Archaeological Oceanography

INTRODUCTION

Robert D. Ballard

Why the title Archaeological Oceanography? Why not Marine Archaeology or Nautical Archaeology or Oceanographic Archaeology? Good question.

The ocean covers 72% of the earth’s surface, with an average depth of 4000 m. Since the early 1950s and before, archaeologists have been discovering, exploring, mapping, and excavating ancient shipwrecks beneath the sea. The depths they have been working at, however, are shallow by oceanographic standards, restricted until recently to the inner portions of the continental margins, to depths of less than 100 m. The choice has been ambient diving techniques, particularly the use of self-contained underwater breathing apparatus, or scuba.

Until recently, it was widely believed that the ancient mariner hugged the coastline, seldom venturing into the open waters of the world’s oceans. But recent discoveries in deep water far from shore have shown that many ancient mariners were either driven out to sea by storms before sinking or chose for a variety of reasons to take the shortest routes from one destination to the next.

Not only are there many ancient shipwrecks to be found in the deep sea, but various conditions result in those shipwrecks being well preserved. These factors include total darkness, cold temperatures, low rates of sedimentation, and limited human activity, since fishing and diving rarely occur in such remote regions of the world. Ships sink in the deep sea generally due to storm action, which leads to them being swamped instead of striking the bottom, which frequently can tear their hulls open. Deep-water shipwrecks tend to sink in an upright position, coming to rest in a low-energy environment instead of being further damaged in shallow water during subsequent storm periods.

Another critical factor leading to their preservation is the thick layers of soft mud into which they settle when reaching the bottom. Deep-sea sediments commonly consist of fine-grained clay that is saturated with salt water. As a result, shipwrecks sinking into such a bottom commonly come to rest with their main deck near the bottom/water interface. Since deep-sea mud quickly becomes anoxic, the majority of a shipwreck and its contents, including its human occupants, are thrust into highly preserving anoxic conditions minutes after sinking.

Although this is all interesting, let us return to the question of the title: Archaeological Oceanography. Oceanography, like marine archaeology, is a relatively new field of research, a child of the 20th century. Unlike marine archaeology, oceanography is expensive and reliant upon costly resources, such as large research ships, submersibles, and advanced undersea vehicles, including remotely operated vehicles. It is not uncommon for an oceanographic expedition to cost $30,000 to $40,000 a day. As a result, one month at sea can cost $1 million.

To the world of marine archaeology this is prohibitively expensive and even more so when compared to archaeological programs carried out on land. If one were to use these comparisons in making decisions regarding the allocation of scarce resources, the shipwrecks of the deep sea would never be studied—that is, if you expect the traditional archaeological community and its traditional sources of funding to finance work in the deep sea. But that need not be the case since there is no reason why archaeological oceanography could not be supported by the same sources of funding that support other fields of oceanographic research.

It is important to point out that oceanography is not a separate discipline such as physics, chemistry, or geology. It is an arena in which these disciplines work, bonded together by common needs such as the need for unique facilities that are required to carry out these separate lines of research. It is common for these various disciplines to work together on oceanographic field programs, no different than multidisciplinary programs carried out on land or in outer space. Oceanographers come from all fields of research in the physical and engineering sciences, fields of research that could easily be expanded to include the social sciences of maritime history, archaeology, and anthropology.

More importantly, it is a young enough science to be inclusive, commonly accepting new disciplines into its fold. The history of marine geology is an excellent example. It started in the 1930s and was dominated by sedimentologists concentrating on the continental margins of the world. But the evolving theory of plate tectonics in the 1960s took the earth sciences into the deeper ocean basins, bringing in petrologists, volcanologists, and structural geologists. The discovery of hydrothermal vents on the Mid-Ocean Ridge in 1977 saw an influx of chemists, geochemists, and a broad range of biologists entering the field, placing increasing demands on access to the expensive tools of oceanography.

When we first began to discover ancient shipwrecks in the central Mediterranean Sea in 1988, it was thought to be a rare occurrence. But in subsequent years, as this book documents, more and more ancient shipwrecks were discovered in other deep-water locations. More recently, professional salvage companies have obtained the necessary technology to carry out commercial recovery programs.

