D.C. Mosher, Geological Survey of Canada - Atlantic; R.G. Currie, Geological Survey of Canada - Pacific; D. Sullivan, Environment Canada Ocean Disposal Control Program, North Vancouver, BC

Ocean disposal of approved material (dredges or excavated material, organic material of natural origin) at designated sites is regulated by Environment Canada. One of the most active sites on the west coast of Canada is just outside Vancouver Harbour, off Point Grey in the central Strait of Georgia (Figure 1). Environment Canada requires that all vessels proceeding to the site report their positions to Vessel Traffic Management and ensure that they are within the site boundaries before commencing disposal activities. Water depths at the site (210-250 m) present a challenge for conducting monitoring surveys of the seafloor to determine if material reaches the bottom within the site boundaries. Earlier papers by the Geological Survey of Canada reported evidence of dumping well outside the prescribed area (Hart, 1992; Collins, 1994), but dating of this activity was not possible. The use of repetitive sidescan sonar mosaicing of the ocean floor, combined with video inspection using a remotely operated vehicle (ROV) are two monitoring tools tested in this trial.

Figure 1: Location diagram for Point Grey Disposal Site in Strait of Georgia, off Vancouver, British Columbia, Canada.

The Point Grey ocean disposal site was officially designated as a disposal area by the Canadian Ministry of Transport in 1968, although dumping has been occurring in the region since 1938. The area is a circle with a radius of one nautical mile (1.852 km) centred about 49°: 15.4'N and 123°: 22.1'W (10.78 km²). Most material dumped at Point Grey is dredged during routine maintenance projects at forest industry sites on the Fraser River and channel maintenance of the river for navigational purposes. The other main component is clean excavation material from the British Columbia Lower Mainland. During the period of 1989-1993, 2.8 million Tonnes (T) of material were disposed of at Point Grey. Approximately 9.0 million T have been dumped since its designation. Material proposed for ocean disposal is tested prior to loading on barges, and must meet chemical screening limits to be approved for open ocean disposal. Recent geochemical studies by the Geological Survey of Canada and Environment Canada have shown no correlation between anomalously high metal concentrations within the sediment and the location of dumping.

At the request of Environment Canada, the Geological Survey of Canada conducted two sidescan sonar surveys of the Point Grey disposal site; in September and in October, 1996. The intent was to determine if sufficient seafloor detail could be discerned using this technology to image disposal activity that had occurred during this one month period. In addition, if digital maps could be produced, then a quantifiable "difference" map could be produced that might highlight recent disposal activity. Backscatter information from the sidescan sonar data can be related to the spoils materials because detailed records of disposal are maintained by Environment Canada. Selected areas of the site were investigated in June, 1997, using a tethered ROV equipped with video cameras.

Sidescan sonar is an acoustic technique for mapping the seafloor. It uses a repetitive process of emitting a fan-shaped, or swath acoustic emission (narrow in the fore and aft plane, but very wide in the vertical plane) from an underwater towbody. A trace of reflected energy versus time for each emission is plotted (see Fig. 2). The frequency of these emissions can vary from 6.6 kHz to 500 kHz; in the case of this survey 120 kHz was used.

Figure 2. Conceptual drawing of the operation of sidescan sonar. It shows how seafloor topography creates changes in the amount of acoustic energy reflected back to the sidescan sonar towbody. This variation yields the light and dark areas that can be interpreted by the geologist. The data are recorded on paper charts and digitally recorded.

There are two sources of reflected energy that comprise a sidescan sonograph: (1) specular reflected energy, such as that reflected from seafloor surface relief, and (2) backscatter reflected energy resulting from physical property characteristics of the seafloor (e.g. hardness). The technique is analogous to shining a beam of light at a low angle along the ground. Relief in the ground surface or objects above the ground reflect light back and cast a shadow behind them, just as they reflect sound and cast a sound "shadow" behind them. Brighter objects reflect more light than darker ones, just as harder materials will reflect more acoustic energy. In terms of sidescan sonar, the resulting acoustic image is a swath of the seafloor to the port and starboard of the ship's track.

