A Review Archaeological Geophysical Remote Surveying for the Seafloor

Transcription

1 A Review Archaeological Geophysical Remote Surveying for the Seafloor C. R. Bates, Sedimentary Systems Research Group, University of St Andrews Over the past 20 years the offshore survey community has witnessed remarkable advancements in the way in which the seafloor is remotely investigated. Progress has been stimulated by a desire both to investigate the most inaccessible parts and depths of the oceans and also the desire to obtain greater detail of conditions in the near shore. To meet these desires, geophysical researchers, contractors and equipment manufacturers have taken advantage of the advances in digital technology, increases in computer power and the accessibility of high power computing offered by tumbling prices. This presentation will review the basics of geophysical remote sensing and the recent developments with the view to the benefits offered to the archaeological community. The review begins with a discussion on navigation issues as precise navigation and positioning is key to the good use of the digital geophysical equipment. This is followed by descriptions of individual techniques for seafloor surveying. The basics of marine geophysical surveying can be found in standard text (Jones, 1999, Kearey and Brooks, 1991) Navigation Critical to the correct use of all modern marine geophysical survey equipment is a precise navigation input string. For archaeological purposes it is often vital that positions are known to less than 0.5m. Traditionally navigation in the near shore has been provided by trigonometry based on electromagnetic waves generated by shore-based transmitters. These have fallen into two types those based on pulse travel time between the survey vessel and shore transmitters whose geographical positions are know and those based on a phase comparison of the waves broadcast on a continuous basis from a fixed transmitter. Examples of these systems include Loran-C and Decca at low frequency (10-300kHz) and Trisponders and Micro-Fix at high frequency ( Mhz) (Jones, 1999). The higher frequency systems give high precision for positions in near shore situations but have limited range and are subject to interference if line of sight is not possible. In the early 1990 s the US Department of Defence launched a series of satellites in various orbits. These formed the basis of the Navstar (Navigation Satellite Timing and Ranging) Global Positioning System that today gives global coverage for navigation purposes (Hartl, 1989). With these satellites it is possible to obtain a horizontal position to about 50m on the Earth s surface. While this is sufficient for most deep ocean uses it is too coarse for archaeological research. For this it is necessary to use a system of Differential GPS which utilises an international network of reference sites connected by landlines to control stations. The information from the reference stations is re-broadcast by the Inmarsat Satellites to receivers on the vessel and compensations can then be made to obtain horizontal position to about 1m (Ashkenazi et al., 1997). For positions less than this it is necessary to set up a dedicated relay station near to the survey area and provide specific corrections to the survey ship. Seabed Imaging Two main types of image of the seabed are requested in most surveys, namely the bathymetry and an image or photograph-type view of the seafloor. Historically, two separate systems have been developed for each of these objectives. The echo-sounder has been used to measure bathymetry and the sidescan sonar to obtain images. The current trend in technology is to produce equipment that can meet both these functions within the same instrument. The survey instruments can be either deployed within towed instrument houses known as fish or on mounts that are permanently attached to the vessel. Both types of deployment have advantages and disadvantages for each type of instrument. In general, it is difficult to predict where a towed fish is in the water and thus there is always an increased location error associated with this type of survey mode. However, it is possible to place the fish nearer to the target, thus increasing the sensitivity of the instruments in the fish to the seafloor. On a fixed mount the exact location of the instruments are known but they will be more adversely effected by boat noise. Further data about the seafloor has been acquired through the use of other geophysical sensors such as magnetometers and electromagnetic instruments. Both of theses are usually mounted in a fish for tow behind the survey vessel. The historical development of instruments and some technical background will be briefly given here in order to assess the current survey instruments and practice.