It has become increasingly clear that the deep sea can be of great importance to the social fields of archaeology, anthropology, history, and art, to name a few. But how can this interest turn into a meaningful and viable research program?

The term archaeological oceanography sounds like the former is subordinate to the latter but that is not the case. A geological oceanographer is a geologist working in the ocean. An archaeological oceanographer is an archaeologist working in the ocean as well.

It is encouraging to see the willingness on the part of the leadership of the National Oceanic and Atmospheric Administration’s Ocean Exploration Program to support this budding field of archaeological oceanography and our only hope is that other federal funding agencies that support oceanography will follow suit.

PART ONE

The Technology and Techniques of Archaeological Oceanography

1

Oceanographic Methods for Underwater Archaeological Surveys

Dwight F. Coleman and Robert D. Ballard

Geophysical prospecting techniques for land-based archaeological studies are fairly well established. For the most part this is true for marine archaeological studies as well (Oxley and O’Regan 2001). Oceanographic survey techniques that focus on mapping and exploring the marine environment are also well established, but traditional oceanographic methodologies are not typically applied to marine archaeology. A major limiting factor that influences this is the high cost. For example, the current operational cost for using an ocean-class research vessel can be more than $20,000 per day. Deep submergence vehicle systems and advanced geophysical survey equipment that are used with these research vessels can cost more than $10,000 per day. The total cost for one day of shipboard operations could be enough to fund an entire season of a terrestrial archaeological site excavation. But this example really does not represent a fair comparison. Such daily costs associated with doing research at sea are typically devoted to the study of natural history phenomena in the oceans and on the ocean floors. The following questions can be asked: Is the study of human history beneath the sea just as important as the study of natural history beneath the sea? Are cultural resources as significant as natural resources? Should federal dollars be equally spent to protect these resources? Should archaeology be federally funded to the same level as other oceanographic sciences? If the answer is yes to any of these questions, then we can justify the cost of conducting archaeological oceanography. Many of the well-established geophysical tools and techniques that have been employed by archaeologists in shallow water can also be used on larger ships and in deeper water, thereby employing an oceanographic approach. Deep-water oceanographic techniques do not differ greatly from shallow-water techniques, but a focus here is to present methodologies for surveying that optimize time on board expensive scientific research vessels.

Archaeologists have used side-scan sonar, subbottom profilers, magnetometers, and visual imaging techniques, although not nearly as extensively as scuba techniques, to search for and map submerged sites, especially shipwrecks (Oxley and O’Regan 2001). For exploration and mapping of terrestrial sites, use of ground-penetrating radar (GPR) has become more widespread to acoustically image the subsurface details of sites. Collection of sediment cores to ground-truth the GPR data and to characterize the depositional context of terrestrial sites is also common in terrestrial site surveys. In a similar manner to the way these geological techniques have been employed to investigate terrestrial archaeological sites, oceanographic techniques are now being employed to investigate underwater archaeological sites. These techniques (discussed below) are all commonly used during oceanographic research and exploration cruises and represent important methodologies employed to characterize underwater archaeological sites and landscapes.

Established Archaeological Survey

Archaeological survey strategies and techniques, particularly for terrestrial sites, are well established (Banning 2002) and include different approaches for exploration, reconnaissance surveying, and intensive site surveying. Regional scale surveying techniques (Dunnell and Dancey 1983) and sampling strategies (Nance 1983) are also well established, but these are also mainly for terrestrial archaeology. Archaeological survey can involve different techniques and methodologies, depending on the site. From a theoretical standpoint, there should be almost no difference between surveying on land or under water, except for the obvious logistical differences. For example, on land aerial photographs can be used as a base map similar to the underwater use of side-scan sonar mosaics. From a practical standpoint, however, there are significant differences. Firstly, many underwater sites are in regions of very poor visibility, so surveyors must rely more on acoustic strategies than visual strategies. Secondly, survey techniques for shipwreck archaeology differ from the techniques for surveying terrestrial (including inundated) sites. Ancient shipping trade routes or more modern naval battle locations—regions where shipwrecks would be expected, for example—would have well-defined boundaries that would bias the survey strategy. Thirdly, and perhaps most importantly, underwater surveys are much more difficult logistically, and the rigid limits set by cost, time, and weather for work at sea could significantly influence survey strategies.

