319

Hanumant SinghWoods Hole Oceanographic InstitutionWoods Hole, MA

Jonathan AdamsUniversity of SouthamptonSouthampton, UK

David Mindell

Brendan FoleyMassachusetts Institute of TechnologyCambridge, MA

In this paper we examine underwater imaging for archaeology and especially the use ofmultiple acoustic and optical sensors at varying degrees of resolution for reconnaissance andmapping sites on the seajloo1: Specifically, we look at the role of side-scan sonar for locatingsuitable sites of interest, which may then be mapped quantitatively with bathymetric sonars.We also discuss the role of conventional digital and video cameras in providing high resolu-tion redundant imagery, and how, after compensating for lighting and other artifacts, suchimagery may be assembled into a photomosaic of a large site.

IntroductionArchaeology is increasingly moving into deep water

(over 100 m) and sites several thousand meters below thesurface are now within reach through robotic technology.In this paper we focus on the methods for locating anddocumenting archaeological sites in the deep sea. Thechal-lenge, from archaeological and engineering persptctives, isto achieve or exceed the standards in mapping and docu-mentation used by researchers on land and in shallow wa-ter. In order to do this, we must review the different phas-es of site discovery and mapping, especially methods of de-ployment, their limitations and strengths, and the comple-mentary nature of the optical and acoustic sensing modal-ities (Stewart 1991).

The authors have conducted numerous archaeologicalexpeditions in shallow and deep water, including repeatedexpeditions to Skerki Bank in the Mediterranean (McCannand Freed 1994), exploratory surveys in the Black Sea, andsurveys of the Hamilton and Scourge in Lake Ontario(Stewart 1991: 10-22). Other scientists have participatedin the location and survey of a number of considerablymore modern shipwrecks such as RMS Titanic (Ballard1987) and the German battleship Bismarck (Ballard 1990).For the deeper water sites, technologies utilized includetowed side-scan sonar systems, the tethered JASON re-motely operated vehicle (ROY) system, and the U.S.

N avts NR-l nuclear submarine. The purposes of these ex-peditions have varied enormously. Some such as Titanic,Bismarck, and the surveys at Guadalcanal were mounted forthe sole purpose of locating the wrecks and recording themwith video; others (Hamilton and Scourge, Skerki Bank)were archaeological investigations. These projects com-bined quantitative acoustic mapping, high resolution opti-cal imaging, robotic object recovery, and conservation. Themulti-year investigations at Skerki Bank suggest that the re-gion was a crossroads of intensive marine traffic duringseveral periods in antiquity, and have yielded significant in-formation about the patterns of ancient Mediterraneantrade.

Conducting archaeology in a harsh deep water environ-ment necessitates sophisticated oceanographic ships andsubmersibles, leading to operational costs of tens of thou-sands of dollars per day. The tremendous pressure at greatdepth, the corrosive salt water environment, difficulties as-sociated with accurate positioning of vehicles, and limitedvisibility underwater present significant obstacles for the

archaeologist.This paper reviews the use of these deep ocean tech-

nologies and sensors for archaeology. We describe the useof optical and acoustic sensing, associated sensor artifacts,and the manner in which such sensor-specific distortionsmay be minimized. We emphasize that the navigation and

320 Imaging Underwater for Archaeology/Singh et at.

control of the sensor platform greatly affect the quality ofthe imaging data, but a complete discussion of these tech-nologies is outside the scope of this paper. The interestedreader may pursue these topics elsewhere in detail (Ballardet at. 1991; Whitcomb et at. 1998; Yoerger 1991).

Acoustic Imaging

Where photography involves light, acoustic imaging "il-luminates" the seafloor with sound energy and measuresthe reflections. As sound travels considerably further in sea-water than does visible light or other electromagnetic en-ergy, acoustic sensors can perceive much larger areas thanoptical cameras. Side-scan sonar has been used for decadesto search and map the seafloor for archaeological and oth-er purposes and recent developments in transducer design,pulse shapes, and signal processing have enabled acousticimages to achieve the quality of photographs.

