3.5 Underwater excavation techniques

Underwater excavation techniques are crucial for uncovering submerged archaeological sites. From shipwrecks to sunken cities, these methods allow researchers to explore and document human history hidden beneath the waves.

Archaeologists use specialized equipment and strategies to overcome challenges like limited visibility and water pressure. Careful mapping, artifact recovery, and conservation are essential for preserving underwater cultural heritage and piecing together our maritime past.

Types of underwater sites

• Underwater archaeological sites encompass a wide range of human activity and history submerged beneath the water's surface
• Studying these sites allows archaeologists to learn about past maritime cultures, trade networks, and patterns of human settlement near bodies of water
• Types of sites can be categorized based on their original function, depth beneath the surface, and the characteristics of the surrounding aquatic environment

Shipwrecks vs submerged settlements

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• Shipwreck sites are locations where sailing vessels have sunk due to storms, navigational errors, warfare, or other causes, preserving a snapshot of the ship's construction, cargo, and crew at the time of sinking
• Submerged settlements are sites where past human habitation is now underwater due to rising sea levels, coastal subsidence, or intentional flooding (reservoir creation), offering insights into the daily lives and architecture of past cultures
• While shipwrecks often represent a single catastrophic event, submerged settlements can show long-term patterns of human adaptation to changing coastal environments over time

Shallow vs deep water sites

• Shallow underwater sites, typically in waters less than 30 meters deep, are more accessible to divers and can often be investigated using traditional archaeological methods adapted for submerged contexts
• Deep water sites, such as shipwrecks in the abyssal plains of the ocean, require the use of remote sensing technologies (sonar, ROVs) and specialized diving equipment for manned exploration (saturation diving, submersibles)
• The depth of an underwater site affects the logistics, costs, and risks associated with archaeological investigation, as well as the degree of preservation of the site's features and artifacts

Freshwater vs saltwater environments

• Freshwater underwater sites, such as those in lakes, rivers, or cenotes, are generally characterized by better preservation of organic materials (wood, textiles) due to the absence of marine organisms and lower salinity
• Saltwater sites, located in seas and oceans, often have poorer preservation of organics but can exhibit excellent preservation of metal objects (coins, cannons) due to the formation of protective concretions
• The chemical composition and biological activity within the surrounding water heavily influence the deterioration and conservation challenges associated with artifacts recovered from underwater sites

Challenges of underwater archaeology

• Conducting archaeological investigations underwater presents a unique set of challenges that require specialized equipment, techniques, and training to overcome
• These challenges stem from the physical properties of water, the limitations of human physiology, and the dynamic nature of aquatic environments
• Addressing these challenges is crucial for ensuring the safety of archaeologists, the integrity of the archaeological record, and the quality of the data recovered from underwater sites

Limited visibility in water

• Water clarity is often reduced due to suspended sediment, plankton, and dissolved organic matter, making it difficult for archaeologists to visually inspect and document underwater sites
• Visibility can vary depending on factors such as depth, water chemistry, and seasonal changes in biological activity, requiring archaeologists to adapt their survey and excavation strategies accordingly
• Techniques for improving visibility include the use of high-intensity underwater lighting, water dredges to remove suspended sediment, and photogrammetry to create 3D models of sites in low-visibility conditions

Effects of water pressure

• Water pressure increases with depth, exerting significant physical forces on both archaeologists and the equipment they use underwater
• At depths greater than 30 meters, the effects of nitrogen narcosis can impair cognitive function and motor skills, making it necessary for archaeologists to use specialized breathing gas mixtures (trimix, heliox) or work from submersibles
• Water pressure also affects the buoyancy and handling characteristics of excavation tools and recording equipment, requiring modifications to ensure their functionality and precision underwater

Preservation of artifacts underwater

• The low oxygen levels, constant temperatures, and reduced biological activity in many underwater environments can lead to exceptional preservation of organic materials that would normally deteriorate quickly on land (wood, leather, textiles)
• However, the presence of saltwater and marine organisms can also cause accelerated corrosion of metal artifacts and the degradation of ceramics and stone through physical and chemical processes
• Archaeologists must carefully consider the preservation conditions of underwater sites when planning excavations and artifact recovery, as well as develop appropriate conservation strategies for treating waterlogged finds

Underwater survey methods

• Underwater archaeological surveys aim to locate, map, and characterize submerged sites without causing disturbance to the archaeological record
• These non-invasive methods rely on remote sensing technologies that can penetrate through the water column and provide detailed information about the seafloor and sub-bottom features
• Advances in underwater survey methods have greatly expanded archaeologists' ability to discover and study underwater sites, particularly in deep water environments beyond the reach of conventional diving

Side-scan sonar for site mapping

• Side-scan sonar systems emit acoustic pulses from a towfish towed behind a survey vessel, creating detailed images of the seafloor based on the strength and timing of the returning echoes
• These sonar images, or sonographs, can reveal the presence of shipwrecks, submerged structures, and other anomalies on the seafloor, as well as provide information about the surrounding sediment type and bedforms
• Side-scan sonar is particularly useful for mapping large areas of the seafloor efficiently and identifying targets for more detailed investigation using other methods (visual inspection, excavation)

