INFLUENCE OF BOTTOM TRAWLING ON THE NORMAL- INCIDENCE REFLECTION COEFFICIENT

Transcription

1 INFLUENCE OF BOTTOM TRAWLING ON THE NORMAL- INCIDENCE REFLECTION COEFFICIENT P. A. van Walree a, M. A. Ainslie a, and J. Janmaat a a TNO, Oude Waalsdorperweg 63, P.O. Box 96864, 2509, JG The Hague, The Netherlands Contact author: Paul van Walree, the Netherlands Organisation for Applied Scientific Research TNO, Underwater Technology Group, Oude Waalsdorperweg 63, P.O. Box 96864, 2509 JG The Hague, The Netherlands. Facsimile: paul.vanwalree@tno.nl Abstract: A North Sea area characterized by muddy, sandy, and gravelly sediments is surveyed with a single-beam echo sounder operating at 12, 38, and 200 khz. Closely spaced survey legs and a high repetition rate of five sounder pings per second allow the production of echo energy maps with a high spatial resolution. These energy maps reveal a series of string-like features, which are believed to be due to beam trawling activities. The trawler plough marks feature a dramatic increase in the echo energy, up to 12 db above that of their surroundings. This effect, the strength of which is frequency dependent, may be used to monitor fishing activities and demersal habitats. The present paper illustrates the marked acoustic signature of the furrows and discusses several candidate responsible physical mechanisms. Disturbance of the mass density gradient in the benthic layer is considered the most likely candidate. Keywords: Echo sounder, bottom trawling, reflection coefficient 1. INTRODUCTION Research on the physical and biological impact of trawler fisheries is an active field in marine sciences [1,2]. In addition to diver or camera observations, remote sensing with acoustic means has revealed morphological signatures in the form of furrows. Scars left by the trawl doors have a particularly strong signature [2,3]. Such morphological features are observed with sidescan or multibeam sonar systems, which measure backscatter at oblique incidence. The present paper adds another acoustic signature of the plough marks. A 909

2 strong impact is observed on the reflected energy of single-beam echo sounders. It is suggested that the effect is not due to morphological features, but to some alteration of the seafloor along the full width of the plough marks. 2. ACOUSTIC SURVEY Sea trials were conducted in the North Sea Cleaver Bank area in November A dense pattern of east-west tracks was sailed for a survey with a multibeam echo sounder. Single-beam echo sounders were switched on in parallel to collect additional data at normal incidence, and at three frequencies. This paper considers echo energies of the single-beam sounders, operated at 12, 38, and 200 khz. Pertinent sounder parameters are tabulated in Table 1. Frequency Transducer model Beam width Footprint diam. at 40-m depth Pulse length Ping rate 12 khz Simrad m 1024 s 5 s khz Simrad m 256 s 5 s khz Simrad E 7 5 m 256 s 5 s -1 Table 1: Echo sounder parameters. During the survey, sounder data were digitally stored after baseband conversion. Echo traces were corrected for propagation losses in the form of spherical spreading and absorption. Bottom returns were detected and cropped from the echo traces. The selected time window extends over ten pulse lengths around the peak echo value. Subsequently the echo energy is computed as the integral of the intensity envelope over the selected time window. No further processing is performed for the work presented in this paper. Results are shown only for the energy reflection coefficient. The top graph in Fig. 1 shows a geological map produced in 1987 [4] together with the legs of the 2004 survey. The map, which is based on a sparse sampling programme, roughly distinguishes sandy Mud (left side), Sand (right side), and sandy Gravel (top). The designations sandy Mud, Sand, and sandy Gravel are adopted from Folk [5]. Ref. [6] gives a more detailed description of the Cleaver Bank geology and sedimentology. 3. REFLECTED ENERGY AND PLOUGH MARKS The close spacing of the survey legs, and the large number of echoes ( 7 10 at each frequency), support a map of reflected energy with full seafloor coverage. To this end the relevant section of the seafloor is divided into bins measuring m. For each bin the mean echo energy is calculated and subsequently converted to a decibel scale. The bottom three panels of Fig. 1 show colour maps of the measured echo energy thus produced. In the absence of calibrated sounders the energies are plotted on a relative scale

