The Application of Aerial Magnetometers inMaritime Archaeology

Magnetometers have been used since the 1950sfor terrestrial archaeological prospection.The first application of a magnetometer in a

marine environment was in 1965 in Turkey (Hall, 1966).The results were inconclusive because of equipmentproblems, but in October 1966, as part of an Anglo–Israeli survey of Akko Harbour, Hall located severalarchaeological targets including a wreck-site using aproton magnetometer (1966: 37). Since then, marineproton magnetometers have been widely used for thelocation of underwater cultural heritage (Arnold andClausen, 1975; Arnold, 1976, 1987). The proton mag-netometer suffers from a number of limitations: a slowcycle-rate that affects the speed of the survey; generalinstrument and electromagnetic noise affects the signal-to-noise ratio; and microphony, or noise generated bymechanical vibration of the cable, also decreases thesignal-to-noise ratio. As a result, the instrument is onlyusually able to resolve anomalies greater than about 10nano tesla (nT). Various attempts have been made touse an airborne proton magnetometer using helicopters(see Ingelman-Sundberg, 1978; Arnold, 1981). Theproton magnetometer, however, is difficult to use on anaircraft because of the slow cycle-rate and the speed atwhich the aircraft has to travel.

Today marine proton magnetometers have largelybeen replaced by optically pumped caesium vapourmagnetometers and Overhauser effect magnetometers,both of which provide greater sensitivity and muchfaster cycle-rates. Nevertheless, anyone who hascarried out a marine magnetic survey will be aware thatsuch surveys are fraught with logistic problems.Weather conditions are probably the most significantfactor that can delay survey work and tie up expensiveboats and equipment. In general, a survey vessel canonly operate at relatively slow speeds (8–12 knots), sothat surveys of large areas can be time consuming;again with financial implications. Navigation is also aproblem, as the ability to maintain a steady course andto run closely spaced parallel lanes is often difficult(Camidge et al., 2010).

In 2000, Prospero Productions, a Western Austra-lian TV documentary company, approached theDepartment of Maritime Archaeology of the WesternAustralian Museum with a suggestion of sponsoringnew maritime archaeological research projects. One ofthe projects that came out of this initiative was to testif it was possible to locate maritime archaeological sites

using the airborne magnetometer that is commonlyused for geological prospection and to compare theeffectiveness of this method with the marine magnet-ometer. Two different types of environment werechosen to test the system: shallow-water sites; anddeep-water sites.

Deep-water sitesThe Rottnest Deep Water Graveyard was chosen asthe deep-water site. It lies 20 nautical miles west of theport of Fremantle on the Western Australian coast in adepth between 80 and 100 m (Fig. 1). The area (No. 7)was designated under the Beaches, Fishing Grounds andSea Routes Act 1932 (Cwlth) as an area seven miles indiameter (Plunkett, 2003). This is where vessels thatwere to be scrapped or disposed of could be officiallydumped (Richards, 2008: 89). Historical research indi-cated that a total of 39 vessels were scuttled in theRottnest Graveyard between 1917 and 1994 (Garratt,1999). The 1917 date showed that the area was beingused well before the designation of the Act (Fig. 2), andsome vessels were dumped outside the Graveyard afterthe Act.

Of the vessels scuttled in the Graveyard, 31 have aknown tonnage in the range 50–2100 tonnes (Fig. 3),and the main types of vessel scuttled were hulks. Largequantities of military material were also dumped in theGraveyard under the Second World War Lend/LeaseAgreement, whereby the US lent equipment to AlliedNations on the condition that, at the end of the war, itwas either to be returned or destroyed. Such equip-ment, including Catalina flying boats, vehicles, ammu-nition and miscellaneous equipment, was disposed ofin the post-war period in the Graveyard together withother non-Lend/Lease surplus military equipment.

Early attempts to locate these sites had proved dif-ficult since the recorded position of scuttling had beenapproximate, usually a compass bearing on RottnestLighthouse and an estimation of distance. With thegrowing use of GPS, the Museum started to receivereports of positions of suspected wrecks from peoplefishing in the area, wreck-sites being well-known fish-aggregation places. However, in 2000 the depth wasbeyond normal diving operations, so it was not feasibleto investigate these reports. Sidescan sonar was used toattempt to locate sites and obtain imagery but thisproved to be ineffective, given the Museum’s resources.