To complete a well-planned archaeological survey, whether on land or under water (shipwrecks or inundated terrestrial sites), the entire region of interest should be mapped and investigated, even if there are no suspected sites in parts of the survey region. The absence of sites in particular locations provides scientific data and evidence to support the regional archaeological interpretation. For example, to search for shipwrecks along suspected trade routes, surveyors must also search away from the suspected trade routes to verify working hypotheses about delineation of the suspected routes.

Established guidelines for underwater survey exist, primarily, for the purposes of cultural resource management. Several federal agencies in the United States, such as the Army Corps of Engineers, the Minerals Management Service, the National Park Service, and the National Oceanic and Atmospheric Administration, either suggest or require that survey operations follow their guidelines. These guidelines vary depending on the particular archaeological sites and the scope of work. For the most part, the survey guidelines were established to protect submerged cultural resources from being damaged by activities that involve disturbance of the seabed, such as dredging, construction projects, and oil well drilling. For these activities in U.S. waters, compliance with the National Historic Preservation Act of 1966 is a requirement, and a complete site survey and characterization is necessary and must be approved prior to further site activity. Many individual coastal states have rules and regulations in addition to the requirements by federal law. Organizations such as UNESCO (United Nations Educational, Scientific and Cultural Organization) have worked to develop international guidelines and codes of ethics for conducting archaeology under water.

A variety of geophysical methods have been used in land-based archaeological exploration and surveying, including, but not limited to, satellite remote sensing, airborne imaging, ground-penetrating radar, and magnetic techniques (Renfrew and Bahn 2000). These are primarily tools for prospecting. Other terrestrial archaeological methods that involve geophysical techniques include archaeo-magnetism, radioisotope studies, dendrochronology, palynology, paleontology, and provenance studies. These are primarily analytical methods and are useful in absolute dating and understanding past environmental conditions and archaeological associations. For buried terrestrial archaeological sites, a regional sampling strategy can be employed to test for potential sites (Nance 1983). This could include coring or excavating test pits situated in high-probability locations.

For the marine environment, similar sets of prospecting and analytical techniques exist. The primary focus here will be on marine geophysical exploration and surveying techniques, but techniques analogous to those used on land can be used once an archaeological site is identified for higher-resolution investigations. Intrusive techniques have been employed as part of the survey phase of underwater archaeology (Oxley and O’Regan 2001). This primarily involves limited sampling of material from the site to better characterize and understand its nature, such as the collection of organic material for radiocarbon dating. Excavation, such as trial trenching on land to test whether a site exists, is intrusive and can be very destructive, and this is not very practical for investigating underwater sites (Oxley and O’Regan 2001). Techniques for underwater site excavation are well established (e.g., Green 1990), and typically involve intensive surveying to carefully map the site prior to excavation and subsequent site disturbance. New oceanographic methodologies that employ remotely operated vehicle systems for high-precision site surveys can now be utilized in both shallow- and deep-water settings (Foley and Mindell 2002).

Archaeological Oceanographic Surveys

The nature of the survey strategy is dependent on whether archaeological sites are known to exist within the region to be surveyed. Exploratory and reconnaissance surveys can take many forms, but for targeting inundated archaeological sites, certain methodologies work better than others. A full range of geophysical methods can be applied to archaeological oceanography, and these methods help to define this new field. Interpretation of the survey data will help to delineate sites for further exploration and detailed investigation. These oceanographic methods include bathymetric mapping, side-scan sonar surveying, high-resolution reflection surveying (including subbottom profiling and lower-frequency seismic methods), magnetometer surveys, and visual imaging surveys using remotely operated vehicle (ROV) systems (Oxley and O’Regan 2001). Other geophysical methods, including electrical resistivity and marine gravimetry methods, can be used to explore for and characterize underwater archaeological sites.

For the survey of submerged terrestrial sites, the strategy must be different from shipwreck mapping surveys because prehistoric sites are typically buried in the shelf sediment. The process of coastal inundation due to rising sea level is generally destructive to archaeological sites. If sites are rapidly buried, there is a greater chance for preservation of delicate materials. But for the most part, what survives are the nondelicate cultural and human remains—lithic artifacts (stone tools, points), kitchen middens (mammal and fish bones, shells), gravesites (human bones and associated artifacts), stone foundations of dwellings, pottery, hearths, postholes, and other cultural features. Remote-sensing methods would not typically be able to distinguish these cultural remains from natural features on and in the sediments. Visual methods must be used to identify specific anthropogenic features from natural features.

