Body Search and Recovery Using Sonar Photos Mark W. Atherton Mark W. Atherton, Echoes and Images Image courtesy FBI Dive Team Images not for distribution. All photographs and drawings courtesy and Mark W. Atherton, Echoes and Images
The successful conclusion to a sonar search program is dependent on the following conditions: Sonar image resolution Operator experience Seabed conditions Bottom debris Surface reverberation Presence of the target in the search area
The task of acoustically searching for a body is complicated by two main reasons: 1: acoustic reflectivity 2: target size
What do I use? Side-scan, scanning sonar, side-scan, scanning sonar.?? The equipment selected for a specific search program is influenced by: size of the search area bottom topography water depth shoreline geometry wind, waves and current equipment availability
Scanning sonar and side-scan are complimentary tools! Side-scan is a large area search tool and is best utilized for large area searches where the seabed is relatively flat. Scanning sonar is the best tool to guide a diver to target and for area searched where the area is defined, and confined to approximately 1 x1 kilometer (approximately 5/8 X 5/8 mile) and around structures,.
Side-scan and Body Search Mark W. Atherton, Echoes and Images
Scanning Sonar and Body Search Mark W. Atherton, Echoes and Images Associated speedboat wreckage Body Target marker drag marks Target marker Data courtesy Gavin Cullimore, Kongsberg Mesotech Ltd.
The following questions must be resolved prior to commencing operations as they assist in estimating equipment, manpower, and overall program cost. 1. What is the sex and size of the victim, and what were they wearing? 2. What is the suspected cause of death? 3. Why is the search area selected? 4. What is the size of the search area? 5. What is the water depth? 6. What is the bottom topography? 7. Are any other targets or debris associated with incident (boat, aircraft etc.)? 8. Are there any unique local conditions or weather patterns? 9. What onsite logistical support is available?
Range Selection Body size and bottom conditions determine the optimum range scale used for the search. An adult male can be generally detected against a low acoustic reflective bottom (such as soft mud) at ranges to 25m (82 ), and sometimes farther. A small child requires the searcher to use a shorter-range scale. Always plan the search for the smallest dimension of the body (assuming it is intact) - the width of the victim s shoulders or waist.
Skeletal remains, or a body that has been dismembered, are extremely difficult to acoustically detect and interpret under any circumstances. Knowing the suspected cause of death assists in determining target size, and the optimum sonar range selection.
IMPORTANT In the event of a homicide, the body may be wrapped and weighted which offers a better acoustic target. The difficulty is a target may not appear body-like under these circumstances. Investigate all targets that are approximately the same size, and/or that display different acoustic characteristics to that of natural bottom.
Mark W. Atherton, Echoes and Images
Water Depth If published bathymetric information is not available, have someone measure the depth with a depth sounder (fathometer) prior to mobilizing. Record depth measurement at several locations within the search area. If the customer or another party supplies the depths, it is wise to get this critical information in writing.
Umbilical Requirements Photos Mark W. Atherton The length of umbilical needed is different between side-scan and scanning sonar.
How Long an Umbilical is Needed? Side-scan Sonar The cable needed for these speeds is 3 to 4 times longer than the maximum water depth. Scanning Sonar For scanning sonar used in a drop-mount or tripod deployed configuration, the amount of cable needs to exceed the maximum water depth by 25%.
Body detection is relatively straight forward on a benign bottom Mark W. Atherton, Echoes and Images The second tripod drop showed the target at approximately 26m (85') range. Body-like target Body-like target Third tripod drop at 16m (52.5') range snowmobile Lakebed depression Had the target been located in the lakebed depression, it is unlikely the body would have been detected.
The more complex the bottom, the harder it is to find a small target. Mark W. Atherton, Echoes and Images snowmobile On a complex bottom, the diver needs to be directed not only to targets of interest, but also to search the acoustic shadows created by the debris.
Before Starting the Search Ask about the following BEFORE showing up onsite: Current patterns and tide Wind and waves (are there daily patterns) Marine traffic Aquatic plants (milfoil, water hyacinths ) Floating or mid-water target (bogs, moored vessels, log booms, deployed nets) Maximum water depth Surface ice (can it be used as a sonar platform?)
Working from an Ice Platform Photos Mark W. Atherton
Surface Ice can also Refract the Sonar Signal The ribbon-like returns result from a combination of under ice reverberation and acoustic refraction. Note that range is limited in the shallow water due to the sound being refracted upward by the ice which is colder than the water below. body snowmobile Mark W. Atherton, Echoes and Images Data collected by Terrace Water Rescue Team using equipment donated by Kongsberg Mesotech Ltd.