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The International Journal of Nautical Archaeology (2014) 43.2: 436–452doi: 10.1111/1095-9270.12062

© 2014 The Author. International Journal of Nautical Archaeology © 2014 The Nautical Archaeology Society.Published by John Wiley & Sons Ltd. 9600 Garsington Road, Oxford OX4 2DQ, UK and 350 Main Street, Malden, MA 02148, USA.

Since most of the larger known sites were iron ships,it was considered that an aerial magnetometer might beable to detect some of the sites. A local aerial surveycompany, UTS, was contracted to fly two areas: onecentred on the Deep Water Graveyard (24 sq. km); thesecond, further out to sea, centred on the approximateposition of HMAS Derwent, a river-class frigate,

scuttled in 1994 in 200 m of water (8 sq. km). UTS flewthe survey in 2001, taking 1 hour 20 minutes to surveythe first area and about 30 minutes to survey thesecond (not counting the time to and from the surveyareas). The results were surprising. The Graveyardsurvey (Fig. 4a) showed 10 sites and HMAS Derwent, a2100 tonne vessel, showed up quite clearly in thesecond area (Fig. 5a).

The Derwent survey was interesting as it allowed theverification of the Hall Equation (Hall, 1966: 36) sincethe mass and the depth were known (Green, 2002: 125).The survey shows a 14 nT anomaly which is consistent

Figure 1. Map showing the Rottnest Deep Water Graveyard in relation to the coast of Western Australia, the sites so farlocated and the Marine Futures Program multibeam survey area. (Western Australian Museum)

Figure 2. A histogram of the dates of scuttling of vessels inthe Rottnest Deep Water Graveyard.

Figure 3. A histogram of the tonnage of vessels scuttled inthe Graveyard. (Western Australian Museum)

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Figure 4. a) The UTS survey of the Graveyard (raw data); b) The Graveyard with the Earth’s magnetic field variationremoved. (Western Australian Museum)

Figure 5. a) The UTS survey of the HMAS Derwent site (raw data); b) The HMAS Derwent site with the Earth’s magnetic fieldvariation removed. (Western Australian Museum)

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with the predicted 16 nT given by the Hall Equation,which takes into account the length/breadth magnify-ing effect: ΔM = 10 A/B x W/d3, where ΔM is theanomaly in nT, W is mass in kilogrammes, D is dis-tance from the centre of the object in metres, and A/Bis the length (A) to breadth (B) ratio of the object.

Several other interesting features were noted in theresults of the survey. ‘Swell noise’ was observed in bothsurveys. This is caused by the movement of the sea (theswell) relative to the Earth’s horizon. As the sea is aconductor, it moves through the Earth’s magnetic fieldinducing a small magnetic field that adds to the Earth’sfield; hence, the presence of a series of domes in theplots shown in Figures 4 and 5. Using magnetometersoftware (MagPick), it was possible to remove thecomponent of the Earth’s magnetic field and reveal thedetail underlying the swell noise and the wreck-sitesmore clearly (Figs 4b and 5b).

In 2005 a second aerial magnetometer survey wasflown by UTS of the Graveyard area. This survey, while

covering much of the area covered by the first, wasthought to be less successful than the first, initiallyfinding only two new targets. One target in particular,was a very large anomaly that was thought to be theK11, a Dutch Second World War submarine that hadbeen scuttled in the Graveyard in 1946. A large mag-netic anomaly would be expected from a submarinebecause of the large length to breadth ratio (A/B) thatmagnifies the signal. Another investigation of the site,however, a week or so later, with a marine magnetom-eter, showed no evidence of a magnetic anomaly at all,suggesting the original anomaly might have been aRoyal Australian Navy Collins Class Submarine (orpossibly a submarine of some other nation!). Furtheranalysis of the data using the MagPick program showedtwo additional sites, both on the edges of the survey area(Fig. 6a). Comparing the magnetometer informationwith a multibeam survey conducted under the MarineFutures Programme in Western Australia showed asea-bed feature that could, in fact, be the K11 (Fig. 6b),

Figure 6. a) Detail of the second magnetometer survey of Graveyard (Western Australian Museum); b) A detail of the MarineFutures Program multibeam survey corresponding to the area in Fig. 6a. (Marine Futures)

Table 1. Sites from the Rottnest Graveyard aerial magnetometer survey area with known tonnage, depth and condition; the pre-dicted tonnage and depth; and the size of magnetic anomaly and angle of orientation

