SO482A.  Historic Shipwrecks: Science, History, and Engineering

Underwater Surveying
 

About the Presenter:
 

Alexis Catsambis is an underwater archaeologist with the Naval History & Heritage Command’s Underwater Archaeology Branch. In the field, he has participated in underwater and terrestrial surveys and excavations throughout the Mediterranean, Black Sea, Gulf of Mexico, and along the U.S. Atlantic coast. At the same time, Alexis has authored a number of articles and presentations on different aspects of the field and has been involved with the survey, conservation and digital reconstruction of sites and artifacts as an intern with the Warren Lasch Conservation Center (USA) and the NATO Undersea Research Centre (Italy). He is currently pursuing a doctorate degree in the Nautical Archaeology Program of Texas A&M University, supported by a number of scholarships from institutions including the Onassis Public Benefit Foundation and the Institute of Nautical Archaeology.

 

Lesson Objectives:

 

I. INTRODUCTION

Underwater surveying is a multi-faceted inter-disciplinary undertaking that can apply to a number of fields, including underwater archaeology. While the overall premise of surveying might be a simple concept, there are a number of questions that one needs to address prior to taking to the field. In-water time is the most expensive, and many times the most restricted, part of an operation – whether diving or using remote sensing equipment. Therefore, being well-prepared in advance of a survey is the best way to ensure the most efficient operation possible. While it may seem self-evident at first, a successful survey is one that manages to thoroughly cover an area beyond reasonable doubt in the time allotted, whether a target is located or not. This, of course, is often separate from accomplishing a mission objective which centers on locating a target.

II. WHAT ARE YOU LOOKING FOR?

In general terms, there are three possible objectives for underwater survey, although some surveys have overlapping goals. These include:

(1) surveying an area to find a particular target;

(2) surveying a promising area to uncover unknown targets; and

(3) surveying an area to make sure it is clear of targets (eg. mines).

Underwater archaeology tends to focus on the first two objectives, however, an area with no cultural resources present whatsoever is also interesting in itself – Why is there nothing there? Are there environmental factors such as sedimentation? Or human factors such as trawling or dredging? What does that mean for other areas or for the protection of underwater cultural heritage in general? What does that mean for the survey itself? Were the right tools selected? The right methodology followed? Should something be done differently next time around? Absence of evidence is not necessarily evidence of absence.

It is therefore of primary importance to be thorough and systematic in surveying, as presence or absence of targets can both be significant. This begins with knowing how to recognize targets, and what you might find in a particular area. While recognizing wrecks of large WWII destroyers may be relatively evident, recognizing smaller targets or historic wooden vessels that have been exposed to the elements for significant periods of time is more challenging. What is required is a fundamental understanding of what a target (e.g. a shipwreck) originally looked like, and how site-specific formation processes have influenced it over time. The greatest contributing factor in gaining this skill is experience – the more sites you look at, the more you understand how something was originally put together, and the more familiar you become with how sites are impacted by their immediate environment.

Stratigraphy: 

A) Context 1 is later than context 2.  Absolute dates can be placed in the relative dating sequence. 

B) A coin dated AD 79 in context 3 indicates that 3, 2, and 1 arrived after AD 79.

C) A floor constructed in AD 1322 indicates that the contexts below it must have accumulated before then.

(Based on original artwork by Ben Ferrari)

Image courtesy of the Nautical Archaeology Society (2009)

 

SITE PRESERVATION

The term “preservation” (as opposed to “conservation”) of submerged cultural resources such as wrecks and structures pertains to the condition a historic site is currently in and its rate of degradation. Understanding how materials such as wood, rope, textile, leather (commonly referred to as organic material) or metals such as iron, copper alloys, and lead, are affected by the marine or fluvial environments is very important, as each material is impacted differently. Materials science and archaeological conservation, the science of stabilizing the condition of artifacts, are fields that are dedicated to understanding these impacts. In general terms, the presence or absence of oxygen, sediment, wave action, and currents are among the primary environmental factors that impact preservation, particularly of organic materials.

