THE USE OF THE PROTON MAGNETOMETER IN UNDERWATER ARCHAEOLOGY
Post on 30-Sep-2016
THE USE OF THE PROTON MAGNETOMETER IN UNDERWATER ARCHAEOLOGY
BY E. T. HALL
A. GENERAL CONSIDERATIONS The use of the Proton Magnetometer for archaeological land surveys is now
well established and many papers have been published in this Journal* and others (Black and Johnston 1962) describing this type of survey. Its use at sea, however, for archaeological purposes has been limited. Several operators in the treasure hunting business have been using instruments for some time: for obvious reasons the experiences of these teams have not been divulged in any detail.
The Proton Magnetometer and in particular the device developed from the Oxford Laboratory prototype (Aitken 1958, Waters and Francis 1958) and has been used commercially in many systems at sea, but only or geophysical and geological research. However, this has meant that much of the necessary hardware had already
FIG. 1. Recording Magnetometer Console
+Various papers. Archaeometry Vol. 4, 1961, pp. 71-84. Various papers. Archaeornerry Vol. 5 , 1962, pp. 123-138
A R C H A E O M E ~ R Y 33
been developed. For instance the hydrodynamically stable detector fish and non- microphonic cable with a high breaking strain were already in existence, when the laboratory contemplated operations at sea.
The Instrument Figure 1 shows the apparatus installed in its sea-going console. The three
electronic niodules are contained in a plastic-veneered water-resistant plywood case. These three component parts consist of the following:
a) The Magnetometer is precisely similar to the land-based instrument. The reading obtained on the five read-out meters is inversely proportional to the magnetic field intensity. An output for recording purposes is available at the front panel.
b) The Digital to Analogue converter is connected via a twelve Pin plug to the magnetometer unit. As mentioned above the read-out of the magnet- ometer is on meters in digital form. In order to dispIay this on a chart recorder it is necessary to convert the reading to a voltage or analogue form. This function is accomplished by this intermediate unit entirely electronically,
FIG. 2. Magnetometer record. Dardanelles. Note high gradient atld the use of fiducial marks.
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using in the final stages weighted resistors. The resultant output voltage is built up across a small (
A R C H A E O M E T R Y 35
The Detector and Cable Figure 3 shows the detector which has been developed for sea use. Incidentally
it is also used in precisely the same form for air-borne surveys and is stable both in water up to 15 knots and in the air up to 120 knots.
It will be noticed that the detector is pulled through the water from a towing saddle situated some six inches above the centre of gravity. It has been found necessary to use this point of tow for stability. In early experiments it was thought that towing from the nose would be best; in fact the fish was unstable in this mode, and the cable would not have withstood the continuous flexing. The detector itseif consists of a standard (land-based) polythene bottle containing methyl alcohol with iron alum added. The amount of the addition is so adjusted that the polarisation time is reduced whilst the proton signal decay is still slow enough to obtain a reliable reading to within k 2y. The bottle is wound with about 1400 turns of 20 swg copper wire,* and is completely encapsulated in epoxy resin and glass fibre. At the same time a cast aluminium fin is moulded into place.
Connections from the bottle to the towing cable are made within the hollow fin cavity and a neoprene seal affected between the cable and fin to keep out sea water in depths up to 500 feet. This seal is most important and considerable research was needed before a reliable demountable seal was produced. A second line of defence is provided by filling the fin cavity with viscous silicone oil.
The cable used must both carry the signal and provide a means of towing. It is specially made to be non-microphonic and does not produce noise when it is moved violently in the water. It is of coaxial construction, the screen being stout cadmium-copper braiding. The overall covering of PVC is about 0.55 inches diameter. Its breaking strain is about 700 Ibs. The length of cable used will depend on the type of ship used for towing and the depth to which the detector is designed to operate. If a steel ship is used for towing, the length of cable should be at least 2) times the length of the ship. It is expected, however, in archaeological surveys that wooden or fibre glass boats would be used and the length of cable dictated by the boats motor would be no more than perhaps 30 metres. It is quite likely that a considerably longer cable might be necessary to get the detector near the sea bottom when a length at least 13 times the maximum operational water depth would be required. The problems associated with this will be discussed later.
Magnetically Detectable Objects 1. Steel Wrecks.
It is, of course, stretching archaeology to include steel ships under this head- ing. but there are historical problems which might be solved by finding a steel wreck and the principles involved for less easily detectable objects are largely the same.
