It can be difficult to locate ancient remains, often even more challenging in water than on land. Of course, searching large areas in dark and cloudy waters takes quite a long time for a maritime archaeologist. But thanks to modern technology, vast swathes of water can be searched faster and more efficiently than ever.
Dive down and document
When a relic or find of any kind is examined, the job must be well planned and effective – the time under the surface is limited, after all.
Once down at the seafloor, archaeologists photograph or film objects using different kinds of cameras. They measure, record and sketch environments and objects on water-resistant drawing film attached to a drawing slate. A pencil works fine.
It’s rare for a maritime archaeologist to bring objects or finds up to land. When it does happen, it’s in order to document the find on land and then put it back – where it belongs. Samples, on the other hand, can be brought up and used when determining the age of different objects.
ROVs – A diving aid
A remotely operated vehicle, or ROV, is a small robot equipped with a lamp, film camera, GPS and grapple arm. An ROV has virtually unlimited diving time and is invaluable in places where it is dangerous to dive or very deep.
Archaeologists can control the robot from a boat or on land, and see images and film footage that the ROV sends up via a cable. Simpler tasks can be done using the grapple arm, but it is a bit clumsy and doesn’t work so well when the job involves taking samples taken or handling sensitive materials and fragile objects. An experienced diving archaeologist is also better at assessing and evaluating remains directly on site.
Most of the tools and instruments used today measure in different ways how long it takes for a sound wave to bounce off the bottom of the sea or against an object. The search instruments utilise sound waves that spread faster underwater than in air, as opposed to light and radio waves.
Sonar, relatively simple and inexpensive
With sonar, a beam of audible signals is sent straight down and reflected off the bottom so that the distance can be measured. A computer converts the signals and produces a profile of the depth – a digital image that can be displayed on a computer screen.
This is a simple and fairly affordable method, but sonar only measures straight down. You have to pass right over a wreck for it to be registered. It takes a long time to search a water area with sonar, and it can be tricky to locate wrecks that barely stick up above the bottom.
Side scan sonar shows contours of the seafloor
Side scan sonar can send and receive signals both downwards and off to the sides. With the values it records, it is possible to create a digital image of the contours of the sea bottom where whatever sticks up is visible. The instrument is sometimes called side-vision sonar.
Side scan sonar is often used to create draft maps of various kinds. In maritime archaeology, the instrument is often towed after the survey vessel while emitting and receiving audible signals. The instrument records strips of the seafloor that can be a few hundred metres wide.
It serves as an effective instrument for searching large water surfaces. The vessel can travel at about four knots, and the sonar can usually see up to about 300 metres off to each side. The best results when using side scan sonar are achieved when the seafloor is firm and relatively flat.
Multibeam for creating images in 3D
Multibeam emits a large number of sound waves simultaneously in the form of a cone. With the help of the information it records, it is possible to create an almost three-dimensional image of the bottom which can be rotated and flipped.
Multibeam is time-efficient because the survey vessel can maintain a higher speed. It also covers a larger search width than a side scan sonar. Although the instrument is mainly used for measuring water depth, it’s also very useful for obtaining images of ship remains that protrude above the bottom. But since it is not that delicate, things that just rise up a bit from the bottom can be easily overlooked.
Magnetometers – efficient but limited
A magnetometer measures the natural magnetic field on the ocean or sea floor. It can be towed after a boat, and when the instrument passes big iron objects, the magnetic field is disturbed. This interference is recorded by the magnetometer and displayed on a printer or screen.
The magnetometer is ideal for finding covered wrecks that other instruments have difficulty recording. It’s useful for locating modern ships, but the benefits are limited if the goal is to find older wooden vessels.
Metal detectors require permission
Metal detectors work well in water and can be used by divers looking for objects in demarcated areas. When the diver swims with the detector above a metal object, a disturbance is recorded in the geomagnetic field. The interference can be transmitted as a beeping tone in the diver’s headphones.
Permission from the county administrative board is needed to use certain metal detectors. Permits are issued for well-defined areas for a limited time only, and the restriction applies both on land and under the water.
Sounding lead and probe – older instruments
A sounding lead is a long, marked line that is weighted with a lead at one end. They were used in the past when measuring water depth. There are still bodies of water in Swedish charts whose depth measurements are based on long-past sea-depth measurements using sounding leads.
The classic lead had a small hollow in the bottom which could be filled with wax. While the depth was being measured, a sample of the bottom material could be taken. Anders Franzén used a home-made probe when he finally, after many years of research, located the huge wooden wreck that turned out to be the Vasa ship.
Dive searches – in circles, grids or along lines
Underwater searches carried out by divers are usually limited to smaller areas. The diver searches along lines laid out on the bottom (line search), in circles around a reference point (circle search), or in a grid pattern on the bottom (grid search). The appearance, size, visibility conditions and number of divers all influence affect which technique is best.
For larger areas, towboards and sleds can be useful. The devices are towed with the diver on board at a few knots’ speed behind a boat. The diver regulates the depth. Today, sonar or echo sounding is most often used instead.
Aerial photos, satellite photos
Shallow areas are often difficult to navigate for the ships or boats needed to map the seafloor using sonar methods. In such cases, aerial and satellite photos can be used to search for sunken ships and other remains like docks, ballast piles and pile barriers.
Today, images that are openly accessible to the general public are also used to take inventory of maritime remains, via services like Eniro and Google Maps. They often contain aerial images produced by the Swedish mapping, cadastral and land registration authority.
Sub-bottom profiles, scanning in sediment layers
A penetrating sonar, called a sub-bottom profile, is specially developed to detect objects down in the bottom sediments. So do conventional and side scan sonar, but only to a limited extent.
A sub-bottom profile works the same way as sonar but uses a much lower frequency, usually between 1-7 kHz. If the instrument is connected to a side scan sonar, the information from both can be presented in the same image. Like the sonar and magnetometer, the bottom-penetrating sonar is usually towed behind a boat.