Multibeam Water Column Imaging : Improved Wreck Least-Depth Determination

Water Column imaging, using hydrographic-grade multibeam echo sounders is now available commercially. Such a capability was originally developed primarily for fisheries imaging, but provides significant advantages for hydrographic data quality control. Most immediately apparent is the ability to view the near 2-D scattering field around wrecks or other man-made objects. Robust bottom tracking algorithms have to be optimized for likely seabed geometry. To avoid excessive outlier density, mistracking on spurious echoes is often discouraged by the use of gating or neighbour-proximity rules. Such methods, however, can fail spectacularly on abrupt non-continuous surfaces, commonly found over submerged manmade structures like wrecks or oil and gas infrastructure. These structures are often unsuspected and imaged only in a random pass of a regular survey. Based on prior experience, the least depth determination is often questionable from the real-time bottom tracking solutions. The main submerged superstructure is usually apparent but concern over protruding features can result in the need for bar or wire sweep investigations. Nevertheless, if the complete water-column trace from each beam is retained it is possible to review the full volume of scattering targets in the vicinity of the suspected object during post-mission feature examination. The interpretation of such imagery is prone to error unless the operator fully comprehends the imaging geometry. Particularly significant is the role of sidelobe echoes, both from the transmit and receive beams, producing secondary, ghost-like targets in the vicinity of point scattering features, such as masts or abrupt hull forms. This paper will review the imaging geometry and resulting scattering field that would be produced by such targets. Real examples of wrecks, imaged using EM3002 and EM710 sonars, are presented as case studies. Introduction Multibeam sonars have revolutionized the practice of hydrography. By providing nearcontinuous bathymetric coverage of the seafloor, they have increased the accuracy of and confidence in hydrographic charts. It was, however, quickly recognized that there were limitations to these systems, most notably in achieving 100% coverage (Miller et al., 1997) and in detecting objects as small as, or smaller than the beam footprint (Hughes Clarke, 1998, Hughes Clarke et al., 1998). Canadian Hydrographic Conference 2006 1 May, 2006 Multibeam Water Column Imaging 2 Hughes Clarke, Lamplugh and Czotter The most extreme target detection problem is that of discontinuous, small cross-section targets such as protruding mast and spars from shipwrecks. Despite their small size, these targets are disproportionately important in hydrography, as they often represent the minimum clearance required to be represented on the chart. Prior to the advent of the multibeam sonar there were four common approaches to establishing the minimum clearance: 1. tracking the apex of hyperbolae seen in broad-angle single beam soundings. 2. measuring shadow length from sidescan imagery. 3. diver investigations. 4. bar or wire sweeping. The first approach takes advantage of the broad angle of a single beam, and relies on the tip of the mast being a strong scatterer. Even if not directly below, the scattering point should show up within the beam cone and would produce a characteristic hyperbolic series of echoes from a moving vessel. By moving randomly over the suspected centre of the wreck, a minimum depth from the apex of the hyperbolae can be used as an estimate of the least-depth. The second approach assumes that the sidescan is of sufficiently high resolution and stable enough to detect the shadow cast by the mast. Using flat seafloor assumptions, the maximum protrusion can be estimated. However, even if a mast is seen, the horizontal position of the top of its top is not precisely known, as it is rarely vertical anymore. Thus the distance over which the shadow is cast cannot be properly estimated. The third and fourth approaches are the least ambiguous, but require the largest investment in ship-time and pose a risk to livelihood. Depending on the exposure and the depth, these may not be practically employable. All the methods of course, required prior knowledge of the wreck. Multibeam sonar bathymetry is an ideal method for locating wrecks, as the main superstructure will reliably show up. The first pass however, rarely provides reliable detections above the wreck. Because of the finite beam widths and spurious sidelobe artefacts, singular outliers around the wreck cannot be used with confidence. The quality of the soundings on the wreck depends primarily on three factors: • the physical apertures which control the beam widths, • the side-lobe suppression and • the bottom detection algorithm. To date, by far the best published results have been obtained by the RESON 8125 which, until recently, had the narrowest beamwidths available