A brief review of acoustic sensing underwater will beuseful. Acoustic propagation underwater is a function offrequency (Urick 1975), with lower frequencies propagat-ing further than higher frequencies, although lower fre-quencies have less resolving power so there is a trade-off as-sociated with range and resolution. For archaeological pur-poses, useful frequencies range from 10kHz to 1 Mhz withcorresponding swath widths from several kilometers downto 10m and corresponding resolution from tens of metersdown to sub-centimeters respectively. Lower frequencysystems may be ship-mounted or towed and are appropri-ate for broad area survey. Higher frequency systems may beemployed for very precise microbathymetry. Based onthese resolutions, acoustic systems are capable of distin-guishing objects as large as shipwrecks or as small as an am-phora. The signal received at the sonar is governed by thesonar equation (Urick 1975), which measures all quanti-ties in decibels, in turn implying that we are dealing withquantities of very high dynamic range. One can utilize thisdynamic range in two fundamentally different ways-todigitize the intensity of a signal with very high fidelity inthe hope of examining minute differences in the return (asin the case of side-scan sonars) or alternatively, to detect thepresence or absence of objects with very high spatial reso-lution (as in the case of bathymetric profiling, pencil-beamand multibeam sonars capable of resolving with centime-ter-level precision).

Side-scan Sonar Systems

Side-scan sonar is the primary tool for locating sites andgauging their extent. Side-scan sonars (Klein 1967; Rus-sell-Cargill 1982) are designed to image the seafloor overwide areas with twin, fan-shaped beams. Two sonar trans-ducers are usually attached to a torpedo-shaped carrier, or

"fish;' which is towed behind a vessel as the sonar scans outto both sides and measures the return from the seafloor.The fan-shape of the beams yields a narrow scan line of re-turn from each ping, and when many such lines are plottedin sequence (as the fish moves forward), an image is creat-ed. The small gap between the two beams directly belowthe fish produces the characteristic line down the middle ofside-scan sonar images. The height of the fish above thebottom is usually 10% of the swath width (a typical setupwould be 30 m off the bottom for a swath 150 m on eachside for a total of 300 m). A well-designed side-scan sonaroperating in an optimal environment can yield vivid resultssuch as the image of the Scourge (1813) in Figure 1.

Typically, however, the results are considerably less strik-ing, so the image data must be analyzed and interpretedcarefully. The most common interpretation errors arisefrom artifacts associated with geometry (Cobra 1990) andradiometry (Mitchell and Somers 1994; Reed and Hus-song 1989). The geometric artifacts are a function of theimaging of the object's actual three dimensional structure,which differs dramatically from the assumption of a flatbottom that is associated with side-scan sonar.

The presence of "shadows" in side-scan sonar traces is akey element in data interpretation, as "noise" and interfer-ence in sonar data often closely resemble real targets, butdo not produce shadows. Shadows indicate that a sonaranomaly has vertical relief and often convey the shape of atarget in greater detail than its actual acoustic return. Fig-ure 1 provides a good illustration of this phenomenon, asthe shadows highlight the structure of the masts, bowsprit,and gun ports much more clearly than the actual returnfrom these objects. Beam pattern effects are another com-mon source of error, but are easily identified as bright anddark bands in the imagery (FIG. 2), corresponding to thedifferences in the projected energy as a function of angle,for a particular transducer. These bands may move in or outdepending upon the altitude of the fish. Other problemscan be introduced by excessive pitch, roll, and yaw of thefish, any of which smears the return. An unstable fish mayrender the data useless and often the only recourse is toreacquire the data.

Radiometric artifacts are more subtle than geometric ar-tifacts. They result from the dependence of target strengthon the angle at which an object is ensonified by the sonar.This, for example, can preclude the operator from distin-guishing rocks from other materials and therefore compli-cates the task of accurately identifying objects on the bot-tom.

In summary, the two crucial points that affect the use ofside-scan sonars in the field are first, the wide area of cov-erage that makes them suitable as a first tool for locating

Journal ofFieldArchaeologyjVol. 27, 2000 321

Some of the major impediments for underwater imageryare the low contrast nature of the images and, in the caseof color imagery, the nonlinear attenuation of the visiblespectrum which introduces a preponderance of blue in theimage. In practical terms one of the biggest limitations isthe inability to acquire large objects in a single frame.