Since a steep ship will certainly give a strong magnetic anomaly, often it will be unnecessary to tow the detector near the sea bottom and a length of cable just long enough to be out of the tow-boats magnetic field will be sufficient. Moreover, if we are looking for a sizeable anomaly its effect will persist for a considerable distance and we would be able to search at an increased speed (up to 15 knots) and our search lanes could be more widely spaced. *This copper wire is specially prepared for magnetometer work. It is iron free and is drawn under special precautions through diamond dies. These precautions prevent heading error build-up as mentioned in a previous article (Hall 1962). The heading error now does not reach a value of even 1 y after several years use.
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An approximate estimate of detection distance is required when undertaking a survey since this knowledge will affect the planning of the search; it will alter a number of factors such as length of towing cable, speed of towing boat and width of search lanes.
The precise calculation of anomaly to be expected from a given wreck is complicated by the fact that a wreck or other iron object will not have an ideal shape for calculation. However, if we assume that we will be detecting the object from a distance which is large compared to its physical dimensions we can assume it will behave as a short bar magnet or an evenly magnetised sphere; this will mean that the anomaly produced will vary with the cube of the distance.
A M = ~ O * A . W (1) A useful formula for such calculations is as follows:
B- ds where A M = magnetic anomaly in gamma
- ratio length to width of object A B w = weight of object in gm. d = distance of object in cm.
As an approximation we may assume that in normal conditions the smallest anomaly we can reliably detect would be 57 and for a ship the ratio of length to breadth would be 5; hence we can say
d3 = lo. w
or d = 3\-L/10.w -__
or if W = weight of object in tons D = distance of object in metres D = 3 ~ 1 0 4 ~
As exaniples we can calculate the following: A 10 ton ship should be detectable at 45 metres A 100 ton ship ,, ,, ,, 100 metres A 1,000 ton ship ,, ,, 9 , ,. 200 metres A 10,000 ton ship ,, ,, ,, ,, 450 metres
From such calculations it should be possible to arrange ones search procedure according to the type of object expected.
2. Hi.rtorica1 Wooden Wrecks Such ships as 17th, 18th and 19th century warships would always be carrying
cannon and if these were cast iron, they would be detectable; moreover, bronze cannon used cast iron balls and when present in large numbers would provide a mass of iron for detection. Iron anchors and fittings should also provide anomalies where cannon were not carried.
From the formulae 1 and 2 above we may calculate detection distances to be expected from individual objects:
A 20 lbs. cannon ball should be detectable at 3 metres A 2 cwt. anchor ,, ,, , I ,, 10 metres A 2 ton cannon 1, ., ,, 27 metres
When there is more than one object, the situation will become more complex, since the individual magnetic moments of the objects may add or may cancel each other.
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3. Archaeological Wooden Wrecks These wrecks will be the most difficult to find, since the amount of free iron is
likely to be small or non-existent. In fact, classical Greek and Roman wrecks often do contain a certain amount of iron fittings which may be detectable. There is also another possibility of detection; often such ancient wrecks carried large numbers of amphora as cargo-in some wrecks several thousand lie in a com- paratively tight heap. When these amphora were made they would have been fired at a high temperature which would have caused the magnetic domains in the raw clay to line up with the earths magnetic field direction. This will result in each amphora having an induced magnetic field. By measurement we have found that a single amphora may give a magnetic anomaly of 5 gamma at 3 feet. The effect of a mass of amphora is still unknown. It is proposed during the next season (summer of 1967) to test a magnetometer system over some known amphora wrecks so that we have some data on which we can base our future surveys. Survey Procedure
As in land based surveys it is most important that the area to be investigated is covered in a systematic way. Moreover, the plotting of position must be organised most carefully; it is in this area of planning where sea-going surveys are so much more difficult than land based ones-to find the underwater anomaly on a chart recorder requires much patience and some skill, but to relocate it particularly in deep water may become almost impossible in the present state of the art.
1. Laying out the area. The four corners of a square should be located as nearly as possible to predetermined locations with reference to an Admiralty chart of the area. Standard navigational methods (e.g. hand bearing compass or hori- zontal sextant angles) can be used for this, since very precise absolute positions are unimportant. The corners of the square should be buoyed with large flags. It is most important to select buoys and/or flags for easy visibility from within the square even in choppy weather; orange, red or yellow flags are most easily visible, white or blue or nearly useless.