3 Fig. 1: Geological map (top); shaded relief map produced with the multibeam echo sounder (greyscale); single-beam echo energies at three frequencies. 911

4 15 12 khz 10 5 Echo energy (linear scale) i ii iii iv v 38 khz 200 khz Longitude (degrees East) Fig. 2: Echo energy along a segment of the southernmost leg in Fig. 1. At each frequency the energy is normalized such that the mean value between and degrees East amounts to unity. A 7-ping moving average filter is applied to the data. Fig.3: Illustration of beam trawling. Image courtesy of 912

5 Offsets are adjusted so as to render approximately the same colours for the mud trench at the left side of the maps. A global comparison of the three maps shows that the energy reflection coefficient is frequency dependent. The contrast between the sandy Mud and sandy Gravel areas increases with the frequency. There are also significant differences between the rendering of the Sand and sandy Gravel areas. The contrast is strong at 38 and 200 khz, but almost absent at 12 khz. A first conclusion is that the measurements are not accounted for by the Rayleigh reflection coefficient of a discrete interface, which is independent of frequency. Further inspection of the energy maps reveals the presence of string-like features running in arbitrary directions. These features have a strong acoustic signature at 38 and 200 khz, whereas they are barely visible at 12 khz. It is hypothesized that they correspond to plough marks caused by fishing gear dragged over the seafloor. Beam trawlers were witnessed during the November 2004 acoustic survey, and are known to operate regularly in this part of the North Sea. The hypothesis is corroborated by a morphological map (not shown) produced with the multibeam echo sounder. A close-up examination of this map reveals a crisscross of plough marks normally associated with trawling, precisely matching the coordinates of the string-like features. Further evidence for the beam trawler hypothesis is provided by Fig. 2. This figure zooms in on the echo energy along a short section of the southernmost leg, traversed by at least five string-like features in Fig. 1. At 38 and 200 khz the energy rises significantly above that of the surroundings. In some furrows the excess energy amounts to 12 db. Furthermore the energy peaks tend to occur pairwise, with a spacing of approximately 30 m. These observations point to involvement of beam trawlers such as depicted in Fig. 3. The distance between the two trawls depends on the type of fishing ship, but a value of 30 m is not uncommon. The width of the individual trawls also varies, up to a maximum opening of 12 m. As the water depth in the Sand regions is about 40 m, the sounder footprints (see Table 1) are comparable to the width of the plough marks. The plough marks have a strong acoustic signature in the Sand and sandy Gravel regions, but are absent in the sandy Mud trench at the west part of the survey. The trench is either unfished, or it is fished but plough marks in mud have no detectable signature. 4. MECHANISMS UNDERLYING THE ENERGY INCREASE To inquire into the origin of the enhanced reflection coefficient, we first conduct a literature survey on the impact of trawls on the seafloor. Subsequently we examine echo envelopes captured inside and outside the plough marks, and subsequently we discuss candidate responsible mechanisms Literature survey Research on the physical effect of beam trawls on the seafloor has shown that the penetration depth is typically of order 5 cm in sandy sediments [1,7]. A record of beam trawl disturbance in another part of the North Sea [8] shows that the surface roughness in a fishing sector characterized by sand waves and some ripples was reduced, but that the particle size distribution was not altered. Similar conclusions were obtained for an experiment in the Adriatic Sea [9]. A recent cruise in the Bering Sea [10] revealed mean 913