on the market (1.0° transmit by 0.5° receive), and thus had the best chance of completely occluding the beam on a narrow target like a mast. Even with these results however, one is faced with editing an unattributed cloud of solutions, many of which appear to define linear features, but need not be contextually related. Subjective decisions are required to accept or reject a singular solution. Statistical methods developed for cleaning soundings (e.g. Ware et al, 1991, Canadian Hydrographic Conference 2006 2 May, 2006 Multibeam Water Column Imaging 3 Hughes Clarke, Lamplugh and Czotter Eeg, 1995, Calder, 2003) all base their sounding confidence on an association with others on an assumed continuous surface. Such statistics will obviously not be appropriate here. Another way of validating outlying solutions is clearly required. Water-column imaging from the newest generation of multibeam sonars provides one plausible means of achieving this. Beam Occlusions Ultimately, for a single beam of a multibeam to unambiguously lock onto the top of a narrow cross-sectional target like a mast, the echo from that mast should be the strongest, and preferably the only one at that beam elevation. The reality however, is that the projected solid section of the mast is likely to be smaller than the main lobe of the beam. Therefore some of the main lobe and most of the surrounding sidelobes will completely miss the mast. Figure 1: simulation showing the full ensonification pattern of 1°, 2° and 4° beams on a wreck with typical dimension (30m depth, 50cm diameter mast with a beam steered at 40°). Note, the illustrated main lobe footprint is drawn to the first null ,NOT to the 3dB limits and thus appears significantly wider. If some of the energy in the main lobe, and/or significant energy from side-lobe contributions make it past the target, the energy would be able to scatter from the more distant surface, resulting in more than one echo in a time series. There is thus a case for Canadian Hydrographic Conference 2006 3 May, 2006 Multibeam Water Column Imaging 4 Hughes Clarke, Lamplugh and Czotter tracking multiple solutions for a given beam boresite. Examples given below illustrate how echoes from protruding targets such as masts clearly coexist with later echoes at the same apparent elevation angle. This is indicating that much of the projected energy is bypassing the intended target. Instrumentation Simultaneous, hydrographic-grade bottom tracking and multibeam water-column imaging is now offered from Kongsberg (EM3002, EM710, EM302) and RESON (7000 series). Herein, examples presented will be from the EM3002 and the EM710 multibeam sonars installed on the CCGS Otter Bay and the CCGS Matthew respectively. The EM3002 (KSM, 2004) is a single sector multibeam operating at ~ 300 kHz. The transmit beam width is 1.5°, and the receive beam width is 1.5° at broadside (growing with steering angle to be 3° at 60° off nadir). The EM3002 forms 164 physical beams. In the usual mode of beam forming, (termed, High Definition (HD)), the beams are spaced in an equi-angular geometry. The HD beamforming provides more bottom detection solutions (256) than physical beams. Such an approach, however, cannot be used for the water column imaging and thus only 164 radial channels are recorded for that purpose, irrespective of bottom detection approach. The data herein is collected using a roll – stabilized 130° sector with the physical beams spaced at ~ 0.8°. The EM710 (Kongsberg, 2005) model used was operated in two modes: • the 2.0° transmit, 2.0°* receive version (June 2005) and • the 0.5° transmit, 1.0°* receive version (May 2006) (*: the receive beam width again growing with steering angle) The exact beam widths depend on the centre frequency of the sector used, being slightly wider for the lower frequency sectors. The three sectors are at 97kHz (centre), 71kHz (port) and 83 kHz (starboard). As with the EM3002, more bottom detection solutions than physical beams can be achieved, but for water column imaging purposes there are 135 (270) receiver channels for the 2° (1°) version. In the HD mode used throughout, the physical beams are spaced in an equi-angular manner. Bottom detection cannot usually be achieved past ~ 70°, but the water column data can be acquired out to the full roll-stabilized +/-75°.Under this geometry, the beams are spaced at 1.1° (0.55°). One should be aware though, that the outermost receiver beams are approximately 4 times wider than the nadir receive beams at that steering angle. For the angular sectors presented (+/-65° and +/45°), the physical beam spacing was 0.96° (0.48)° and 0.7°, (0.35) °for the 2° (1°) beam widths, always corresponding to tighter than half the 3dB beamwidths. The EM3002 uses a 0.15ms pulse for all operations and the EM710 uses a 0.167 ms pulse for the water depths from which these examples are taken. The beam forming Canadian Hydrographic Conference 2006 4 May, 2006 Multibeam Water Column Imaging 5 Hughes Clarke, Lamplugh and Czotter cha