Name TonnageActual Depth

m ConditionPredictedTonnage

Predicted depthm

Magneticanomaly nT

Angle of anomalydegrees

1 Cape Otway 996 86 Good 1432 91 59.55 512 HMAS Junee 790 81 Good 902 84 47.38 483 Unknown — 89 Poor 725 91 34.77 604 Unknown — 88 Poor 584 95 25.90 665 Kos VII 253 80 Good 217 79 14.00 376 Unknown — 64 Poor 635 68 52.51 627 Unknown — 100 Poor 257 104 9.93 198 Unknown — 98 Poor 275 102 11.32 359 Eucla 574 104 Good 706 111 22.34 56

10 Unknown barge — 93 Good 62 69 6.19 4111 Commalies 263 60 Medium — — — —12 HMAS Derwent 2100 200 Unknown — 172 17 Dipole

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however because the anomaly is on the edge of thesurvey area it is difficult to determine its exact size.

The sitesThe magnetic survey located 12 sites of which six havesubsequently been identified by visual inspection: CapeOtway; HMAS Junee; Kos VII; Commalies; Eucla andHMAS Derwent (Table 1). (For further informationon these sites go to http://museum.wa.gov.au/maritime-archaeology-db/wrecks and enter the vessel’sname.) The magnetic signature of each of the vessels isof interest particularly as some are almost completelyintact, albeit it should be remembered that all vesselswould have been salvaged for any useful metal and alsoblown up in the scuttling process (Fig. 7 shows theCape Otway, a typical site in the Graveyard). Table 1shows that some of the predictive capabilities of thisprogramme are quite effective, certainly showing a rea-sonable indication of tonnage. This is importantbecause it enables some opportunity before mounting amore ambitious visual inspection to determine whatthe anomaly is likely to be. This also demonstrates theadvantage of the aerial magnetic survey over a marinemagnetic survey, the whole operation taking onlyabout an hour to cover about one square km.

The Rottnest Graveyard has been the site for otherinnovative research; in April 2012 a group of technicaldivers dived on HMAS Derwent in 200 m of water,marking a milestone in the exploration of deep-waterwreck-sites. It took six minutes to descend to the site,nine minutes was spent on the site and 7 hours 45minutes was spent decompressing.

Shallow-water sitesGiven the success of the work in the Rottnest DeepWater Graveyard, the Correio da Azia wreck-site wasselected as a shallow-water test project for the aerialmagnetometer. The Correio da Azia was a Portuguese

advice boat lost in 1816 off Point Cloates, in thenorth-west of Western Australia, en route from Lisbonto Macau. The loss of this vessel on Ningaloo Reefcame to light in 1987 when a journal published in Goain 1817 was found, describing the search for the site bythe Portuguese a year after the loss (Green, 2011:83–101). Five Western Australian Museum expeditionsto the relatively remote Point Cloates area had failed tofind evidence of the site. This was surprising, given thata further journal describing the actual loss of the vesselhad been subsequently found in Lisbon and both jour-nals had extensive navigational and topographic infor-mation about the position of the wreck. The account ofthe loss describes the vessel running onto a reef andsinking with its masts still above water, and the posi-tion was estimated to be within an area on NingalooReef measuring about two nautical miles. Thus, it waslikely to be in shallow water in an area where there area large number of coral outcrops that make systematicsearches extremely difficult and dangerous.

The Point Cloates area has a number of knownwreck-sites, mostly vessels engaged in the China Trade.Such vessels, like the Portuguese Correio da Azia,would have been heading for the Strait of Lombokin order to make for Canton. Point Cloates was a

Figure 7. A view of the Cape Otway site in Graveyardshowing extent of damage caused in scuttling. (WesternAustralian Museum)

Figure 8. The Point Cloates area showing the extent of theFugro survey. (Western Australian Museum)

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notorious navigational hazard because it is low lyingand extends to the west of the main coastline, thustending to trap vessels that were sailing too close to thecoast. In the 17th and 18th centuries Point Cloates wasthought to be an island and its position had been vari-ously plotted in different longitudes.

Prospero Productions and the WA Museumapproached the aerial survey division of Fugro Surveyswith a proposal to search for the Correio da Azia, andthe company agreed to carry out an aerial magneticsurvey for the site covering an area of 220 sq. km probono public (Fig. 8). The results again were surprising.There were six known sites in the area and the surveyshowed up two additional magnetic anomalies inapproximately the area where the Correio da Azia wasthought to have been lost (Fig. 9). The anomalies weresubsequently investigated, taking only a few minutes tofind the two new wreck-sites. One was the Correio da

Azia, from coins found dated to 1815; the other was an,as yet, unidentified late 19th-century site. Thus themagnetometer survey was able to achieve what fiveseasons of visual survey in the area had failed to.

