MultibeamTheory

Figure Chapter 4 - -31: Position of the Echo in a Single Beam

The launch angle represents the angle at which an echo was received by the survey ship. The TOA of the echo represents the time it took the ping to travel to the position of the echo and back to the ship. Using these two values, you can compute the exact location of the echo in latitude and longitude and depth if you know the sound velocity characteristics of the ocean between the surface and the bottom.

Sound Velocity and Ray Tracing

The sound velocity characteristics of a typical ocean environment are very complex. Sound velocity in water depends on the salt content (or salinity) of the water, its pressure, and temperature. While salinity is a fairly stable quantity throughout the oceans, and changes in water pressure with depth follow well-understood laws, the temperature of ocean water can change from place to place, between different depths in the same place, and even vary from day to day. The most accurate bathymetry results only from a sonar system which has up-to-date information about the changes of the sound velocity as a function of depth (called the sound velocity profile) for the area in which it is operating.

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The sound velocity profile of an area of ocean can be approximated by a set of layers, each with a different sound velocity. Sound traveling through different layers moves at different speeds. In addition, when changing from one layer to another, the direction of the sound changes. When traveling from a region of high sound velocity to one of lower sound velocity, a sound wave bends toward the vertical. When traveling from low to high speed regions, it bends away from the vertical. If you start with the launch angle and TOA of an echo, you can follow the path an the echo must have taken from the bottom, changing its angle based on the sound velocity profile (see Figure Chapter 4 - -32). Within each layer, you subtract the amount of time it would take the echo to travel through the layer at the appropriate angle (twice— once down, once back) from the TOA. Eventually, you are going to run out of time, yielding the location of the true echo. This process is called ray tracing. Using the location of the true echo, you can get the true depth to the echo, and the bearing offset— the distance between it and the position directly below the sonar. Combining the bearing angle (see Figure Chapter 4 - -31) and the navigation information of the survey ship with the bearing offset will give the exact longitude and latitude of the echo position.

Figure Chapter 4 - -32: Ray Tracing to Find the Bottom

The SEA BEAM 2100 system uses a sound velocity profile with up to 29 layers of different sound velocity, which are described by 30 depth/sound velocity value pairs entered by the operator. Ray tracing is performed for each of the echoes reported by the BDI and/or WMT processing, yielding up the 151 echo depths and locations for each ping. This process makes the assumption that the velocity gradient is linear. Interested readers can refer to Acoustical Oceanography by Clarence S. Clay and Herman Medwin: John Wiley & Sons, 1977.

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Chapter 5 - Sidescan Sonar

Previous chapters in this document presented sonar as a tool for finding water depths using the time delays of acoustic echoes. Yet this is only one function of sonar and only one of the two ways sonar is employed in the SEA BEAM 2100 system. The other way in which the

SEA BEAM 2100 may be used is as a sidescan sonar. This section provides a description of sidescan sonar— how it works and what it is used for. It includes a detailed description of the SEA BEAM 2100 sidescan sonar system including an examination of the methods used for collecting and processing data.

Understanding Sidescan Sonar

Instead of measuring the depth to the ocean bottom, a sidescan sonar reveals information about sea floor composition by taking advantage of the different sound absorbing and reflecting characteristics of different materials. Some types of material, such as metals or recently extruded volcanic rock, are very efficient at reflecting acoustic pulses. Clay and silt, on the other hand, do not reflect sound well. Strong reflectors create strong echoes, while weak reflectors create weaker echoes. Knowing these characteristics, you can use the strength of acoustic returns to examine the composition of the sea floor. Reporting the strength of echoes is essentially what a sidescan sonar is designed to do.

Combining bottom-composition information provided by a sidescan sonar with the depth information from a range-finding sonar can be a powerful tool for examining the characteristics of an ocean bottom. Figure Chapter 5 - -1 shows an example of such a combination of data types. The seafloor canyon that was mapped in bathymetry in Figure 1-1 is shown mapped in sidescan. The bathymetry map clearly shows the shape of the canyon, while the sidescan indicates that the material forming the floor of the canyon, which is highly reflective, is different from that of the canyon walls and from the surrounding sea floor.

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Figure Chapter 5 - -1: Example of Sidescan Mapping

How Sidescan Sonar Works

Sidescan sonar employs much of the same hardware and processes as a conventional depthsounding sonar. Pulses are transmitted by a transmitter using a projector (or array of projectors), and hydrophones receive echoes of those pulses from the ocean floor and pass them to a receiver system. Where sidescan sonar differs from a depth-sounding system is in the way it processes these returns.