2 Single Beam Echo-sounding A conventional single beam echo-sounder uses an acoustic pulse generator such as a piezo-electric crystal to produce energy in the water. The echo-sounder measures the time taken for this acoustic energy to travel from the generator to the seabed and back to the source. By knowing the velocity of the energy in water, the distance to the seabed can be calculated. The frequency range of acoustic energy is kHz for which there is little attenuation of the acoustic waves. The velocity of acoustic energy in water is dependant on the temperature, pressure and salinity of the water all of which can vary significantly in deep ocean work however, for shallow applications this is not usually a large issue. The beam pattern of an echo sounder can also result in a large footprint of the echo for deep ocean work but again at shallow depths this is less of a problem. By continually sending out signals and listening for the returned energy along a line track, a picture of the topographic seafloor variations is achieved along the track. Single beam echosounders are standard equipment on the majority of survey vessels today with their output displayed to the helmsman in either digital or graphical format. Much of the ocean topography has been mapped using conventional echo-sounders however considerable uncertainties exist in the data on line to line extrapolation or correlation. The extrapolation between lines often breaks down in complex topographic areas such as exist at many archaeological sites. For this reason techniques have been developed to look not below the survey vessel but to the side of the vessel. Two types of sonar area available for this, namely the sidescan sonar and the multibeam and swath-sounding sonar. A third sub-class of sonar is presently being developed based on amalgamating all of the preceding types of sonar but this will not be discussed. Sidescan Sonar The sidescan or side-looking sonar as its name suggests uses arrays of transducers to send out energy to the side of the instrument. A continuous image of the seafloor is achieved by recording sequential bursts of energy from each side of the instrument along a line track. In contrast to the echo-sounder, the transmission beam of the sidescan sonar is a narrow fore and aft beam that is wide in the transverse direction. This results in a large area of sea-floor that is ensonified at every transmission burst or ping. Typical frequencies range approximately kHz with pulse lengths of tens to hundreds of microseconds. Older sonar used a single frequency however the modern sonar is typically dual frequency with some of the latest developments being made with chirped multi-frequencies (Blondel and Murton, 1997). With sidescan sonar, the travel time of the pulses are recorded together with the amplitude of the returned signal as a time series. The recorded amplitudes are a measure of the acoustic backscatter and specula reflection from the sea floor which is in turn dependant on the sea floor type or roughness, and nature of objects on the sea floor. The sidescan sonar is typically towed at some distance behind the survey vessel which can introduce positional errors to the system. However, for many shallow (less than 10m deep) applications it is often possible to deploy the sonar as a pole-mount off the side of the vessel. Older sidescan systems consist of analogue recorders with real time data output to a printer. Today, most sidescan sonar acquisition is done with digital data recording to a digital storage medium that can be replayed as well as output in real time. Modern digital sidescan sonar record swaths of the seafloor many tens of metres wide in a single pass of the sonar with swath widths up to 10 times the flight height off the bottom. After the acquisition of each swath, the swaths can be joined together in a mosaic to give full coverage of the seafloor. The full coverage is geo-referenced within each swath and as a whole mosaic for objects on the seafloor with locations known to errors that are a function of the distance away from the sonar. As such in shallow water it is possible to locate objects to less than 5m and with multiple passes of the sonar to better than 2m when the sonar is used in conjunction with a DGPS. Typical survey objectives for modern digital sonar are wreck sites, surface geology and biology. An example of the high resolution possible with a digital sidescan sonar is shown in Figure 1. Archaeological applications are mostly for recognisance surveying prior to the use of divers on a site for surveying sites beyond the typical dive depths for divers or for surveying in poor visibility conditions. Many examples are given in the literature, for example Quinn et al., 1998 and 1999.