A number of other smaller sinkers with markers (coloured polystyrene blocks?) should be carried by the boat. These will be used not only for marking suspected anomalies but also reference points along two opposite sides of the square as the survey progresses. If the square is perhaps + mile on each side, it may be appre- ciated that i t will become increasingly difficult to locate each lane the correct distance from the preceding one. It is convenient therefore if every 100 meters or so a buoy is thrown out at each end of a particular traverse; location can then proceed from these latest two buoys.
2. Lane spacing. The distance between each lane traversed will depend on the type of object sought. Theoretically the distance between each lane can be twice the maximum detection distance as calculated in the formulae above. For instance if we are looking for 18th century warship wrecks containing cannon, we should be able to have our lanes 50 metres apart. In practice however, not only are the formula approximate, but the accuracy of steering the boat parallel on each course may be impossible. Probably a more realistic figure is to keep the lane spacing down to once the maximum detection liniit. In this way verification of the object should be obtained on the return run.
3 . Boat speed. This will depend on two factors-the size of anomaly expected and the depth of the detector. When large steel ships are expected the magnetic
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change will persist over a much larger distance than if we were looking for a single cannon when detection would only persist over 50 metres. The magnetometer, in underwater conditions when the cable noise may be significant, will require com- paratively long polarisation times; in practice we have found one reading per 3 seconds the best compromise between accuracy and repetition speed. If we travel at 5 knots (i.e. approximately 23 metres/sec.) we will travel some 7+ metres between readings. In this instance therefore. we should see the cannon over some 6 magnetometer readings. To be sure that a change of reading on the recorder really represents an object a magnetic profile is much more convincing. A change in a single reading could be due to a number of erroneous causes, e.g. a small but close magnetic object such as a tin can, a jolt on the detector giving it a sharp twist at the moment of measurement, a burst of noise or even an erroneous reading by the electronics. With experience the shape of the magnetic profile becomes very significant and characteristic.
4. Control of Detector Depth. When looking for comparatively small anomalies, this becomes one of the most important and difficult problems. A number of different approaches have been tried or considered during the 1966 season.
a) The easiest and most obvious is to tow the detector with no attempt at stabilisation of depth or distance from the sea bottom. For large objects such as large steel ships or where the water is shallow this is all that is necessary. Incidentally, it is impossible to know by observation the depth of tow of the detector fish. The line of the cable gives no indication since the lay of the cable under water is never straight. It would seem, however, that when unweighted the fish never goes more than some six feet below the surface.
b) A stainless steel strop is fitted to the cable and can be made to slide up the cable to any set position. Lead weights may be fastened to this to make the fish travel lower in the water. Combinations of speed, lengths of cable and attached weight will determine depth of tow. Construction of such tables are impossible without the use of a depth gauge attached to the fish. It is intended that such tables should be constructed during our next season, when we will have the correct apparatus. When using this technique it is desirable that the water should have an even depth; a steeply shelving bottom in the direction of travel would make this system unworkable.
c) A similar method consists of using a very heavy weight towed on a separate line which takes the detector to the bottom. The detector itself is held up from the bottom by a small submersible buoy. This method is probably the most practical, but the shape of the lead weight is most important. At Ashdod in Israel this summer we used a 40 lb weight in the form of a lump 15in x 4in x 2in. This weight was apt to catch in rocks and to spin in one direction so destroying the lay of the towing rope. It has been suggested that a better method is to use lead piping some 13 inches in diameter which may be threaded onto the magnetometer cable itself or towed separately. Such a method would be less likely to snag and would be easier to handle.
d) The most sophisticated method which has not been attempted would be to have a fish fitted with an echo sounder and servo system driving a pair of trimming fins. The complications involved would almost certainly make this system impracticable.
A R C H A E O M E T R Y 39
5 . Fixing Position of Anomaly. Having obtained a significant reading on the magnetometer recorder it is of course necessary to mark this position accurateIy so that the cause of the anomaly may be investigated by divers. We have adopted two approaches-one simple for shallow water, and the other much more compli- cated for deep water.
a) If we are working in comparatively shallow water up to 20 metres, we would be using a towing cable only some 30 metres long and its tow path would very nearly coincide with that of the boat. In such cases we would have somebody ready to throw out a buoy and sinker as soon as it was obvious we had passed an object of interest. That lane would then be completed without changing speed. An exactly reciprocal lane would then be run so that the fish runs over the ground in the opposite direction. When the chart record had again detected the anomaly another buoy and sinker would be thrown. Exactly half way between these buoys would represent the position of the object. In practice it has been found expedient to make runs at right angles also so as to obtain a more accurate centre point. This method of location was found to be most successful during the expedition to Akko harbour area in October 1966.
b) It should be appreciated that in deep water up to 200 feet (or if special diving techniques are contemplated in even deeper water) we might have up to 400 feet of cable out. This would mean that the path of the detector might be very different from that of the boat due to wind and tide. For this reason fixing the position of the boat might have little relevance on the position of the located anomaly.