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To explain the processing of a sidescan sonar, recall the description of a simple, single-beam echo sounding system in Chapter 2, “Sonar Concepts“ (see Figure Chapter 5 - -2). To briefly review, the transmitter of the single-beam echo sounder produces an acoustic pulse, or ping, which is transferred into the water by the projector. The ping expands as a spherical wave from its source, striking the sea floor at the point closest to the projector source. The ping is then reflected in a return spherical wave, part of which is detected by the hydrophone. The receiver records the returned echo, which can be illustrated by a plot of amplitude as a function of time (see

Figure Chapter 5 - -3). The time between the transmission of the ping and the reception of its echo is used to compute the range to the sea floor.

Figure Chapter 5 - -2: Single-Beam Echo Sounding System

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Figure Chapter 5 - -3: Plot of Amplitude as a Function of Time

This simplified picture does not consider what happens to the transmitted pulse after it first strikes the bottom, because a single-beam echo sounder is only interested in the time between transmission and the earliest return echo. Yet to a sidescan sonar, the first returned echo only marks when things start to get interesting. Look at what happens to the transmitted pulse as it continues its spherical propagation.

Figure Chapter 5 - -4 shows the single-beam echo sounder schematic at a time shortly after the ping first intersected with the bottom. At this time, the first return echo is on its way back to the hydrophones. But the ping is still expanding, and in our schematic is intersecting the bottom at two points. These points also create echoes. They are weaker than the first return (remember, as the spherical pulse expands and propagates, it loses power), and they occur after it does (because it took the ping longer to get to them). It is easy to see that as the pulse front continues to propagate, it will produce a continuous series of weakening echoes in time. If the receiver continues to collect these echoes, it will see a amplitude versus time sequence similar to the one pictured in Figure Chapter 5 - -5, with a strong first bottom echo followed by a declining slope of continuous returns.

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Figure Chapter 5 - -4: Single-Beam Echo Sounder Schematic

Figure Chapter 5 - -5: Amplitude Versus Time Sequence

Thus far, this discussion has ignored the characteristics of the ocean bottom. Although unstated, the assumption to this point has been that the bottom is perfectly flat and perfectly uniform. Consider now what happens when some detail is added to the ocean bottom. Figure Chapter 5 - -6 redraws the schematic (shown in Figure Chapter 5 - -4) with a spherical pulse front some time after the first bottom echo. In this case, however, a box represents detail on the bottom. This box is more efficient at reflecting acoustic energy than the surrounding uniform bottom. Because of this, the energy of its echo is slightly higher than that of the surrounding bottom, which causes a small jump in the amplitude measured by the receiver at the time of its return. An observer looking at the amplitude versus time sequence in Figure Chapter 5 - -7 would be able to discern from this jump that some discrete feature exists on the bottom.

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Figure Chapter 5 - -6: Schematic with a Spherical Pulse Front with a Detailed Bottom

Figure Chapter 5 - -7: Amplitude Versus Time Sequence

In reality, of course, ocean bottoms can be very complex. They may be composed of many different components, all with different acoustic reflection characteristics. True amplitude versus time sequences observed by receivers are similarly complex.

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A Basic Sidescan Sonar

As a practical instrument, the simplistic sidescan sonar described above is not very useful. While it provides the times of echoes, it does not provide their direction. Returning to the example presented in Figure Chapter 5 - -6 and Figure Chapter 5 - -7, the sidescan sonar detects a bottom feature (the box). From the amplitude versus time plot in Figure Chapter 5 - -7, an observer can tell there is a highly reflective feature on the bottom. From the time difference between the first echo (which is presumed to be due to the bottom directly below the sonar system) and the echo of the reflective feature, the observer can compute the range to the feature from the sonar. But remember, this is all the information the observer has.

Figure Chapter 5 - -8 shows the bottom in this example redrawn from an overhead view. The “X” marks the point on the sea floor where the ping strikes first, creating the first return echo. After striking at the “X,” the spherical pulse front continues to propagate, intersecting the bottom in an expanding circle, eventually striking the bottom feature represented by the box. The observer can compute the range R between the “X” and the box, but otherwise does not know where on the ring of the expanding wave front the box is. In the real world, where sidescan sonars are used to locate objects such as rock formations or sunken vessels on the bottom, it is easy to see that this simple depth sounder amplitude information is too limited to be useful.

Figure Chapter 5 - -8: An Overhead View of the Bottom

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