Mosaicing

As the vessel traveled around its survey grid, the sidescan sonar imaged a 400-m wide corridor of the seafloor. Combining each corridor image or strip into a large composite image is known as mosaicing. Mosaics can clearly show large structures, debris patterns or geological features that may be very difficult to interpret from individual sidescan strips. Until recently, mosaics were usually generated by physically placing each strip of paper from a survey line beside the record from an adjacent line, and ultimately photographing the resultant collage. If the lines overlapped, or the ship path was not exactly straight (which it rarely is) the paper had to be cut and warped in an elaborate and often inaccurate process. Today, mosaics can be generated digitally with the integration of sidescan sonar and navigation data. These digital mosaics can be far more useful because they are georeferenced and quantifiable.

Equipment

The two surveys were conducted with a Simrad Mesotech 992 dual frequency sidescan sonar system. Although this system operates at 120 and 330 kHz, the 120 kHz signal with a ping rate of 3.12 pulses/second and a 200 m range was selected for this project. The analogue signal produced by this system was digitized and logged on 8 mm digital tape using a MUSE digital data acquisition system. Ship navigation was obtained from a Northstar differential GPS receiver. The Canadian Coast Guard transmitter at Point Atkinson was used as the source of differential corrections. All sidescan and navigation data were time tagged to allow post survey compilation. The Canadian Scientific Submersible Facility (CSSF) remotely operated vehicle ROPOS, equipped with video cameras was used in the 1997 survey to obtain images of the ocean floor.

Survey methods

A grid of north-south survey lines was established to provide 100% coverage of the Point Grey disposal site. These lines were run at a nominal speed of 3.5 knots with the sidescan sonar sensor at approximately 30 metres above the seafloor. The height of the sensor above bottom is affected by ship's speed, sensor dynamics and currents. Sensor height above the seafloor is controlled by adjusting the amount of cable between the ship and sensor and this quantity is logged to allow the sensor to be positioned with respect to the ship. The first survey was conducted from CCGS Vector over the time period 1848 PDT September 23, 1996 to 1859 PDT September 24, 1996. The second survey was conducted from CCGS John P. Tully over the time period 2229 PDT October 28, 1996 to 1437 PDT October 29, 1996. The ROV work was conducted between 10-12 June, 1997, following NE/SW and NW/SE transect lines, developed after review of the sidescan images.

Navigation

Navigation data were transferred to a Unix work station and reviewed for obvious errors such as spikes, dropouts and unlikely speeds. This was done both under program control and graphically. Once the navigation data were clean they were smoothed with a simple low pass filter. The water depth, cable out and sensor height (obtained from sidescan processing described below) records were then merged with the navigation data. The sensor position was then calculated with respect to the ship using cable out, water depth, sensor height, appropriate offsets and under the assumptions that the sensor follows the ship and that simple geometry can be applied.

Sidescan sonar

Sidescan data were transferred to a Unix work station and reviewed for obvious errors. The first step was to pick and log the first arrival (i.e. bottom pick). This is required for both the slant range correction and sensor positioning as discussed above. Slant range correction reestablishes the true geometric relationships of objects on the seafloor, which are distorted on the raw image because of the beam angle of the sonar swath. The next step was to calculate a beam pattern correction which is used to smooth out angular variations in the transducer response. At this point, the sidescan data were merged with the navigation data, slant range and beam corrected. The adjacent lines were compared to assess navigational uncertainties and adjusted as appropriate. Once all adjustments were made, the lines were merged to create a digital, geo-referenced mosaic with a pixel size of 1 metre for this project.

Results

A mosaic of the Point Grey disposal site was produced for each sidescan sonar survey (Figs. 3 and 4). Evidence of abundant disposal activity can be seen on these mosaics. Subtraction of one image from another yields a "difference" mosaic (Fig. 5). Approximately 85,000 m3 (76 scow loads) of dredged and excavated material were disposed of at the Point Grey site during the month between the two sidescan sonar surveys. ROV observations show the disposed material to consist of wood waste spoils (Fig. 6), excavation material form construction sites (Fig. 7) and bundle wire from saw mills (Fig. 8). Dredged wood waste is a mix of river sediment and wood debris and is normally removed from the barges by front-end loaders leaving clearly defined "strings of pearls" on the mosaics. Excavation material is normally carried to the site in split-hull barges and disposal occurs in a matter of minutes leaving large white "splotches" on the sidescan mosaic. Bundle wire is acoustically highly reflective and leaves bright, well defined targets on the sidescan records.