The vessels, alongside their magnetic signatures andestimated mass, are given in Table 2. Most of thesurvey area had a background noise of around 2 nTand the area was not affected by swell noise.

The comparison of the targets with the known sitesshows that it is doubtful if the anomaly of 0.6 nT of theRapid could have been identified as a wreck-site againstthe background noise, particularly because the site lieson the edge of the area to the east where there is con-siderable disturbance in the magnetic field (Fig. 10).This disturbance is likely to be a geological feature,possibly an old river-bed, containing ferrous materialthat has been washed down from the iron-ore fieldsinland.

Figure 9. Aerial photograph showing the position of the Correio da Azia site and the unidentified site. Tracks show previousvisual and marine magnetometer surveys undertaken in the 1990s. Two as yet un-located anomalies are shown with whitearrows. (Western Australian Museum)

Table 2. Wrecks in the Point Cloates area with date, construction, tonnage and size of anomaly

Name Date Construction Tonnage Size of anomaly nT

Rapid 1811 Wood, 1 anchor — 0.6Correio da Azia 1816 Wood, 1 anchor, 2 small cannon, iron ballast — 1.9Stephano 1875 Wood, 2 anchors and chain — 1.2Unidentified late C19th Wood, 2 anchors — 0.8Perth 1887 Iron 499 26820.0Benan 1888 Iron 1415 138122.0

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By comparison, the slightly larger anomalies of theunidentified late 19th-century site and the Stephano allcan be easily identified against a relatively uniformbackground magnetic field. There are a number ofsmall anomalies that are yet to be identified. It isknown that the Correio da Azia lost an anchor as ittried to go about prior to the wreck and two smallanomalies are visible near to the wreck-site (whitearrows Fig. 9). One to the north is in an area which isvery difficult to access as it is on top of a thick coralarea. The other, to the SSE of the wreck, has beensurveyed visually and using a marine magnetometer,both of which have failed to find an anchor.

ConclusionsThe results of the aerial magnetometer surveys haveproved to be successful in locating iron shipwrecks indeep water and small wooden shipwrecks with smallquantities of iron in shallow water. The limitation indeep water is the effect of swell noise and ultimately thesensitivity of the magnetometer to background noise.A 2100-tonne vessel was easily detected in 200 m ofwater. It is interesting that Hall’s (Littlemore ScientificEngineering, 2006: 34, fig. 1) predictions (Fig. 11)

showing anomaly/distance/mass is surprisingly goodfor the Cape Otway (50 nT predicted against 59 nTrecorded) but a bit low for the Derwent (10 nT/17 nT)showing that this is a useful guide in determining thefeasibility of a search. In shallow water the influence ofunderlying local geological magnetic structure is likelyto affect the ability to locate small magnetic targets.The sort of problem seen in the Correio da Azia surveymakes it impossible to locate anything small in thedisturbed area. It can be seen that if the system has asensitivity of about 0.5 nT, then objects of about 50 kgor more can be detected when the height from target toaircraft is 50 m. It is unlikely that a marine magneticsurvey would fare any better, since the underlyinggeology such as found on the Correio da Azia survey islikely to mask very small sites. On the other hand,where the geology is on a larger scale, it is possible todifferentiate between the two.

An aerial magnetometer is not a cheap option;however, it is worth considering when there is a largearea to be covered that is difficult for a marine opera-tion, such as with the Correio da Azia survey. Clearly,there are distinct advantages to the aerial option whencompared with the cost, time and effort of a marineoption.

Jeremy GreenDepartment of Maritime Archaeology, Western

Australian Museum, Cliff Street, Fremantle, WA6160, Western Australia

AcknowledgementsThe author would like to thank Ed Punchard from Prospero Productions for help and assistance in both these operations.Also, Jan Kramer of Fugro Surveys, who sponsored the extensive airborne magnetometer survey for the Correio da Azia.Geoff Glazier, from OmniStar for help with GPS work. The late Mike Capelhorn, for allowing access to his survey data ofthe second UTS survey. Mikhail Tchernychev, for his extensive and generous help with the MagPick program. The MarineFutures Program for permission to use their data. I would like to acknowledge the authors Camidge, et al., 2010, who notedmy error in Green (1990: 63) in regard to the Hall equation (the correct version is shown above). Finally, I would like toacknowledge the inspirational work of my supervisor, the late Professor E. T. (Teddy) Hall, who was the genius behind themarine magnetometer.

