By Alex Kilpa Maritime Archaeology Masters Student
So you’ve done all your homework, you’ve consulted all the relevant historical documentation and you know its approximate location, but where exactly is that illusive shipwreck? This is a typical scenario that maritime archaeologists are confronted with when trying to locate cultural materials deposited in an underwater environment. One remote sensing device that can assist in detecting shipwrecks and other cultural materials is the magnetometer. In essence, a magnetometer is an instrument that measures “magnetic force field intensity and direction”. This is done at the sensor data collection point where the measurements are taken. (Hine 1968:125; Ripka 2001:xvii).
What Magnetometers Detect
As many of you would be aware the earth has a slight magnetic field. The presence of ferromagnetic materials such as iron influences this field and produces what is referred to as an anomaly, which is the variation in the magnetic field caused by a magnetic target. These variations in magnetic field strength are measured in units of nano Tesla (nT) (Camidge et al 2010:10). What a magnetometer does is measure the extent of this anomaly, producing data that is recorded on a console similar to the one shown in figure 1. This raw data can then be used to produce a contour diagram highlighting the position of where the anomaly was found on the seabed (Green 2006).
Figure 1. A magnetometer recording console from the 1960s used for maritime archaeological purposes. Similar apparatus are still in operation today (Hall 1966:32).
Types of Magnetometers
There are many different types of magnetometers available commercially.
The proton magnetometer (see figure 2) was once highly regarded for conducting magnetometer surveys, but this form of technology has largely been superseded by more modern types. Equipment such as the Overhauser and caesium-vapour magnetometers can generate a much higher sample rate than a proton version, producing a higher density of data. The higher the density of data, the more accurate the sampled values will represent the actual variations in the magnetic field strength (Camidge et al. 2010:23).
Figure 2. A proton magnetometer (detector ‘fish’) designed for underwater archaeological survey work (Hall 1966:34).
Calibration of Instrumentation
An important aspect of any archaeological survey using a magnetometer is the calibration of the instrumentation. An approximate estimate of the detection distance is required as it will affect the planning of the search. In particular, consideration should be given to the length of towing cable, speed of the towing boat and width of search lanes (Hall 1966:36).
It is also desirable that some estimate of the weight and size of the object should also be taken into consideration to optimize the magnetometers detection potential. Bevan (1999) provides a comprehensive list of the magnetic properties associated with archaeological materials that can help estimate the size of a magnetic anomaly that ‘might’ be expected from a feature and assist in interpreting quantitative data in the post magnetic survey process, “by converting magnetic moments into estimates of the objects mass or volume”.
Although maritime archaeologists have used magnetometers primarily to locate iron shipwrecks, they can also be used indirectly to locate wooden vessels as well. This is done by setting the magnetometer search parameters to ferrous materials that may be associated with a wreck as such anchors, cannon or even amphora (Hall 1966:26). The latter of these may seem strange but it is believed that ceramics develop their “thermo-remnant magnetic properties” when the “magnetite bearing clay is heated to a relatively high temperature and cooled in the presence of the earth’s magnetic field” (Green 2004:63; Breiner 1973:46). Also, it should not be taken for granted that all ‘iron based’ materials are detectable with this form of instrumentation. Some grades of stainless steel such as the austenitic 300 series for example are non-magnetic and therefore not detectable by a magnetometer (Selwyn 2004:98).
Operating from a Boat
Historically, the most common method for conducting magnetometer surveys at sea has been by boat with a proton magnetometer in tow. When conducting magnetometer surveys by this method it is important to have the ‘detector fish’ some distance away from the pilot vessel, otherwise it is likely that the noise generated from the boat’s engines will interfere with the data collected.
Another critical factor that needs to be taken into consideration is the position of the Global Positioning System (GPS). If for example a GPS is mounted on a boat and the layback is 200 meters (as in figure 3), all GPS readings will have to be adjusted by that amount to reflect the true position of the anomaly.
Figure 3. Schematic relationship between the pilot boat, magnetometer (detector fish) and GPS (Green 2004:69).
Close-Plot Magnetometer Surveys
Although the magnetometer has frequently been used to locate iron shipwrecks from distance, they have on occasion been utilised underwater for close proximity survey work as well. An example of this is the Kyrenia shipwreck, a vessel found off the north coast of Cyprus and dating from the fourth century B.C. The two instruments available for this survey were the Proton Magnetometer and an underwater metal detector developed by the Oxford Archaeology Research Laboratory (Green et al. 1966:48).