To offer an example, if a wooden wreck is buried in silt with little current to stir it up, during an initial period of rapid degradation, the oxygen surrounding it is consumed as wood-boring organisms and bacteria which devour organic materials thrive. In the new anoxic environment that develops, the same marine organisms can no longer survive, and so the rate of degradation drops significantly, leaving a hull with good long-term preservation prospects. The hull will remain in this equilibrium until its immediate environment is disturbed and new oxygen-carrying water is introduced.

In contrast, if a wooden ship hits a reef surrounded only by a few pockets of sand, the combination of the powerful wave action (often referred to as a high-energy zone), plentiful oxygen, and plethora of marine organisms, as well as the limited prospects for burial, will result in a poorly preserved vessel. However, traces of that site are likely to remain, more often than not in the form of concretions or artifacts made of non-perishable materials such as clay (pots, jars, etc.).

Marine encrustations, or concretions, form around ferrous artifacts and parts of the ship like iron fittings, rigging elements, iron frames, and even iron hulls. They are based on a combination of rust, shell, marine organisms, calcium carbonate, and other materials that form a layer (of varying thickness) around objects as the iron oxidizes and reverts back to its natural state. Different metals respond differently to the marine environment, with lead being among the most impervious to the elements, copper and copper alloys fairly tolerant, and iron objects among the most heavily affected by seawater. The result is a site that can be composed of multiple concretions, of various sizes, many of which look like they could potentially belong to the natural setting. In many instances, with the passage of time the magnetic signature of iron artifacts diminishes to beyond note – in other cases, concentrated pig iron ballast can be detected from tens of meters away using a magnetometer.

Therefore, submerged craft may take on varying shapes, sizes, and degrees of preservation. Knowing the environmental parameters in the survey area is important not only when selecting remote sensing tools, but also when trying to ascertain the current condition of targets.

 

Simplified version of the site-formation process (top left to bottom right). The vessel lies across the prevailing current, which results in mechanical and biological degradation, leading to the breakdown of the superstructure and localized scouring.  At the same time, the vessel sinks deeper into the sea-bed.  In time the site stabilizes until human interference, which results in new scour patterns and infill that should be easily identifiable in the stratigraphic record. (Drawing by Graham Scott)

Image courtesy of the Nautical Archaeology Society (2009)

 

 

REGIONAL OVERVIEW

Taking a step back from specific sites and targets, when preparing for a survey it is important to have a good grasp of the overall region and specific survey area.  Below are some variables to consider.

Above the surface of the water:

A diver inspects the wreck of the USS Macaw off Midway Island.  The accessibility of medical assistance was undoubtedly one of the considerations in planning this mission.

Image courtesy of the National Oceanic and Atmospheric Administration

In the water column:

Poor visibility in the North Sea makes the fantail of this wreck difficult to see.  The soft white coral growing on the wreck further impedes the investigation. The image was taken by Submarine NR1 in June 2008 on its search for the Bonhomme Richard.  June, July, and August are the best weather months for an expedition in the Bonhomme Richard search area.

Image courtesy of the Ocean Technology Foundation

 On the seafloor:

 

In this side scan sonar image, we can see that the geology of the seabed should be taken into account when surveying in this area.

Image courtesy of the Ocean Technology Foundation

 It is also important to tap into knowledge that already exists about surveying in the area to gain the following information:

Learning from past efforts is key to maximizing the effectiveness of an underwater survey.

The historical past itself is a great source of information; learning how an area has been used through time will indicate the kinds of wrecks that may be present. Ancient mariners were on the whole very competent – if areas were avoided that is because there was and may still be a danger to ships present; on the other hand there may be a reason why a certain bay became a key trading port, or a geographic feature is named the “Black Cape.” Often times the reasons only become apparent with additional research.