A new methodological approach is presented here that involves both remote-sensing and visual inspection techniques. The remote-sensing strategy is used to identify potential archaeological environments on the seabed. For shipwreck exploration this involves using geophysical prospecting techniques to locate man-made targets on the seabed. For submerged terrestrial sites the remote sensing strategy is used to identify paleoshorelines, ancient river channels, tidal inlets, lagoons, and embayments. Once these features are located through mapping and geomorphologic analysis, a systematic approach to develop the visual survey is used to target regions where submerged archaeological sites are predicted based on the environmental setting. One aspect of the archaeological oceanographic survey methods presented here that is common to all techniques is accurate and precise navigation. By employing the Global Positioning System (GPS), navigation can be accurate to within a couple of meters using differentially corrected signals. For the shipwreck and submerged landscape studies presented in later chapters, the geophysical techniques utilized are bathymetric mapping, side-scan sonar imaging, subbottom profiling, video and still camera photography, and geological sampling. These are described below.

Side-scan Sonar Surveying

Side-scan sonar is commonly used to acoustically map the seafloor. A side-scan sonar towfish is typically deployed off a survey vessel and towed behind the ship through the water at a given altitude above the seafloor (figure 1.1). Echo is an example of a side-scan sonar towfish (see figure 2.7). For use of this particular system, which is capable of operating in deep water, the towfish is tethered to a depressor weight that acts as a heave compensator, thereby allowing the tow-fish to be unaffected by the ship’s vertical motion. The system emits acoustic pulses at set intervals that are focused with a defined beam pattern according to the design of the sonar transducer. The range, or imaging distance to either side of the centerline, can also be set, as can other data acquisition parameters. Because the instrument images to both sides, twice the range indicates the effective swath width that represents the width of seafloor that is mapped along track (figure 1.1). The acoustic sonar pulses are transmitted with a set frequency. Some dual-frequency side-scan sonar systems exist (Echo, for example), which enable high- and low-resolution data to be collected at the same time.

By keeping careful track of the layback, or the horizontal distance behind the ship that the instrument is towed, which is a function of the amount of cable that is spooled out, features on the seabed can be precisely located. The layback represents an offset that can be used to compute the GPS position of features on the seafloor. Acoustic targets stand out on the sonar record as anomalous features. These targets are either natural or man-made features that can be later inspected using visual surveying techniques, as discussed later. Large targets that have vertical relief will have an associated acoustic shadow that is clearly visible on the sonar record. The shadow results from a loss of acoustic information on the seabed because the target is essentially blocking the sonar pulse from reaching points inside the shadow. As with most acoustic data, the interpretation of targets relies on groundtruthing by visual inspection, to determine if the features are natural or man-made. An exception to this is modern shipwreck targets that typically can be recognized solely by their acoustic character. To fully characterize shipwreck targets, visual inspection is still required, however. For more information on side-scan sonar theory and operation, refer to Fish and Carr (1991).

Figure 1.1. Use of side-scan sonar (such as Echo, shown in figure 2.7). Refer to text for explanation.

Bathymetric Mapping

The initiation of a terrestrial archaeological survey typically involves examination of topographic maps, aerial photographs, and satellite remote-sensing images (Banning 2002). Marine archaeological surveying must commence with a similar data set. Multibeam (swath) and single-beam (echo sounding) sonar mapping methods are used in reconnaissance surveying to initially interpret the seafloor topography to give a first-order depiction of the submerged landscape. Multibeam bathymetric sonar systems use hull-mounted acoustic transducers and receivers, with signals that sweep through a swath beneath the ship (figure 1.2). Typically, a hull-mounted multibeam sonar system can resolve features on the seafloor on the order of tens of meters in size, depending on the water depth, acquisition parameters, and characteristics of the sonar transducers. Advances in this technology, particularly with deep-towed and robotic systems, have resulted in much more detailed bathymetric maps, with centimeter-scale spatial resolution (Singh et al. 2000). In addition to collecting new data, preexisting bathymetric data from older surveys are useful and there are excellent resources for large data sets, although at much coarser resolution. The NOAA National Geophysical Data Center is an excellent resource for processed bathymetric data sets. Global bathymetric grids based on satellite gravity data exist, but with a resolution of only a few kilometers (Smith and Sandwell 1997). Higher resolution grids from shipboard surveys exist for most U.S. waters. Once the general bathymetric features are determined, more detailed geophysical work can commence.

