backscatter is the measure of the amplitude of a reflected acoustic signal bouncing off the seafloor (or riverbed, etc.). Reflectivity refers to the reflective properties of the seabed. Thus, the strength of any given target depends on both the reflective property of the target and the extent to which it contributes to the backscattered signal. We assume that the surface is Lambertian in the sense that it reflects incoming sound waves equally in all directions: Figure 4 - Constant spatial distribution of radiance after ideal diffuse reflection at a Lambertian surface (left) versus diffuse reflection of a surface/substrate with a directional component. For more details see or your . Thanks to Jonathan W. for finding this information. From an engineering perspective, there are several systemic parameters that can be controlled to maximize sidescan sonar performance. Operating the port and starboard arrays at slightly different frequencies reduces the amount of crosstalk (signal from one side that is recorded as a mirror image on the opposite side) in the data. As demonstrated by numerous investigators (including the Vogt paper that is assigned and our own Dr. Appelgate), there is a trade-off between system resolution and areal coverage: Figure 5 - (Appelgate s own) Comparison showing the trade-off between resolution and coverage.

4 Experience has demonstrated that the optimal altitude for towing a sidescan sonar is about one-tenth of the swath width desired; therefore, tow at 10m altitude to produce a 100m wide swath and 100m altitude to produce a 1km wide swath. Obviously the sonar s frequency comes into place in maintaining the 10:1 swath_width-to-altitude ratio; the echoes produced by high-frequency systems will attenuate once the range to the seafloor is too great. Similarly, for lower frequency sonars, it is not possible to maintain the 10:1 ratio if the water depth is too shallow. As stated previously, sidescan sonars record echo strength (also called amplitude or magnitude) and time. Figure 6 shows an example ping collected by the MR1 sonar system. Figure 6 - One ping of data collected by the MR1 sidescan sonar. In this figure, from 0.0 until 6.5 seconds after the ping, there is very little sound being received by the transducers because the seafloor is so deep that the outgoing pulse has not had sufficient time to travel to the seafloor and echo back to the transducers. At ~6.5 seconds the transducers receive a very strong return that we refer to as the bottom detect to indicate that it is the first echo returning from the seafloor. It is widely assumed that this first return comes from directly under the sonar vehicle, but in fact in regions having significant topographic relief (mid-ocean ridges, continental slopes, fracture zones) this is often an inaccurate assumption. Note that with time the strength of the echo diminishes with time, rapidly at first, which results from the diminishing specular (mirror-like) return as a function of increasing angle from the towfish. Just after the 13 second mark there is another noticeable peak in the data stream this corresponds to the first multiple return; that is, the point when sound waves making two roundtrips from the vehicle to the seafloor (and one round trip from the vehicle to the sea surface), takes the same amount of time as one round trip from vehicle to seafloor with a higher angle of incidence. When the seafloor is relatively flat and the sonar is towed at an altitude approaching the total water depth (typical for the MR1 system), the first multiple will be observed at an ~60 angle from straight down. By building up an image of successive sidescan pings, it becomes possible to make an image of seafloor textures, which are typically projected onto a flat surface (Figure 7). With the resulting sidescan imagery, scientists can investigate processes that contribute to seafloor creation and modification such as volcanism, tectonism, sedimentation, mass wasting, etc.

5 Figure 7 - Swath of MR1 sidescan data showing round volcanic cones and tectonic lineations. The labels on the left side of the figure indicate where it is hard to interpret data collected by a sidescan sonar (near nadir and at the swath edge) due to distortions caused by the physics and shape of the outgoing pulses. One of the most common mistakes made by novices to sidescan sonar data interpretation is to assume that the magnitude of the return echo has a geological meaning; that is, that magnitudes in such-and-such a range correspond to volcanics and in another range correspond to coral reefs. Although significant effort has been expended to quantify the backscatter in order to systematically characterize seabed substrate, to date these experiments have not been very successful for sidescan sonars. Assigned Reading Johnson, H.P. and M. Helferty, 1990, The Geological Interpretation of Side-Scan Sonar, Rev. of Geophys., 28, pp Vogt, P.R. and B.E. Tucholke, 1986, Imaging the Ocean Floor: History and State of the Art, in The Western North Atlantic Region, Decade of North American Geology vol. M., 5, Geo. Soc. of Am., Boulder, CO, pp