3 Multi-beam and Swath-sounding systems Multi-beam and swath sounding systems are both digital acquisition systems that use an array of transducers to look to the side of the sonar. The sonar is usually mounted as a pole-mount either to the side of the vessel, through a hull-mount or as a bow-mount. The position information for the vessel is fed directly into the acquisition software together with corrections to account for roll, pitch and heave. The information from the reflected energy from the seafloor is processed in each technique in a slightly different manner but with the intention of producing a contoured bathymetric plot of the echoes thus defining in 3D the topography of the seafloor along each line or swath. The survey is conducted so that swaths from different lines overlap and thus a mosaic map of total seafloor coverage is produced. With good DGPS and real-time survey corrections for the sonar, it is possible to survey locations on the seafloor to better than 25cm, however typically better than 50cm is achieved (Goodfellow, 1996). An example of a bathymetric chart surveyed at the H.M S. Invincible Site and a view of a chart for Loch Earn is shown in Figure 2. With all recently developed systems, the acoustic energy is measured for travel time and amplitude strength and thus both bathymetric values and a form of sidescan image are produced. The advantage of this sidescan type image over that of a true sidescan is the fact that the reflecting point is exactly known on the seafloor. This gives the geophysicist the ability to correct the amplitudes for water column and seafloor reflectance angle losses thus ending up with a true picture of seafloor backscatter. However the disadvantage is that the sonar is usually attached to the boat and therefore the slant range is limited giving poorer profiles of objects on the seafloor. A recent development of these instruments has been for seafloor characterisation and this is discussed later. The benefits of a swath-sounding system for an archaeological survey are immediately apparent. The bathymetric 3D full coverage map of the seafloor with sub-metre locations can be used as a first pass at many site plans. When this is combined with an amplitude map or sidescan-like map an image of the seafloor can be used for classifying the areas with most likely wreck potential, hazard and instability or biological cover. These surveys are rapidly performed and while the technology is still developing fully and hence is relatively expensive, the large data coverage possible can offset the cost. Furthermore, routine surveying over a period of years can give quantification to any changing conditions on a site such as the amount of sediment cover. Seabed Classification Characterising the seafloor has been a secondary objective for marine users such as fishermen, engineers and resource extraction companies for many years. Classifying the seafloor is also of use to the archaeologist for defining areas of different sediments, areas of high biological activity or biological cover and areas that may represent wreck locations. A number of systems have been developed for seafloor classification mostly based on single beam echo-sounders. While these systems have proved useful for large site evaluation, they suffer from similar problems to those for the construction of bathymetric charts using echo-sounders, namely that interpolation is necessary between lines. Some progress has been made with classifying sidescan images based on dual frequencies (Ryan and Flood, 1996) and on textural, fractal and power spectral analysis (Reut et al., 1985; Tamsett, 1993). DERA have made significant progress in automatic object recognition on sidescan images. However, it is likely that the most significant advances with bottom classification will be made using the multi-beam and swath-sounding techniques. With these techniques it is possible for the first time to be able to record a co-located amplitude and depth point on the seafloor. The advantage of this is that the amplitude can be fully corrected for geometric and other acoustic losses in the water column resulting in a true record of the amount of energy loss and backscatter on the seafloor. Therefore overlays of the bathymetric charts can be produced for bottom reflectance which can then be correlated to bottom type. This work is in its infancy at present with only a handful of trials showing the potential benefits (Hadden and Green, 1999). Magnetometry and Electromagnetic Surveying Mapping of magnetic anomalies on the seafloor has been practised since the 1950 s and has contributed to some of the most significant discoveries in geology such as Plate Tectonics. The use of magnetics in archaeology has also been significant over the last couple of decades with many land sites identified by subtle magnetic variations due to burn pits and other manmade alterations of the landscape (Woolfman,