Figure 1.2. Geophysical surveying and prospecting equipment, including multibeam bathymetric mapping, subbottom profiling, and towing a magnetometer. Refer to text for explanation.

Subbottom Profiling

High-frequency seismic reflection methods, also called subbottom profiling (figure 1.2) typically use lower-frequency acoustic signals than side-scan sonar to map features below the seafloor. Very low-frequency seismic reflection methods are used to image deep within the earth’s sedimentary sequences and crust, but this will not resolve shallow buried features. Oil companies employ this technology for hydrocarbon exploration. Echo (figure 2.7) is equipped with a high-resolution (Chirp) subbottom profiler. Typically these systems are towed behind a survey vessel. A sound pulse is transmitted vertically through the water column, and any density changes within the seafloor sediments produce reflections that are recorded to produce seismic-stratigraphic profiles. High-resolution subbottom profiling data can be used to map anomalous features buried below the modern sedimentary cover and to interpret the recent sedimentary history. Data collected in this manner typically need to be postprocessed to correct for ship and towfish navigational parameters, and to remove artificial noise. The processed data, combined with seafloor mapping data (bathymetry and side-scan sonar), can be used to produce a complete three-dimensional picture of the geological and archaeological landscape. These systems do not have high enough resolution to image sites that are buried in the shallow sediments. However, a recently developed ROV-mounted subbottom profiler has been successfully employed to investigate shipwrecks (Mindell and Bingham 2001). As with side-scan sonar targets that must be visually verified, seismic data can also be verified by groundtruthing. This typically involves the collection of marine geological samples (usually sediment cores) along a seismic-stratigraphic profile that correlate to the subbottom imagery. For more information on the application of subbottom profiling to the investigation of submerged archaeological sites refer to Stright (1986). For theory and operational details of high-resolution (Chirp) subbottom profiling, refer to Schock et al. (1989).

A marine magnetometer can be towed in tandem with a side-scan sonar or subbottom profiler (figure 1.3), or towed by itself. The magnetometer measures the strength of the earth’s magnetic field. Magnetic objects on the seafloor will locally disturb the earth’s magnetic field and create an anomaly measured by the magnetometer (figure 1.3). The anomaly is displayed on the shipboard acquisition system, and the location of the object or feature on the seafloor represents the magnetic target, similar to a side-scan sonar target. Again, ground-truthing is important for target identification and characterization. For more information on the application of marine magnetometers to shipwreck archaeology, refer to Clausen and Arnold (1976).

Figure 1.3. Magnetic field anomaly measured by a magnetometer, created by a target on the seabed. Refer to text for explanation.

Visual Imaging Techniques

Visual imaging of acoustic and magnetic targets on the seafloor is a necessary component of archaeological oceanographic surveys. The visual identification of objects first identified by other means (such as side-scan sonar or magnetometer) determines whether the object is natural or man-made and enables archaeologists to evaluate the object’s archaeological significance. A number of diverse methods can be used to accomplish this, including scuba diver inspection, inspection by remotely operated vehicle systems, and inspection by towed video camera systems (Oxley and O’Regan 2001). In addition, human-operated submersibles can be used for visual imaging. ROVs are particularly useful, especially when deployed from a research vessel equipped with a dynamic positioning (DP) system. This system of thrusters provides enhanced maneuverability and enables the ship to hold to a fixed location above the seafloor while the ROV is driven directly to the target (figure 1.4). Advances in subsea navigation, lighting, video imaging, and manipulation have improved the quality and general capabilities of ROV systems, especially for archaeological survey.