Magnetometers for detecting metal objects

• Magnetometers are instruments that measure the strength and direction of magnetic fields, allowing archaeologists to detect the presence of ferrous metal objects (iron, steel) buried beneath the seafloor
• In underwater archaeology, magnetometers are often towed behind a survey vessel in a grid pattern, creating a map of magnetic anomalies that can indicate the location of shipwrecks, anchors, or other metal artifacts
• Magnetometer surveys are often used in conjunction with side-scan sonar to provide complementary data about the nature and extent of underwater archaeological sites

Sub-bottom profilers for sediment layers

• Sub-bottom profilers use low-frequency acoustic pulses to penetrate through the seafloor sediments, creating cross-sectional images of the sub-bottom stratigraphy and buried features
• These instruments can detect the presence of buried shipwrecks, submerged landscapes, and other archaeological features that may be obscured by overlying sediments
• Sub-bottom profiler data can help archaeologists understand the geological context of underwater sites, reconstruct past sea-level changes, and guide targeted excavations to expose buried artifacts and structures

Excavation techniques in water

• Underwater excavation involves the systematic removal of sediments to expose and record archaeological features and artifacts in their original context
• Due to the challenges of working in an aquatic environment, underwater excavation techniques must be adapted to ensure the safety of divers, the stability of the site, and the precise documentation of the archaeological record
• The choice of excavation technique depends on factors such as the depth of the site, the nature of the sediments, the fragility of the artifacts, and the research questions being addressed

Dredging vs airlift excavation

• Dredging involves the use of a suction pump or propeller to remove sediments from the excavation area, allowing archaeologists to expose buried features and artifacts
• Airlift excavation uses a compressed air jet to create a Venturi effect, lifting sediments and small artifacts up a pipe to a screening platform on the surface
• While dredging can quickly remove large volumes of sediment, it can also cause damage to fragile artifacts and lacks the precision of airlift excavation for recovering small finds

Gridding and mapping underwater

• Underwater excavations are typically conducted within a grid system of metal frames or lines, providing a spatial reference for recording the positions of artifacts and features
• Mapping underwater involves the use of measuring tapes, rulers, and waterproof slates to record the dimensions and orientations of archaeological remains within the grid
• More advanced mapping techniques include the use of underwater photogrammetry to create 3D models of the excavation area and the use of acoustic positioning systems to track the movements of divers and artifacts

Recording artifact positions in 3D

• Accurate recording of artifact positions is crucial for understanding the spatial relationships and depositional context of underwater archaeological sites
• Traditional methods involve measuring the depth, distance, and angle of artifacts relative to fixed datums within the excavation grid, allowing for the creation of two-dimensional site plans
• Advances in underwater photogrammetry and structure-from-motion (SfM) software now enable archaeologists to create high-resolution 3D models of excavation areas, capturing the positions of artifacts and features in three dimensions

Diving for underwater archaeology

• Diving is an essential skill for underwater archaeologists, allowing them to directly access, observe, and excavate submerged sites
• The type of diving equipment and techniques used depends on factors such as the depth of the site, the duration of the dive, and the specific tasks being performed
• Proper training, safety protocols, and communication systems are crucial for ensuring the well-being of diving archaeologists and the success of underwater fieldwork

SCUBA vs surface-supplied air

• SCUBA (Self-Contained Underwater Breathing Apparatus) diving involves the use of portable air tanks and regulators, providing divers with greater mobility and flexibility for exploring underwater sites
• Surface-supplied air systems, also known as hookah diving, provide air to divers through a hose connected to a compressor on the surface, allowing for longer dive times and reducing the risk of equipment failure
• The choice between SCUBA and surface-supplied air depends on factors such as the depth and duration of the dive, the availability of support personnel, and the specific requirements of the archaeological task

Safety considerations for diving archaeologists

• Diving archaeologists must be properly trained and certified in the use of diving equipment, safety protocols, and emergency procedures
• Potential hazards include decompression sickness (the bends), nitrogen narcosis, hypothermia, and entanglement in underwater debris or fishing gear
• Safety measures include the use of dive computers to monitor depth and bottom time, adherence to decompression schedules, the use of redundant air sources and cutting tools, and the presence of trained support personnel on the surface

Communication between divers and surface

• Effective communication between diving archaeologists and support personnel on the surface is essential for coordinating tasks, monitoring safety, and relaying important information
• Traditional methods of underwater communication include the use of hand signals, slates, and line pulls to convey basic messages
• More advanced communication systems include the use of underwater acoustic modems, through-water radio systems, and full-face masks with integrated microphones and speakers

Artifact recovery and conservation

• The recovery and conservation of artifacts from underwater archaeological sites require specialized techniques and facilities to ensure their long-term preservation and study
• Waterlogged artifacts, particularly organic materials, are highly fragile and can easily be damaged during recovery and transport to the surface
• Conservation strategies must take into account the unique chemical and physical properties of waterlogged artifacts, as well as the specific environmental conditions of the underwater site