Today’s mariners are often just as valuable a source of information as those of the past. Where do fishermen tend to fish? Where are common net hangs? Marine life tends to congregate around artificial reefs formed around wrecks. Are there times of the year where the sea is more amenable to survey? It is, however, not always easy to gain the trust of local populations – protecting an important shipwreck may mean in some cases forming marine preserves where fishing is not allowed. However, this should not discourage attempts to approach local mariners, as they are among the best sources of practical information about their region.

Finally, it is also important to consider what other institutions operate in the area. Are there other organizations that could partner with you on the survey? Could they provide assets such as tools, equipment, data, or people that could be valuable resources? As you have seen with the Bonhomme Richard survey, academia, the non-profit sector, federal agencies, and foreign counterparts have all come together with a common goal.

All these questions, and more, directly impact the types of tools, skill sets, and survey methods that are best suited for each particular operation.

III. SELECTING YOUR TOOLS AND YOUR PEOPLE

The usual approach to selecting the appropriate tools and equipment for a project is to compare the ideal set-up to what resources allow for. Single tools may produce satisfactory results for simple projects. More complex projects may require a suite of simultaneously-operating data collecting equipment. The first step is to clearly identify the objective of the mission – the second, to establish how available resources can best accomplish that objective.

BASING TOOL AND TEAM SELECTION ON THE PRIMARY OBJECTIVE

The wider-reaching, more comprehensive surveys tend to be those that are geared towards surveying an area, either to identify targets or make sure there are none, rather than surveys oriented towards surveying for a particular target. This is not always the case, and complex target surveys can also be undertaken, particularly when the location of the target is known, but that shifts the operation to site assessment rather than surveying.

A general area survey would ideally involve as many different tools as possible – a multi-beam survey to record seafloor morphology or map specific targets, a side-scan sonar as the main survey tool to record anomalies on the seafloor, a magnetometer to identify sites that might be buried, and a visual capability (ROVs or divers) to assess targets further or effectively survey a shallower area. To these, one could add the sub-bottom profiler, however, this tool is best suited in its current form to establish if a target on the seafloor is man-made (and therefore resting on the surface) or natural (the tip of a much deeper rock outcrop). A target-specific survey may emphasize one tool over another – if you are looking for a semi-buried historic period wreck, focusing on locating its iron ballast pile with a magnetometer may be the best approach; if looking for a WWII submarine, a high-resolution sonar survey may be all that is required.

As these tools are covered in detail in another lesson, the focus of this lecture is on survey strategies, and not on the capabilities of each tool. However, tool selection is intrinsically part of survey strategy. As aforementioned, the foundations for this selection are a) the information compiled through the regional overview research and b) the type of target/targets you are looking for and its/their presumed state of preservation. One of the most important things to remember, though, is that a survey is always constrained by its weakest link – this applies to tools, data, as well as people, and is key when preparing for a project.

TYING PROJECT PLANNING TO RESOURCES

In most underwater surveys, resources are almost always limited – whether this is due to time constraints (e.g. a survey must begin immediately or can only last for one week), geographical constraints (e.g. your preferred research vessel is in San Diego while you wish to survey off New England), financial constraints (among the most common), or personnel constraints (e.g. your chief scientist is only available during a certain time window). It is with this in mind that the focus usually turns from “What would be the ideal survey set-up?” to “What can I accomplish given my available resources?”

Everything in a survey is interrelated. The research vessel (also called platform) has to accommodate the remote sensing equipment or divers (e.g. a multi-beam works best if hull-mounted, an A-frame needs to be capable of handling the weight of an ROV, a hyperbaric chamber may be needed for diver safety, etc.); the tools are useless without properly trained personnel, including technicians, data processors, and science crew, which need to berth in the research vessel. Space is often extremely limited. The tools themselves are also interrelated. There is little reason to opt for the highest resolution multi-beam echo sounder if your positioning capabilities are limited and the vessel lacks D-GPS capabilities. Dynamic positioning, which allows a vessel to remain stationary within a very limited footprint, is usually a pre-requisite for high-accuracy ROV operations, such as entering a wreck. If a magnetometer will be the primary search tool, there is no reason to opt for side-scan sonar with a low resolution and wide swath, as magnetometers tend to have among the narrowest swaths. It is therefore important, when trying to match tools to resources, to remember the simple question – What is the weakest link? Everything else builds around that.

