Excavation and Recovery Techniques for Underwater Wreck Sites

Flashcards covering survey methods, excavation tools, recording techniques, lift bag calculations, and archaeological principles of interpretation for underwater wreck sites.

What is the general definition of a survey in underwater archaeology?

The general term for various methods of recording, including visual inspection, measurement, or graphic means to document the dimensions, orientation, and items within a site.

Describe the 'Circular Search' method used by divers to locate a site.

A buddy pair swims around a central mark holding a rope or tape measure, completing a full circle before moving out a predetermined distance to increase the radius for the next circle.

What is a 'jackstay' in the context of site surveying?

A fixed reference line held in place by weights used for searching; it can be a single line or a movable system consisting of two parallel fixed lines and one perpendicular movable line.

What is a 'baseline'?

A control point consisting of a fixed, straight, and taut line attached securely to two datum points, serving as the basis for all site measurements.

Define the 'Baseline Offset' surveying technique.

A method where measurements are taken at $90^\text{o}$ from the baseline to the specific point being measured.

What is the difference between Trilateration and Triangulation in underwater archaeology?

Trilateration uses three or four measurements from the baseline to a point, while Triangulation uses two compass measurements from control points.

Why is Triangulation often ineffective on shipwreck sites?

Compasses are easily affected by iron, which is commonly found on wreck sites.

According to the transcript, when should the removal of artefacts be considered?

As a last resort, and only if the artefacts are at risk of further degradation.

What is 'Probing' in underwater excavation?

The use of narrow, hollow tubes that push down water or air at low pressure to loosen sand and develop contact with buried structures.

What is 'Hand Fanning'?

The use of a diver's hand to fan away loose sediment from the surface of an artefact.

What are water dredges and airlift dredges used for?

Narrow hoses sent to the bottom of the ocean to suck, remove, and deposit sediment covering a site in another location.

Under what circumstances are explosives used in excavation?

Rarely, in specific controlled situations to loosen very hard materials or to map areas using shockwaves.

How is a 'Propulsion Vehicle' (underwater scooter) used for excavation?

The diver turns the scooter away to use the fan-like end to blow away sand from large items.

What are the three types of information a recording system needs to capture?

Spatial information (positions), descriptive information (properties), and temporal information (what happened and when).

Define 'Photomosaic'.

A large-scale detailed picture or map built up by combining photographs of smaller areas.

How is 'Photogrammetry' used in recording?

It creates 3D models from overlapping 2D images by analyzing common points to calculate depth, shape, and position.

How does a lift bag function for recovery?

A bladder is attached to an object and filled with air until neutral buoyancy is achieved; divers guide it up, using a purge valve to release air as volume increases due to decreasing pressure.

What is the formula for 'Mass of Water Displaced'?

$$\text{Mass of Water Displaced} = \text{density} \times \text{volume}$$

How is 'Apparent Mass' calculated?

$$\text{Apparent mass} = \text{Mass in Air} - \text{Mass of Water Displaced}$$

What is the assumed density of seawater for calculation problems in the transcript?

$$1.03\,kg/L$$

Calculate the air required to lift a $600\,kg$ anchor that displaces $100\,L$ in seawater ($1.03\,kg/L$).

$$482.5\,L$$

Calculate the air volume needed for an artefact weighing $200\,kg$ with a volume of $100\,L$ in seawater ($1.03\,kg/L$).

$$94.2\,L$$

What is the difference between Absolute Dating and Relative Dating?

Absolute Dating associates an object with a specific date or range, while Relative Dating determines if an object is earlier or later than another.

Define 'Stratigraphy' in the context of shipwrecks.

The principle that lower layers were deposited before upper layers; for shipwrecks, it is useful for hypothetical reconstruction of the relative placing between artefacts.

What is 'Typology'?

The study of the shape and development of objects to place similar items in chronological order based on style (simple to complex).

What is 'Seriation'?

A relative chronological method where the quantity of similar objects found on a site may indicate the time period they were used.