4 1984; Clark, 1986; Wynn, 1986). However the major use of magnetics in marine archaeology has been for metal detection REF. Two main types of magnetometer are available to day, the proton-precession magnetometer and, more recently, the alkali-vapour magnetometer. Both magnetometers are deployed in fish and towed behind the vessel in a similar manner to the sidescan sonar. Like the sidescan sonar, the closer to the bottom the magnetometer is located the more sensitive it is to features on, or beneath, the bottom. As the magnetometer is also only sensitive to objects in the immediate vicinity, lines need to be surveyed at intervals no larger than twice the anticipated object size in order to record the signature from them. The alkali-vapour magnetometers have demonstrated greater sensitivities than the older protonprecession magnetometers and with faster data sample rates more data points are acquired along a line. Electromagnetic survey equipment has also been used on land archaeological sites for some time (Bevan, 1983) but once again its use in marine surveys has been limited by a lack of appropriate instruments. The electromagnetic equipment measures subtle variations in the Earth s magnetic field due to the presence of conductive, usually metallic, objects. The instrument, which is towed in a fish behind the vessel, transmits an electromagnetic signal that is modified by electrically conductive material on the seafloor. The sensitivity of the instrument is inter-dependant on detection range with larger objects detected at greater distances. Typical flight heights for survey fish are a few metres from the bottom. In a similar manner to the magnetic surveys, line surveys with the electromagnetic fish must be spaced no greater than twice the anticipated object size apart. Thus the technique suffers the same limitations. The future AUVs The future of marine remote geophysical surveying has been demonstrated over the last two years by a number of geophysical survey contractors and the work of the Danish Survey with automated underwater vehicles (AUV s) (Ref). These instruments consist of un-tethered fish that containing a package of geophysical instruments such as sidescan sonar, swath-sounding sonar and magnetometers under selfpropulsion. The fish is guided along a predetermined path that is programmed into it or is guided by remote control. These fish have the advantage of rapidly surveying large areas of the seafloor with 100% coverage at any state of the sea surface. However, to date they are extremely expensive and out of consideration for most archaeological investigations. Archaeological surveys currently fall into two categories unfortunately based around available budgets. For those with large resources to draw on, many advanced image techniques are available, see for example the work of Dr. Robert Ballard on the Phoenician ships in the Mediterranean. For these surveys spectacular results are achieved. For most surveys however, smaller resources dictate the use of non-optimal technologies. These can at times prove adequate to meet the objectives on site and the ever reducing costs of the equipment today make their use more readily affordable than ever. Moreover, the real-time, high quality data presentation means that further investigative actions on the site can be made based on the results of the geophysical surveys. References Ashkenazi, V., Chao, C.H.J., Chen, W., Hill, C.J. and Moore, T A new High Precision Wide Area DGPS System. Journal of Navigation, v. 50, pp Bevan, B. (1983). Electromagnetics for mapping buried earth features. Jol;rnaL offieldarchaeology v. 10, p Blondel, P. and Murton, B.J Handbook of Seafloor Sonar Imagery. John Wiley-Praxis, Chichester. Clark, A. J Archaeological Geophysics in Britain. Geophysics, v. 51., n. 7, p Goodfellow, I. T., Analysis of Co-registered Bathymetric and Sidescan Data. Unpublished Ph.D. Thesis, University of Bath. Hadden, S. and Green, C Remote Classification of Seabed Types Derived from Interferometric Sonar Data. Expanded Abstracts AAPG International Conference, Birmingham, UK. P Hartl, P High Precision Navigation with Satellites. In High Precision Navigation: Integration of Navigation and Geodetic Methods, Ed. L. Linkwitz and U. Hangleiter, Springer-Verlag, Berlin. Jpmes, E.J.W. 1999Marine Geophysics, Wiley & Sons, Chichester. Kearey, P. and Brooks, M An Introduction to Geophysical Exploration, 2 nd edn. Oxford: Blackwell Scientific.

5 Quinn, R., Adams, J.R., Dix, J.K. and Bull, J.M., The Invincible (1758) site - an integrated geophysical assessment. International Journal of Nautical Archaeology, v. 27.3, pp Quinn, R., Cooper J.A.G. and Williams, B., 1999.A Remote Sense of the Seabed, Archaeology Ireland, v. 13 (1), Issue No. 47, pp Reut, Z., Pace, N.G. and Heaton, M.J.P., Computer Classification of Seabeds by Sonar. Nature, v. 314, pp Ryan, W. B. F. and Flood, R. D Side-looking Sonar Backscatter Response at Dual Frequency. Marine Geophysical Researches, v. 18, pp Scollar, I., Weidner, B. and Segeth, K Display of Archaeological Magnetic data. Geophysics v. 51, p Stright, M. J. (1986). Evaluation of Archaeological Site Potential on the Outer Continental Shelf Using High-resolution Seismic data. Geophysics v. 51, p Tamsett, D Seabed Characterization and Classification from Power Spectra of Side-scan Sonar Data. Marine Geophysical Researches, v. 15, pp Wolfman, D. (1984) Geomagnetic dating methods in archaeology, in M. Schiffer, ed., Advances in Archaeological Method and Theory, v. 7, pp Wynn, J. C. 1986b. A Review of Geophysical Methods Used in Archaeology. Geoarchaeology: An International Journal v. 1 No. 3 p

6 1a. 1b. Figure 1a. H.M.S. Campania, surveyed using a Datasonics DS2000 digital sidescan in Firth of Forth (30m depth); 1b. Sand ripples (A), rock skerries (B) and mud/silt (C) bottom surveyed using a Klein 2000 digital sidescan with a frequency of 500kHz and a sonar range of 75meters. Data courtesy of Klein Associates Inc.

7 2a. 2b. Figure 2a Bathymetric chart of H.M.S. Invincible Site off Plymouth. Data acquired with Submetrix System 2000 sonar; 2b View of bathymetric chart for Loch Earn. Data acquired with Submetrix ISIS 100 sonar.

8 3. Field Strength (nt) E E E E E E Longitude Figure 3. Magnetic Survey over the Admiral Gardener, Goodwin Sands using an Aquascan AX 2000, data courtesy of the Archaeological Diving Unit, University of St Andrews.