Most underwater cameras are of limited dynamic range(8-bit resolution) and in deep water, all lighting has to besupplied by the imaging platform. Two of the key compo-nents for achieving good lighting are uniform light inten-sity across the field of view and adequate camera-to-lightseparation to minimize backscatter from particles suspend-ed in the water. Even in the case of high dynamic range(12-14 bit) sensors such as those using cooled chargedcouple device (CCD) technology, the imagery is often low-contrast and requires post-processing. Since image qualityis a subjective measure there is no particular post-process-ing technique that can be universally applied. One algo-rithm that frequently works well for image enhancement,however, is adaptive histogram specification and equaliza-tion (Zuiderveld 1994) as shown in Figure 3. Such pro-cessing often yields much fmer detail but it is also respon-sible for highlighting noise in the image. What is more, itoften removes or obfuscates lighting cues that human op-erators utilize to gauge depth in optical imagery. After in-dividual images are processed, they may be combined into

photomosaics (FIG. 4).To collect image data over large areas, the vehicle con-

ducts a survey along a predetermined grid that guaranteescomplete, redundant coverage. Individual images collectedduring the survey are combined into strips, and the stripsare then merged to form larger mosaics, either manually orautomatically. Commercial packages exist for both ap-proaches, helping to resolve issues of spatial distortion andvariations in light intensity from one frame to the next. Fig-ure Sa is an example of a manually constructed photomo-saic covering 30 m by 10m. While photomosaics do pro-vide an overall very high resolution perspective of a scene,they are not quantitative representations. As individual im-ages are added into the mosaic, incremental errors are in-troduced and accumulate over the entire mosaic. These er-rors are primarily due to our inability to accurately modelbottom topography and are present in both manually andautomatically constructed photomosaics. The true shape,location, and size of objects will be distorted to an un-known degree in any photomosaic and many subtle threedimensional features may be invisible. In the Skerki D pho-tomosaic the slight depressions surrounding several am-phoras do not appear (FIG. SA), while they are clearly evi-dent in the micro bathymetric plot (FIG. SB). Archaeolo-gists must be fully cognizant of these errors and data from

Figure 1. A side-scan sonar image of the Scourge. Notice that the shadows provide more defl11ition for the features than the actual returnsfrom the ship.

and characterizing archaeological sites and second, thenumber of sources of error that make image interpretationa subjective exercise. In the field one must compromise be-tween redundant overlapping coverage and the total areacovered by a survey. Redundant coverage provides multi-ple looks at the same objects and thus makes the task ofim-age interpretation and target classification easier. It also en-sures that there are no gaps in the area being surveyed. Fora side-scan sonar operating at hundreds of kilohertz, theoverlap may vary from 25% to 100% depending uponwhether one only requires continuous coverage with nogaps or the best possible look at the site from multiple an-gles. The side-scan sonar survey serves to reveal targets thatcan then be examined more closely with optical systems.

Optical Imaging and Photomosaics

Optical sensing, including fUm, analog video, and digital imagery, is an obvious choice for high resolution imaging underwater, but the rapid attenuation of electromagnetic radiation underwater severely limits their utility

322 Imaging Underwater for Archaeology/Singh et at.

Figure 2. A 19th century Turkish warship off the coast of Sinop in the Black Sea as im~ed by a high fre-quency side-scan sonar. Reproduced with permission from information supplied by David Mindell, Bren-dan Foley, and S. Webster, 1998. Im~e courtesy Marine Sonics Technology, Inc.

all sensors must be analyzed in parallel as the archaeologistinterprets the site.

nated using stereo photograrnmetry, in which any adjacentpair of photographic images can be used to reconstruct thesolid geometry of that part of a subject visible in both im-ages. Software packages have been developed to automatethis process (Rule 1995). First, the co-ordinates and atti-tude of the camera are established for each component im-

Future Applications of Optical Imagery- ThreeDimensional Reconstruction

Potentially, the problems described above can be elimi-

]ournalofFieldArchaeologyjVol. 27, 2000 323

Figure 3. An original low contrast image (top) and the histogram equalized version (bottom). Theimage on the bottom brings out much more detail but removes lighting cues humans rely on for

depth perception.

age relative to a three-dimensional scale or an array ofknown points. From this correspondence the three-dimen-sional position of any other point visible in two or moreimages can be computed. At present, the production of ac-curate models is rather labor intensive although improve-ments that will streamline the process are inevitable. Theuse of such programs, together with digital video, offers

the prospect of three-dimensional recording of underwaterstructures in a fraction of the time taken with convention-al techniques.