Some ROV systems can be operated with two vehicles in tandem to enhance their lighting and imaging capabilities (figure 1.4). Argus (see figure 2.2), is an imaging towsled, lighting platform, and depressor for the ROV Hercules (see figure 2.1) and ROV Little Hercules (see figure 2.6), which can be coupled to Argus via a fiber-optic tether (Coleman et al. 2000). The Argus–Hercules tandem ROV system was modeled after the Jason–Medea ROV system developed by the Deep Submergence Laboratory at Woods Hole Oceanographic Institution (Ballard 1993). Improvements were made to the vehicles’ lighting and video systems, and the equipment was customized for the investigation of submerged archaeological sites, not general-purpose oceanography (Coleman et al. 2000). A sector-scanning sonar instrument mounted to an ROV is very useful for locating targets on the seafloor. The location from a side-scan sonar survey, for example, can be erroneous by up to several tens of meters due to layback and other navigational errors. In deep water or in areas of poor visibility the sonar device can pick up targets acoustically more than 100 meters away, then those fixed locations are used to help navigate the ROV to the targets. Geophysical instrumentation and oceanographic sensors can be mounted on ROVs, and, in conjunction with precision navigation, used to generate very high-resolution maps and images of the seafloor and archaeological sites such as shipwrecks (Singh et al. 2000; Foley and Mindell 2002). Geological and archaeological sampling equipment can also be mounted on ROVs to facilitate the collection of seafloor samples and other data that support the archaeological survey and visual identification.

Subsea tracking and navigation of the ROV systems are critical for locating targets on the bed. An ultrashort baseline (USBL) subsea navigation system can be employed to track one ROV or multiple ROVs at the same time (figure 1.5). A specialized transducer is mounted below the hull of the ship that transmits an acoustic signal that gets repeated by a transponder mounted to the ROV. The resulting time delay and depth and direction of the repeated signal that is received by the ship translates into horizontal and verticle offsets. These offsets are then used to compute the position of the ROV in plan view relative to the ship, giving GPS positions for the ROVs. Sophisticated navigational software can be used to plot the ship and ROV positions accurately on a computer screen in real time. Precise subsea navigation is a critical component to conducting visual surveys and detailed mapping of submerged archaeological sites.

Figure 1.4. Use of remotely operated vehicles (such as the two vehicle system Argus/Little Hercules, shown in figures 2.2 and 2.6). Refer to text for explanation.

Human-operated submersibles (figure 1.6), which have limited capability compared to ROVs because they can stay on site only for short periods of time, can be used for archaeological site investigations, but not typically for reconnaissance surveys. Like ROVs, they can be equipped with scanning sonar, high-quality video cameras, lights, and sophisticated sampling equipment. An advantage to using a submersible is that it can operate independent of the surface ship, which may not be equipped with a dynamical positioning system. The submersible can use sector-scanning sonar to find the target of interest, then drive directly to it and operate with fine precision around the site. Submersibles can also typically lift heavier objects and carry more equipment and/or samples than ROVs, so for some tasks they have advantages.

For visual surveys of underwater archaeological sites, it is advantageous to have a number of different systems and to utilize a number of different techniques. Because of the high cost of shipboard operations, archaeological oceanographers need to avoid down time due to equipment malfunctioning and repair. That is another advantage of the tandem ROV system. If one of the vehicles needs repair, the other can be used independently.

In summary, the oceanographic tools and methodologies mentioned and described briefly in this chapter have been commonplace in marine geological surveys, and also for marine biological transects and habitat characterization surveys. These same tools and methodologies are now being applied to deep-water archaeological studies, in the context of the new science of archaeological oceanography. For shallow-water archaeological studies, many of these tools and methodologies have been employed for several years, but not typically for long cruises operating 24 hours per day, and involving expensive scientific resources. With growing interest in this nascent discipline, we hope expensive assets like ocean-class ships and deep-diving ROVs and submersibles become available to archaeologists for investigating sites of historical significance.

Figure 1.5. Use of precision subsea navigation for tracking an ROV. Refer to text for explanation.

Figure 1.6. Human-operated submersible PC-8B. The Bulgarian Academy of Sciences Institute of Oceanology submersible, sitting on the afterdeck of the research vessel Akademik, is a 3-person submersible equipped with scanning sonar, lights, still and video cameras, thrusters, and a manipulator. (Photo courtesy of Delcho Solakov, Bulgarian Academy of Sciences—Institute of Oceanology)

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