Target Markers Have target markers, floats and rope pre-rigged and ready to deploy. Mark W. Atherton, Echoes and Images snowmobile
Target Markers Mark W. Atherton, Echoes and Images Crab traps are an option to use as target markers. The circular and rectangular traps shown can be detected at ranges of 50m (164') or more on a benign bottom. Crab traps can be used as their shape is unique and stands out against natural bottom targets.
Navigation Requirements Sonar data is of little value unless it is correlated with a known geographic coordinate or relative grid. In most cases, Differential Global Positioning System (DGPS) provides the accuracy to position side-scan or scanning sonar and allow the sonar operator to calculate area coverage and target coordinates.
Search Patterns The type of search pattern is primarily determined by target size, bottom conditions, size of the search area, and sonar system selection. For simplicity, use at least one of the following equipment arrangements: 1. side-scan sonar 2. scanning sonar tripod deployed 3. scanning sonar drop deployed 4. scanning sonar integrated with an observation class ROV
Sonar Search Patterns Before creating a survey grid, take the time to check the GPS receiver for position drift. This test is important when using non-survey grade GPS. Turn on the unit and give it time to acquire the satellites. This may take up to 10 minutes. Let the receiver sit in a fixed position for another 10 to 15 minutes and check the range of positioning errors over that time. When creating the survey grid, take this position ambiguity into account and reduce the distance between tripod drops by the fluctuation error.
Scanning Sonar Search Patterns Use the Trackplotter navigation program to enter survey lines. Follow the directions and create a 20m (66 ) grid. Alternatively the GPS may allow the user to alternatively enter waypoints (known positions). Although a tedious process, this too, allows a grid to be created. If there is any concern about being able to handle the navigation component of the search, hire a survey contractor. As the search expands there needs to be confidence that the body has not been missed due to navigation or positioning errors.
Scanning Sonar Search Patterns Mark W. Atherton, Echoes and Images Before planning the search pattern, turn on the GPS receiver and set it in a fixed position. Give it time to lock onto the satellites and sit an additional 10 minutes. Check for position drift over that time. Make sure to take that position ambiguity into account when creating the grid pattern by reducing the distance between drops by that value. Coverage when tripod is moved every 20m. Coverage when the tripod is moved every 40m Stagger the drops from one line to the next to prevent missing any section of the lake or seabed. For body searches, use a range scale of 25m (82') and create a 20m (66') survey grid pattern. Depending on bottom conditions, move the tripod either 20m or 40m (for a flat benign bottom). If the program permits, mark each drop position.
Side-scan Search Patterns Sonar range selection also depends upon whether other accident-associated debris is expected in close proximity to the victim(s). For example, in a boating accident (depending on bottom, surface conditions, and water depth) a 6m X 2m (20 X 6 5 ) boat will likely be detected at ranges to 125m (410 ) and perhaps farther with side-scan. Use a shorter range for better resolution, but be aware that towfish positioning and overlapping coverage becomes more critical as swath coverage decreases. When laying out the side-scan search pattern, ensure that adjacent lines provide overlapping coverage of the blind area beneath the towfish.
Side-scan Search Patterns Mark W. Atherton, Echoes and Images snowmobile The nadir (black area) must have overlapping coverage as targets in this area are acoustically compressed and may be missed.
Side-scan Search Patterns Areas of limited or no coverage. Mark W. Atherton, Echoes and Images With 25% overlap, areas of limited or no coverage. 25% overlap 75% overlap 90-degree grid pattern snowmobile Select a range where the transverse footprint of the side-scan beam is half the minimum target size at the outer edge of the range. High overlap coverage is used for very small target detection. Grid pattern used for linear target detection when target orientation is not known (overlap coverage may be 25-75% depending on target size). Drawing Mark W. Atherton, Echoes and Images If side-scan is used for the search you need at least a 75% overlap.
How to Deploy the Tripod There are two ways to deploy a tripod; one is the right way! Drawing Mark W. Atherton, Echoes and Images
Setting the Tripod on Bottom snowmobile Mark W. Atherton, Echoes and Images
Rigging the Umbilical GPS receiver wind direction Rig the sonar cable with floats as shown. The vessel does not need to be anchored as long as it is able to hold its position for the time snowmobile it takes to complete approximately three scans. Mount the GPS receiver directly above where the tripod and cable are deployed. Mark W. Atherton, Echoes and Images
Tripod Recovery IMPORTANT Depending on the bottom conditions, the tripod may not have to be recovered to surface at each drop. If it remains in the water, ensure that as the vessel moves to the next adjacent drop position the tripod or sonar umbilical are not pulled into the propeller.
ROV-deployed Scanning Sonar Observation-class ROVs have become increasingly popular with law enforcement agencies. Ease of operation, portability, and low maintenance costs make the smaller-sized ROVs an ideal search tool for water depths to 200-plus metres (656 ), depending upon the system used and water conditions.