Figure 10. Fugro magnetic survey comparing the signalsfrom the iron barque Benan and the single remaining anchoron the site of the Rapid, excavated in the 1980s. (WesternAustralian Museum)

Figure 11. Theoretical logarithmic plot of signal size in nTagainst distance in metres for various masses. (AfterLittlemore Scientific Engineering, 2006)

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ReferencesArnold, J. B. III, 1976, An underwater archaeological magnetometer survey and site test excavation project off Padre Island. Texas

Antiquities Committee Publication No. 3.Arnold, J. B. III, 1981, An airborne magnetometer survey for ship wrecks and associated underwater test excavations, in W. A.

Cockerell (ed.), In the realms of gold. Proceedings of the 10th Conference on Underwater Archaeology, 103–19. Nashville.Arnold, J. B. III, 1987, Marine magnetometer survey of archaeological materials near Galveston, Texas. Historical Archaeology,

21.1, 18–47.Arnold, J. B. III and Clausen, C. J., 1975, A magnetometer survey with electronic positioning control and calculator-plotter

system. IJNA 4.2, 353–66.Camidge, K., Holt, P., Johns, C., Randall, L. and Schmidt, A., 2010, Developing Magnetometer Techniques to Identify

Submerged Archaeological Sites. Theoretical Study Report, Historic Environment, Environment, Planning & Economy,Cornwall Council, Truro.

Garratt, D., 1999, Précis of the wrecks in the ships’ graveyard, Rottnest. Report—Department of Maritime Archaeology,Western Australian Maritime Museum, No. 148.

Green, J. N., 1990, Maritime archaeology: a technical handbook. London.Green, J. N., 2002, The application of side-scan sonar and magnetometer to the location of archaeological sites. Bulletin of the

Australasian Institute for Maritime Archaeology 26, 119–31.Green, J. N., 2011, Shipwrecks of the Ningaloo Reef: maritime archaeological projects from 1978–2009. Special Publication No.

15, Australian National Centre of Excellence for Maritime Archaeology, Perth.Hall, E. T., 1966, The use of the proton magnetometer in underwater archaeology. Archaeometry 9, 32–44.Ingelman-Sundberg, C., 1978, The Dutch East Indiaman Zeewijk wrecked in 1727: A report on the 1978 expedition to the site.

Report—Department of Maritime Archaeology, Western Australian Maritime Museum: No. 10.Littlemore Scientific Engineering, 2006, Proton Magnetometer Recorder Type 7706 Operating Instructions. Littlemore Scientific

Engineering, Gillingham, UK.Plunkett, G., 2003, Sea dumping in Australia: historical and contemporary aspects. Defence Publishing Service, Commonwealth

of Australia.Richards, N., 2008, Ship’s Graveyards Abandoned Watercraft and Archaeological Site Formation Process. University Press of

Florida, Gainesville, Fl.

Fragments of a Roman Mast from the Port of Genoa, Italy

In the spring of 2013, dredging in the port ofGenoa, Italy, allowed archaeologists to investigatestratigraphic layers not previously studied in this

area of the port (for previous work, see Grimaudo andMelli, 2013). In the early hours of 27 March. Themechanical arm of the dredger Maricavor recoveredtwo large parts of a wooden artefact, not far from themodern Calata Gadda dock, at a depth of about 17 m(Fig. 1). According to an operations report and pic-tures taken at the scene, a single timber was found,which immediately broke into two parts in beingmanoeuvred from the dredger’s mechanical arm out ofthe water. Analysis carried out by the author at thetime of the discovery and soon after the artefact wasput into storage prior to conservation, allows it to beprovisionally identified as part of a Roman mast.1

As archaeometric dating is as yet lacking, the findhas been provisionally attributed to the Roman periodon the base of its stratigraphic position. The timbercomes from a layer of marine sand, which containsoccasional unidentified amphora fragments, coveredand sealed by a level of Posidonia oceanica. This layerhas a direct stratigraphic relationship with the higherRoman layers of the so-called ‘Portofranco’ area(Melli and Manganelli, 1996; Melli, 2007: 28–32) and

of the ‘Porto Antico’ (Sanna and Tiboni, 2013), fromwhich pottery and organic finds date from no laterthan the 4th century AD.

Figure 1. Location map. (Author)

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