6 grain size changes of only ~0.02 phi units before and after experimental trawling on a sandy seafloor. A literature search on inspection of bottom trawls with single-beam echo sounders did not disclose direct measurements of the echo energy. There are, however, a number of indirect observations obtained with the RoxAnn bottom classification system [11]. These data concern the RoxAnn parameter E2, which is often associated with the hardness of the seabed and considered a measure of the reflection coefficient. Kaiser and Spencer [8] and Humborstad et al. [12] report a decrease of E2 after trawling, whereas Tuck et al. [3] and Coggan et al. [13] do not measure a difference. Gordon et al. [2] mention a significant increase of E2 after trawling. These contradictory observations may suggest a complex influence of bottom trawling on the seafloor reflectivity, depending perhaps on the type of fishing gear, type of sediment, geological area, etc. However, the interpretation of E2 as a measure of the hardness is questioned by Hamilton et al. [14] and a meaningful comparison with our direct measurements is not possible Echo envelopes A selection of 38-kHz echo signals is displayed in Fig. 4. The figure shows the envelope of five echoes selected in and outside enhanced energy regions. It reveals a drastic increase of the echo amplitude at positions ii and iv. As it appears the excess energy is carried by the initial bottom return; the energy in the echo tail is not noticeably altered. Similar effects are observed at 200 khz, but not at 12 khz (not shown). The dramatic increase of the initial bottom return at the higher frequencies suggests that the energy rise is due to a reflectivity change at the very water-sediment interface, rather than an increase of scattering contributions to the total echo energy. Fig. 4: Selection of echo envelopes at five positions (see the roman numerals in Fig. 2) for the 38 khz sounder. The envelopes share the same normalization factor (black curves) or are individually scaled to an amplitude of 0.95 (grey curves) Candidate mechanisms There are several mechanisms that pop up upon considering possible causes of the strong acoustic signature of the plough marks. In the absence of ground truthing, the proposed mechanisms are discussed in terms of their (im)plausibility. Mechanism 1. The trawl removes a fine-grained surficial sediment layer and exposes a sublayer with larger grains and an increased reflectivity. However, it is unlikely that a surficial layer with a thickness of only a few cm (the penetration depth of a trawl) 914

7 stretches over a large area. Moreover, bottom grabs collected in 2000, sampling the upper 20 cm of the sediment, often yielded only sand in the sandy area of the present survey [6]. The proposed mechanism is also in conflict with the literature reporting that trawling does not noticeably alter the grain size. Mechanism 2. The trawl removes ripples and other unevennesses, thereby flattening the surface, which develops from a diffuse towards a specular reflector. Indeed, literature confirms that such flattening may take place. However, for a spherically expanding sonar beam it is not expected that a smooth reflector delivers a higher integrated energy than a diffuse reflector, except for a very narrow beam or an exceedingly rough surface. The shaded relief panel in Fig. 1 shows that the Sand area features a smooth seafloor, at least on a macroscopic scale, whereas the plough marks stand out acoustically. Mechanism 3. The plough marks are concave, with a radius of curvature of the same order as the water depth. With such a plough mark shape, the reflected sound has a focus in the vicinity of the transducer. Apart from the coincidental shape, a counterargument is that one would also expect a decrease in echo energy at the convex boundaries of the concave furrows, which is not observed. Mechanism 4. In-situ measurements of sediment density and porosity have revealed the presence of a thin transition layer between the water and the bulk sediment [15]. The thickness of this layer is of order 1 cm. The associated impedance gradient acts as an antireflection coating at high frequencies, when the product of wave number and layer thickness is of order unity or higher. Supporting evidence for the importance of the impedance gradient for the normal-incidence reflection coefficient is found in [16]. It is hypothesized that trawling compresses the transition layer, increasing the impedance gradient. Alternatively, upwhirling surface particles may be carried away by currents to deposit elsewhere, promoting the more tightly packed subsurface layer to the new top layer. Either way, destruction of the impedance gradient can thus account for the increased acoustic mismatch. The explicit frequency dependence of this mechanism would also explain why the plough marks hardly show at 12 khz. Mechanism 5. Removal of benthic fauna. Fish are known to scatter sound and a diminution of their number may either increase or decrease the reflection coefficient. Systematic differences of order 10 db, however, are not expected. 5. CONCLUSIONS Seafloor maps of reflected echo sounder energies revealed string-like features, which are in all likelihood due to beam trawling activities. The trawler plough marks feature a strong increase in echo energy at sound frequencies of 38 and 200 khz. At 12 khz the effect is weak. Of the possible causes considered, disturbance of the mass density gradient in the benthic layer is considered the most likely candidate. ACKNOWLEDGEMENTS This work was sponsored by the Defence Research and Development Department of the Netherlands Ministry of Defence. The authors further thank Chris Mesdag for producing the shaded relief map. 915

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