Microbathymetric Measurements

While side-scan sonars provide intensity informationover wide swaths, and CCD and video cameras yield high

324 Imaging Underwater for Archaeology/Singh et at.

Step 1Individual images are processed toremove lighting and other artifactsby being normalized and histogramequalized. These are then mergedinto single strip mosaics byidentifying common features insuccessive images.

Step 0A carefully planned survey is

conducted over the area of

interest to ensure sufficient

coverage and overlap in the

imagery. Image footprints are

then projected on the area of

interest to allow operators to

choose individual images to

be used in the mosaic.

--=s 5Step 2Individual strips are then mosaicked

together using a technique similar to

that used in Step 1.

Step 3Ongoing work is

focused on

understanding and

elaborating on the

quantitive nature of the

mosaicking process.

Figure 4. The process of compiling a photomosaic.

formation one obtains a three-dimensional image of thesite. An example of such an image is shown in Figure 5b,which also illustrates the complementary nature of opticaland acoustic imaging. The photomosaic (FIG. SA) providesa very high level of detail but remains qualitative. Lightingartifacts also tend to mask out changes in microtopogra-phy. On the other hand, the microbathymetric map (FIG.

resolution imagery for photomosaics over large areas, pro-filing sonars provide bathymetric information at high (cen-timeter-level) resolution. These may be single beam, ormore complex multibeam systems. In either case the sen-sor focuses energy into one or more pencil beams in a par-ticular direction in three-dimensional space and records thereturn. By combining platform attitude and navigation in-

Journal ofFieldArchaeologyjVol. 27) 2000 325

Figure 5. A) A ISO-image photomosaic of a 2nd century B.C. Roman shipwreck provides a very high res-olution qualitative global perspective of the entire site, while B) the microbathymetric map of the sameregion is of lower resolution but allows for quantitative measurements.

326 Imaging Underwater for Archaeology/Singh et at.

Ceramic pile before recovery Ceramic pile after recovery

Figure 6. Repeat surveys for detecting change. One can clearly document the absence of amphoras thatwere recovered from this site.

powerful mechanism for addressing the needs of the ar-chaeologist working underwater.

5B) resulting from the profiling sonar is quantitative, and

Conclusions

Underwater imaging is an active field of research. Onedirection of interest to archaeologists involves refiningphotomosaics so that the distortion error is bounded. Thisis a crucial first step toward the goal of quantitative photo-mosaics and eventually may lead to complete photogram-metric three-dimensional site reconstructions. Change de-tection for cultural resource management is another appli-cation for these technologies (Quinn et al. 1998). While re-peat surveys of a site can be conducted, only qualitativechanges can be detected at the artifact level (FIG. 6). Qual-itative detection of change at a very fme scale will be possi-ble once acoustic and optical sensors are fused. Benefits de-rived from these technologies are not limited to the acqui-sition of imagery and the exploration of a site; these ap-proaches also provide a convenient framework for analysisand presentation of results (FIG. 7).

In this paper we have outlined some of the constraintsand evolVing techniques for use in imaging applications inunderwater archaeology. No one sensor is sufficient in it-self but combinations of different sensors provide a very

Hanumant Singh (ph.D., MlT- WHOI Joint Program,1995) is a Scientist at the Deep Submergence Laboratory ofthe Woods Hole Oceanographic Institution. His research in-terests concern system design for underwater vehicles and highresolution acoustic and optical imaging underwatet: Mailingaddress: Deep Submergence Lab, Woods Hole OceanographicInstitution, Woods Hole, MA 02543.

Jonathan Adams, senior lecturer at the University ofSouthampton, England, is the Director of that university'sCentre for Maritime Archaeology. He is involved in severalshipwreck projects in the diving range and has participated inthe deep sea investigation of shipwrecks using robotics and theu: S. Navy's submarine NR-1. His research interests includethe technology of early shipping and the development of ad-vanced methods for archaeology under wate1:

David Minden, Dibner Associate Professor of the History ofEngineering and Manufacturing at the Massachusetts Insti-tute of Technology, heads the DeepArch research group atMIT, specializing in precision robotics and methods for explor-ing archaeological sites in the deepest parts of the ocean.