[Click on an image thumbnail to view a larger image, notice]

Figure 3. Sidescan sonar mosaic of the Point Grey disposal site. Data were collected at the end of September, 1996. In this particular case the mosaic consists of a grid of 15 individual lines, each with a swath width of 400 m, with 50-100% overlap between lines. The seafloor is 100% covered in spoils. White represents highly reflective material. The area within the circle represents the designated disposal site.	Figure 4. Sidescan sonar mosaic of the Point Grey disposal site. Data were collected at the end of October, 1996. This mosaic consists of a grid of 13 individual lines, each with a swath width of 400 m, with 25-50% overlap between lines. Differences between this image and that of Figure 3 can clearly be seen -especially in the western region (see below). The area within the circle represents the designated disposal site.
Figure 5. A difference map of the sidescan sonar mosaics of the Point Grey disposal site. This map was generated by subtracting a normalized raster image of the September mosaic from a normalized raster image of the October mosaic. This result clearly identifies new spoils as white "spots" - most clearly notable in the western region of the disposal site. One trail in the eastern region is noticeable on this map, but is difficult to identify from the previous image. Some artifacts are present where navigation is in error or sidescan towfish positions are different between the two mosaics so the features do not completely "cancel" when producing the difference map (e.g. southeastern corner). The area within the circle represents the designated disposal site.	Figure 6. A ROV video capture image of wood waste on the seafloor of the Point Grey disposal site. The log is about 0.3 m in diameter.
Figure 7. A ROV video capture image of excavation material on the seafloor of the Point Grey disposal site. The pile of material in this image is about 1 m high.	Figure 8. A ROV video capture image of bundle wire. This material shows as highly reflective bright spots with large acoustic shadows on the sidescan images, thus it is easy to identify and locate. The wire is about 1 cm in diameter.

The "difference" mosaic (Fig. 5) should show the dumping that had occurred during the one month period. In order to create a difference map, accurate navigation and positioning of the towed sidescan sonar body are critical. Even differential GPS data require considerable scrutiny. In addition, the relative position of the towed body with respect to the ship is difficult to determine because it is at the end of a cable that is up to 700 m behind the ship and about 200 m below the sea surface. In order to discern quantifiable differences between two repetitive surveys, instrument and recorded gain levels between the two surveys should be identical. This too, is a difficult criteria to meet. Instrument gain levels can be set, but the intensity of reflection is dependent upon not only the properties of the target, but upon the angle of insonification and properties of the medium through which the sound wave travels (e.g. the temperature and density of the water). Post-processing normalization of the gains can minimize the differences.

In practice, there are a few obvious problems resulting in artifacts in the difference mosaic, but new disposal spoils are clearly visible and the area covered by new spoils can be calculated. Barges were requested during the period between surveys to dump in the western half of the prescribed area. It is obvious from Figure 5 that most of the new spoils are in the western half of the region, with the exception of one trail that is clearly in the eastern half of the zone. In addition to new features, old spoils that have been buried and are no longer obvious at the seafloor can be recognized, as they map in black. From repetitive surveying and quantifying these old features, it may be possible to calculate the rate of burial of features. This experiment has shown that the technique of repetitive sidescan mosaicing coupled with ROV video inspection is feasible and can be applied not only to disposal site monitoring but to any situation where changes in the seafloor with time need to be investigated, such as sediment transport studies or mine countermeasures.

Collins, W.T., 1994. Surficial Geology of the Sturgeon Bank and adjacent Strait of Georgia, British Columbia. Contract report to Geological Survey of Canada - Pacific, Pacific Geoscience Centre, Box 6000, Sidney, BC.

Dunn, C.E., Bama, R.G., and Spirito, W.A. 1996. Geochemistry of marine sediments from the Strait of Georgia, British Columbia. Geological Survey of Canada Open File 3052.

Hart, B.S., 1992. Side-scan sonar observations of Point Grey dump site, Strait of Georgia, British Columbia. GSC Current Research, 92-1A, p. 55-61.