Finally, a personal note from experience – man is the measure of all things. An operation may have the best platform and tools available, but may fail to accomplish its mission if it does not revolve around competent people. Data collected may contain a plethora of important targets, but without proper processing and interpretation by a trained eye, it is of little value to the ongoing survey. If it comes to choosing between the best equipment and the best people, opt for the people. Equipment can break, but people can improvise.

The aft deck of the R/V Aegaeo of the Hellenic Center for Marine Research. The vessel is equipped to undertake complex operations including lowering and raising ROVs and manned submersibles, and towing remote sensing equipment.

Image courtesy of the Danaos Project.  

 

PREPARING FOR THE FIELD

Prior to undertaking a survey, it is necessary to verify that all project activities that require permits have received them and that the appropriate official authorities have been briefed. This is often a time-consuming and elaborate process, but a necessary one. Initiating a survey is a very costly operation – having it stopped mid-way because a permit was not obtained is hard to justify.  

Either as part of the permitting process, or in preparation for the survey, a research design needs to be devised. At this point the tools and personnel have been selected, and the overall survey approach determined. Details, including drawing our survey lines, coordinating output products (e.g. who will author the final report and form will it take?; who will process data and according to what time-scale?), and procedures for verifying identified targets (e.g. will the survey operation pause to launch an ROV on each particular target upon discovery or will the team return to a number of related targets upon completion of the survey phase?) need to be put in writing. At this point, it is important to start thinking about details such as ensuring compatibility between systems. For example, the personnel on the bridge need to be able to see on a monitor what the scientific party sees in order to navigate along the survey lines.

On this more practical note, being prepared in advance is among the most effective means of dealing with potential problems. Logistically, make sure to allow for enough time for mobilization and de-mobilization, and ensure that equipment, in whatever form it arrives, can clear customs, be transported, enter and be stored in the port facility, and be loaded onto the research vessel. It is better to give crew and science party an additional day to acclimate, rather than risk missed connections. Create a contact list to facilitate the exchange of information. Most importantly, whenever possible develop a timeline allowing for, depending on the area and time of year, 20-30% down time due to weather, technical difficulties, re-supplying efforts, and other unexpected issues. 

In preparation for onboard operations, it is vital to develop standard operating procedures, shift schedules (often operations run around the clock), and accident response plans. Small details such as how data will be recorded and organized, to larger concerns such as re-fueling stops should all be considered and addressed ahead of time, and back-up plans put in place. It is almost certain that the original plan will be revised during the course of an operation; however, it is much easier to deviate from an existing plan than to come up with one during a moment of crisis.

In discussing moments of crisis, it is not often that an operation goes through without a hitch. Equipment will break down and software will fail. The best approach is to have alternatives – wherever and whenever possible, double-up on important equipment and don’t depend on a single person for a critical task, although this is not always possible for the primary survey tools and key personnel. In such cases, try to include personnel trained in setting up equipment or correcting mechanical difficulties, and make sure you have technical support contact numbers for the manufacturers of tools. It is best to locate regional offices in advance in case a spare part needs to be obtained during the survey. Try to envision scenarios that would jeopardize the mission, along with the best ways to address them. Almost without fail, you will be surprised by one or more difficulties that arise; having a plan to deal with similar foreseen situations, however, will facilitate dealing with the unexpected ones.

An animation illustrating a multibeam echo sounder survey.
Click on the image to begin the animation.

Image courtesy of the National Oceanic and Atmospheric Administration

 

IV. IN THE FIELD AND POST-PROCESSING

It may be surprising that this section of the lecture is shorter than some of the other parts. However, if preparations have been thorough, at this point it is simply time to execute the plan. As mentioned above, it is very likely that during this phase one will stray from the original plan due to unexpected difficulties, changes in priorities, availability of personnel, or the failure of that one piece of equipment that cannot be replaced or repaired. However, forethought should alleviate these difficulties to the extent possible.