Brendan Foley, Ph.D. candidate in the Massachusetts In-stitute of Technology's Program in Science, Technology, and So-

clearly shows the depressions in which some of the am-phorae are lying.

328 Imaging Unde1'Water for Archaeology/Singh et at.

ciety) is a member of MIT)s DeepArch research group and hasparticipated in investigations of ancient deep sea shipwrecks us-ing Remotely Operated Vehicles) Autonomous Underwater Ve-hicles) and the US. Nary's submarine NR-1. His academicinterests include developing methods for archaeology in deepwater and historical research on technological change in theUS. Navy since 1850.

Whitcomb, Louis, D. R. Yoerger, H. Singh, and D. Minden1998 "Towards Precision Robotic Maneuvering, Survey, and

Manipulation in Unstructured Undersea Environments,"in Y. Shirai and S. Hirose eds., Robotit:r Research-TheEighth International Symposium. Berlin: Springer Verlag,45-54.

Yoerger, Dana1991 "Robotic Undersea Technology," Oceanus 34: 32-37.

Zuiderveld, "Karel1994 "Contrast Limited Adaptive Histogram Equalization;' in

Paul S. Heckbert, ed., Graphics Gems Iv: New York: Acad-emic Press, 474--485.

Ballard, Robert D.1987 The Discovery of the Titanic. New York: Warner Books.

1990 The Discovery of the Bismarck. New York: Warner Books.

Ballard, Robert D., D. R. Yoerger, W. K Stewart, and A. Bowen1991 "ARGOjJASON: A Remotely Operated Survey and Sam-

pling System for Full-Ocean Depth," in Proceedings of theIEEE Oceans 91 Conference, Sept 1991. Piscataway, NJ:IEEE Press, 71-75.

Ballard, Robert D., A. M. McCann, D. Yoerger, L. Whitcomb, D.Mindell, J. Oleson, H. Singh, B. Foley, J. Adams, D. Piechota, and C.

Giangrande2000 "The Discovery of Ancient History in the Deep Sea using

Advanced Deep Submergence Technology:' Deep Sea Re-search I47: 1591-1620.

Cobra, Daniel1990 "Estimation and Correction of Geometric Distortions in

Side-Scan Sonar Design:' unpublished Ph.D. dissertation,Massachusetts Institute of Technology, Cambridge, MA.

Klein, Martin1967 "Side Scan Sonar," Undersea Technology 8: 24-26.

McCann Anna M., and J. Freed1994 A Late Roman Ship from Carthage and an Ancient 1rade

Route near Skerki Bank off Northwest Sicily. Journal of RomanArchaeology, Supplementary Series No. 13.

Mitchell, Neil C., and M. L. Somers1994 "Quantitative Backscatter Measurements with a Long-

range Sidescan Sonar," IEEE Journal of Oceanic Engineering14: 368-374.

Quinn, Rory, J. Adams, J. K. Dix, and J. M. Bull1998 TheInvincible (1758) Site-An Integrated Geophysical As-

sessment," InternRtWnal Journal of Nautical Archaeology 27:126-138.

Reed, Thomas B., and D. Hussong1989 "Digital Image Processing Techniques for Enhancement

and Classification of SeaMARC n Sidescan Sonar Im-agery," Journal of Geophysical Research 94: 7469-7490.

Rule, Nick1995 "Some Techniques for Cost-effective Three Dimensional

Mapping ofUnderwater Sites:' in J. WIlcock and K. Lock-year, cds., Computer Applications and Quantitative Methodsin Archaeology. BAR International Series No. 598. Oxford:

B.A.R.,51-56.Russell-Cargill, William G. A., editor

1982 Recent Developments in Side Scan Sonar Techniques.Capetown: ABC Press.

Stewart, William K.1991 "High Resolution Optical and Acoustic Remote Sensing

for Underwater Exploration," Oceanus 34: 10-22.

Urick, Robert J.1975 Principles of Underwater Sound. New York: McGraw-Hill.