Although the magnetometer and a metal detector may seem to do the same thing they work on different principles. Their operational difference being that a metal detector measures conductivity, whereas a magnetometer measures magnetic force.The Kyrenia wreck site was first surveyed with the magnetometer to indicate the overall distribution of ferrous metals present and then resurveyed with a metal detector to get a more precise location of both ferrous and non-ferrous metals. By comparing the survey results obtained by both instruments it was found to be possible to distinguish between metal types. This method led to the discovery of the lead sheathing associated with the ship’s hull (Green 2004:162).
Aerial Magnetometer Surveys
Magnetometers have also been used to survey underwater sites from the air. The benefits of conducting aerial surveys in this manner are that large areas can be surveyed quickly and the noise interference problems associated with towing magnetometers by boat are eliminated. A recent survey of the Rottnest deepwater graveyard that covered 338km linear km took approximately 3.5 hours by air and resulted in locating 34 vessels including HMAS Derwent a ship scuttled by the Royal Australian Navy as part of a training exercise in 1994 (Green 2001; Green 2004:69-70).
Figure 4 shows a contour map of the Rottnest Island ship graveyard. Located approximately 20km west of Rottnest this area was used for 75 years to dispose of old unwanted vessels that had no commercial value. It was also known as a disposal site for munitions, military vehicles, aircraft, and even a Dutch submarine that were disposed of at the end of the Second World War (Green 2011).
Figure 4. Contour magnetic map generated from an aerial magnetometer survey of the Rottnest deepwater ship graveyard off the coast of Western Australia. The dark, close knitted contours indicate anomalies (Green 2004:67).
Understanding the principles behind the use of magnetometers and their operations may seem like a daunting task at first, especially if you don’t come from a pure science background. The selected bibliography attached to this blog provides some easy to read papers that may assist in understanding how magnetometers have been applied to maritime archaeology underwater and the basics relating to their use.
References and Bibliography
2010 Test Excavation of Magnetic and Radar Anomalies, Bunbury Archaeological Brief and Research Design. Report – Department of Maritime Archaeology, Western Australian Museum No. 265. Electronic document, http://www.museum.wa.gov.au/sites/default/files/No.%20265%20Bunbury%20Excavation%20Brief%20.pdf, accessed August 12, 2011.
Bevan, Bruce W.
1999 The Magnetic Properties of Archaeological Materials, Electronic document,
http://www.geometrics.com/geometrics-products/geometrics- magnetometers/magnetometer-information-and-case-studies/, accessed August 14, 2011.
1973 Applications Manual for Portable Magnetometers. Geometrics, San Jose.
2011 Magnetic Search in the Marine Environment. Electronic document,
http://www.geometrics.com/geometrics-products/geometrics- magnetometers/magnetometer-information-and-case-studies/, accessed August 12, 2011.
Camidge, Kevin., Peter Holt, Charles Johns, Luke Randall, and Armin Schmidt
2010 Developing Magnetometer Techniques to Identify Submerged
Archaeological Sites Theoretical Study Report. Electronic document,
http://www.cismas.org.uk/docs/marine_magnetometers_theory_study.pdf, accessed August 12, 2011.
Green, Jeremy N., Edward T. Hall, and Michael L. Katzev
1967 Survey of a Greek Shipwreck of Kyrenia, Cyprus. Archaeometry 10:47-56.
Green, Jeremy N.
2001 Shallow Water Search Options for HMAS Sydney/HKS Kormoran. Report – Department of Maritime Archaeology, Western Australian Museum No. 265.
Electronic document, http://www.museum.wa.gov.au/sites/default/files/No.%20162%20HMAS%20Sydney.pdf, accessed August 12, 2011.
Green, Jeremy N.
2004 Maritime Archaeology a Technical Handbook. 2nd ed. Elsevier Academic Press, London.
Green, Jeremy N.
2006 Magnetometer How to do it Manual No. 1: How to Create a Contour Plot. Report – Department of Maritime Archaeology Western Australian Museum, No. 254. Electronic document, http://www.museum.wa.gov.au/sites/default/files/No.%20254%20Mag%20Contour%20Plot.pdf, accessed August 12, 2011.
Green Jeremy N.
2011 The Rottnest deepwater graveyard. Electronic document,
http://www.museum.wa.gov.au/about/latest-news/rottnest-deepwater-graveyard, accessed August 20, 2011.
Hall, Edward T.
1966 The Use of the Proton Magnetometer in Underwater Archaeology. Archaeometry 9: 32–44.
Hall, Edward T.
1970 A Symposium on the Impact of the Natural Sciences on Archaeology. Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences 269:1193:121-124.
1968 Magnetic Compasses and Magnetometers. University of Toronto Press, Toronto.
2001 Magnetic Sensors and Magnetometers. Artech House Inc, Norwood.
2004 Metals and Corrosion: A Handbook for the Conservation Professional. CMA, Toronto.