During the survey it is important to assign clear duties and responsibilities among the team members. Data collection is a tedious process, however rewarding, and usually requires rotating shifts. It is important to maintain continuity as the shifts change, usually accomplished through documenting actions and decisions throughout the survey, and establishing shift leaders who communicate frequently. Coordination meetings usually occur twice a day and informally over meals, or when decisions have to be made.

It is common practice to complete data collection through surveying parallel lines, appropriately spaced apart depending on the capabilities of the selected remote sensing tools. The orientation of those lines may depend on the coastline, the survey area, or the selected tools (e.g. magnetometers are better able to distinguish anomalies if flown N-S). Establishing an accurate position is critical both in ensuring proper navigation and data overlap, but also in allowing for subsequent target verification. Whether utilizing a dive team or an ROV, being able to efficiently re-locate a position is as important as locating a target in the first place. It is more efficient to complete the survey phase and then switch to target assessment rather than to change operating modes every time a potential target is discovered.

Pre-planned navigation lines indicating planned survey direction, overlaid on top of a multibeam bathymetry map of the sea-floor.

Image courtesy of the Danaos Project

 

 

Animation of a multi-beam echo sounder and side scan sonar survey. Click on the image to start the animation.

Image courtesy of the U.S. Naval Oceanographic Office

 

Data processing and interpretation is on par with effective data collection in terms of importance. On-board processing capabilities allow for archaeologists or technicians to interpret data, assign target priorities, and make critical decisions in the field. One key difference with traditional archaeological projects is that a time-stamp is what ties everything together. Video, remote sensing information, navigation, and other data feeds all need to be tied together to a single timing mechanism, as one needs to make sense of the relationships that exist. Traditionally, the location of an artifact or site, sometimes referred to as provenience, was what this inter-relation was based on. Not all data processing can be accomplished in the field as it is a time-consuming phase of the operation. It is therefore of great importance to collect as much data as possible and record decisions in detail, so that the individual tasked with drafting the final project report is capable of compiling it months later. Preferably, documentation should be so thorough that one could draft the subsequent report even without having been in the field.

Target assessment may or may not be part of any particular survey. While certain projects require immediate results and therefore depend on an effective target identification phase, other projects are multi-phase or multi-year endeavors whereby each project builds upon the success of others. If included in the research design, the target phase usually follows the survey phase. However, this is not always the case, nor should it be – the primary objective is to maximize the use of field time. If, for some reason, a ship cannot survey at night due to limited crew numbers, utilize that time to launch an ROV and assess targets. If poor weather is expected to paralyze the last days of an operation, assess priorities, and if needed, switch gears. A project with an important discovery will likely be in a better position to raise the interest and funding necessary for future operations than a project with a promising series of targets. What is important, however, is not to lose sight of the main project objective. Finally, with all the emphasis on planning, the ability to be flexible and develop innovative solutions should not be overlooked.

The final report mentioned above is usually one of a number of products that follow a survey; preliminary reports, permit-related reports, scientific publications, newspaper and popular magazine articles, and websites are all common. Analysis and interpretation of the collected data can take a number of months, and it is often in the post-processing phase that additional discoveries are made. Archaeology is therefore a field that requires patience of practitioners and, as this brief account of underwater surveying illustrates, attention to detail. However, the field’s ability to bring together the most historical elements of our past with the most advanced technology is a unique trait that captures the imagination.
 

References

Figures 4.9 and 4.11 are copyrighted by the Nautical Archaeology Society and reproduced from:

Bowens, Amanda. 2009. Underwater Archaeology: The NAS Guide to Principles and Practice. Portsmouth, U.K., Blackwell Publishing.

USNA Blackboard for this week's discussion and quiz.

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Last revision 1/25/2011