Raising delicate artifacts to surface

• Delicate artifacts, such as waterlogged wood, leather, and textiles, must be carefully supported and protected during recovery to prevent damage from handling and changes in pressure
• Techniques for raising delicate artifacts include the use of lifting frames, support bandages, and rigid containers to minimize physical stress during ascent
• On the surface, recovered artifacts must be kept in water or humidity-controlled environments to prevent rapid drying and shrinkage

Desalination of waterlogged objects

• Waterlogged artifacts, particularly those from saltwater environments, must undergo a process of desalination to remove harmful soluble salts that can cause deterioration over time
• Desalination methods include soaking artifacts in fresh water baths of increasing purity, or using more advanced techniques such as electrolysis or reverse osmosis
• The desalination process can take months or even years, depending on the size and material of the artifact, and requires careful monitoring to ensure the stability of the object

Special storage requirements for underwater finds

• Conserved waterlogged artifacts require specialized storage conditions to ensure their long-term preservation and accessibility for future study
• Organic materials, such as wood and leather, must be kept in humidity-controlled environments to prevent shrinkage, warping, and cracking
• Metal artifacts may require storage in inert atmospheres or corrosion-inhibiting solutions to prevent further deterioration
• Proper documentation, labeling, and cataloging of conserved artifacts are essential for maintaining their research value and facilitating their study by future generations of archaeologists

Interpreting underwater sites

• The interpretation of underwater archaeological sites involves the analysis and synthesis of data from multiple sources, including artifacts, features, environmental context, and historical records
• Underwater sites can provide unique insights into past human activities, adaptations, and interactions with the aquatic environment
• Interpreting these sites requires a multidisciplinary approach, drawing on expertise from fields such as archaeology, history, geology, and marine biology

Reconstructing shipwreck events from evidence

• Shipwreck sites contain a wealth of evidence that can be used to reconstruct the events leading up to, during, and after the sinking of the vessel
• Analysis of the ship's construction, cargo, and artifacts can provide information about the vessel's origin, purpose, and crew, as well as the circumstances of its final voyage
• Interpreting the spatial distribution of artifacts and features on the seafloor can help archaeologists understand the sequence of events during the sinking and the post-depositional processes that have affected the site over time

Inferring trade routes from cargo

• The cargo recovered from shipwreck sites can provide valuable evidence for reconstructing past trade routes, economic networks, and cultural interactions
• Analysis of the types, quantities, and origins of trade goods can reveal patterns of exchange between different regions and cultures, as well as the economic and social factors that influenced these networks
• Combining cargo evidence with historical records and other archaeological data can help create a more comprehensive picture of past maritime trade and its role in shaping cultural, political, and economic landscapes

Understanding submerged settlement patterns

• Submerged settlement sites, such as flooded coastal villages or inundated city quarters, can provide insights into past human adaptations to changing sea levels and coastal environments
• Analysis of the spatial organization, architecture, and material culture of these sites can reveal patterns of social organization, subsistence strategies, and environmental exploitation
• Interpreting submerged settlements requires an understanding of the complex interplay between human activities, coastal geomorphology, and sea-level change over time, as well as the use of interdisciplinary approaches to reconstruct past landscapes and human-environment interactions

Legal aspects of underwater archaeology

• Underwater archaeological sites are subject to a complex web of legal frameworks and international agreements that govern their protection, management, and investigation
• These legal aspects reflect the competing interests of various stakeholders, including archaeologists, governments, private companies, and the general public
• Navigating the legal landscape of underwater archaeology requires an understanding of the relevant laws, treaties, and ethical principles that apply to specific sites and situations

Ownership of shipwrecks in international waters

• The ownership and jurisdiction of shipwrecks in international waters are governed by a combination of international law, maritime law, and the domestic laws of the flag states of the vessels involved
• In general, shipwrecks are considered the property of their original owners or their successors, unless they have been explicitly abandoned or transferred to another party
• The 2001 UNESCO Convention on the Protection of the Underwater Cultural Heritage provides a framework for the responsible management of shipwrecks and other underwater sites, emphasizing the need for in-situ preservation and scientific investigation

Protecting underwater cultural heritage sites

• Underwater cultural heritage sites, including shipwrecks and submerged settlements, are vulnerable to looting, treasure hunting, and damage from commercial activities such as fishing, dredging, and offshore development
• Legal protection measures for these sites can include the designation of marine protected areas, the implementation of permit systems for archaeological investigations, and the enforcement of penalties for unauthorized disturbance or removal of artifacts
• International cooperation and capacity-building efforts are essential for ensuring the effective protection and management of underwater cultural heritage sites, particularly in regions with limited resources and expertise

Collaborating with salvage companies and governments

• In some cases, underwater archaeologists may need to collaborate with commercial salvage companies or government agencies to investigate and manage underwater sites that are under threat or have already been disturbed
• These collaborations can be contentious, as they often involve balancing the interests of scientific research, cultural heritage preservation, and economic exploitation
• Successful collaborations require clear agreements on the roles, responsibilities, and expectations of all parties involved, as well as a shared commitment to ethical principles and best practices in underwater archaeology
• Building trust, transparency, and mutual understanding between archaeologists, salvage companies, and government officials is crucial for ensuring the responsible stewardship of underwater cultural heritage for future generations