ROV Sonar Search Patterns Sonar coverage issues arise when using an observationclass vehicle without an integrated, surface, sub-surface navigation system. When using a parallel line search pattern (where the vehicle turns to a reciprocal course at the end of its umbilical), the pilot is never absolutely sure of the ROV s position along the new search line relative to the previous. Also, the umbilical drags across bottom in a U- shape, which increases the chance of snagging the tether. A star pattern coverage does not have the same issues.
ROV Star Search Pattern The star pattern is best for calm water conditions when using a medium-sized observation-class ROV. Successful coverage with this pattern depends on keeping the support vessel anchored over the bottom reference marker. This is also the most effective search pattern to use when operating an ROV from land-fast ice.
Star Pattern Search Coverage Mark W. Atherton, Echoes and Images
Determining ROV Search Range It is unlikely that the maximum horizontal excursion distance can be achieved due to limits in ROV power and current affecting the umbilical. A reasonable distance to expect is 90% of the calculated value, or in this case, 141m X 0.9 = 127m (416'). 50 m (164') a c Use Pythagorean Theorem to calculate the maximum horizontal excursion distance. 150 m (492') Maximum horizontal excursion distance = 2 2 2 c - a = b b 2 b 141m (464') Cable floats Maximum horizontal excursion distance snowmobile Use a cinder block or other weight to mark the centre position of the search pattern; start line runs from the marker. Use Differential GPS to mark each location. Deploy a weighted marker with a surface float at each location when the star pattern is completed. Plot each location onto a chart. Tape measurement markers in metres or feet on the ROV umbilical so that the length of deployed cable is known for each excursion. Knowing this allows the distance of each horizontal excursion and total area coverage to be plotted. Mark W. Atherton, Echoes and Images
When a Star Search Pattern Doesn t Work A star pattern is not useful when searching steep slopes. A contour depth pattern, or running search lines perpendicular into the cliff face provides superior coverage. On an irregular shoreline, it is difficult to maintain a constant depth. Consider running the lines perpendicular into the shore. This is far more efficient than running the line from the beach out to deeper water. Searching perpendicular into the shore makes tether management easier as the umbilical does not drag down slope. It also provides better scanning sonar coverage when the vehicle is trimmed 5º bow-up
ROV Steep Slope Coverage An alternative umbilical management method is to clump-weight the umbilical (dashed red line) and use the vessel s echo sounder to monitor its height above bottom. When searching steep slopes, run the ROV on depth elevation or perpendicular to the incline and from deep to shallow. This minimizes the chance of umbilical fouling, and still allows sonar coverage. Positional control needs to be extremely accurate to maintain close spacing between adjacent lines. snowmobile Mark W. Atherton, Echoes and Images
Target Interpretation Look for a target that is approximately the same size as the victim, or is out of character with the surrounding geology/topography. An acoustic shadow is typically observed at ranges to 25m (82 ) from a body-sized target. All targets meeting the size criteria need to be investigated by some sort of visual imaging system (which further supports using divers or a combined sonar/rov package). More important, conduct an organized series of search patterns and ensure that overlapping sonar coverage is achieved, and the target is viewed from different acoustic perspectives.
Target Interpretation On a very soft bottom, it is possible for the body to be partially buried, particularly if dragging has occurred prior to commencing sonar operations. Although this situation is rare the San Bernardino Sheriff s Department reported a case where they surmised the victim, in the panic of not knowing which way was up, unfortunately dug himself into the very soft sediment. When the body was located, only one leg was exposed above the mud line.
Target Interpretation Mark W. Atherton, Echoes and Images
Body Recovery Using Divers Mark W. Atherton, Echoes and Images
Body Recovery Using Divers With a target identified and markers deployed, use the sonar software s measurement tools to determine the distance between the target and marker(s), the sonar to the target, and the sonar to the marker(s). This is especially important if the sonar team has worked ahead of the divers because markers can be pulled off location. With scanning sonar, one or two scans will confirm if the marker is still adjacent to the target; one or two passes are needed for side-scan confirmation. When side-scan is used exclusively, the divers need to complete a circlesearch from the marker. With scanning sonar (assuming the target is relatively close to the sonar tripod or marker), the diver(s) can descend to either the tripod or marker and conduct a circle search, or be quickly guided to target when hardwire or through-water communications are used.
Body Recovery Using Divers Mark W. Atherton, Echoes and Images Data courtesy Gavin Cullimore, Kongsberg Mesotech ltd. Target marker Body Divers at target marker 3 min. 57 seconds from leaving surface. Divers moving to target at 4 min 22 seconds from leaving surface. Divers recover body and leave bottom 8 min. 32 seconds from leaving surface. Even with a minor course correction, divers were directed to the target in 75m (246') water depth and confirmed sighting the body at 5 minutes and 38 seconds after leaving surface. In-water visibility was approximately 4m (13.1').
END Sonar for Body Search and Recovery