For this reason methods of fixing which depend on radar principles are not the answer to the problem since they can only fix the position of the towing boat-they would be a help but not the final solution.
Elsewhere in this issue of Archaeometry will be found a description of a method of location which depends on the measurement of distance of a transducer, attached near the magnetometer fish, from two fixed buoys. The distance measurements rely on the speed of sound in water.
6 ) Magnetic Hazards. The most obvious cause of magnetic noise is the existence of iron rubbish in the path of the survey. Unwanted wrecks, steel cables, oil drums etc. can be very tiresome. The smaller objects can be discriminated against by towing the detector sufficiently far from the bottom, but in modern harbour areas other larger unwanted objects may make a survey impossible. When trying to find ancient wrecks where the expected anomaly will be small the problem will be more acute and magnetometer surveys can only be carried out in clean areas.
Another cause of trouble may be caused by strong geological magnetic gradients caused by igneous or metamorphic rock strata. This applies particularly to tertiary and later volcanic rocks, but even granites may be tiresome if we are looking for small anomalies. A gross example of such a magnetic gradient is shown in figure 2. This curve was typical of those registered in the Dardanelles when looking for battleships sunk in 1915. In such high gradients it would be impossible to find anything but steel ships of some size; these gradients which amounted to up to 5 gamma per metre may be compared with those obtained from the battle- ships, the latter being much sharper when the detector was sufficiently close.
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B. SOME EXPERIENCES 1965 AND 1966 Our experiences particularly in archaeological surveys have been very limited
so far, but a number of lessons have been learnt and it may help others in the field if these are recounted.
1. Bodrum, Turkey (1965) An instrument was despatched from the Research Laboratory at Oxford for
use with the Pennsylvania University expedition under the direction of Mr. George Bass. I was in the area and spent a few days getting the apparatus working correctly. It was June and the temperature at times over 35C and this caused some trouble in the digital to analogue converter showing that all apparatus should be tested to the highest temperature likely to be encountered in practice. The recorder used was a Rustrak percussion type which was not very satisfactory for this type of work since it was difficult to interpret the trace at times.
The detector developed for this purpose was also quite unsatisfactory. We had fixed a standard fish in the centre of a hollow 15 inch diamettr cylinder, which was dome shaped at one end. The cylinder and fish assembly was towed along the sea bed from the nose of the dome. The result was an erratic movement bumping from one point to another. Moreover, the cable although of a non- microphonic type was not as low noise as the newly developed larger diameter cable we now use. The result of this was that it was diflicult to get a low random background trace and small magnetic anomalies were difficult to identify.
The purpose of the survey was to find the location of a Greek classical wreck which it was suspected might be carrying many valuable bronzes. The reason for this belief was that sponge fishermen had during the past few years brought up at least two and probably three remarkable bronzes from the same spot. Was it therefore possible that a ship containing a whole cargo of bronzes was wrecked there? Unfortunately, the depth was between 200 and 300 ft . and so navigatation was extremely difficult and when on a few occasions a likely anomaly was found it proved impossible to relocate it. The necessity for an accurate plotting system was acutely felt. These trials were undertaken by George Basss team in conjunction with the use of an underwater television unit.
The search had negative results, but it is hoped that a new expedition will take place in 1967. The Pennsylvania University team will use side-looking sonar whilst the Research Laboratory will bring their new magnetometer with position plotting equipment.
2. The Dardanelles (May 1966) On March 18th, 1915, the combined British and French Mediterranean fleets
entered the Dardanelles passage in an attempt to force the narrows and so gain entrance to the Sea of Marmara and Istanbul. If they had succeeded the course of the war might have been very different. However, a line of mines had been laid the night before, parallel and about a mile from the Asiatic coast and these were missed by the somewhat demoralised minesweeper force, which was composed mainly of converted North Sea trawlers with civilian crews. The result was both a fantastic chance and a fiasco. Only about twenty mines were laid and five of these succeeded in either sinking or crippling a major battleship.
The French Bouvet was the first to go and she sank whilst still going full ahead, within two minutes of being struck. The reason for sinking so swiftly is still obscure.
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Irresistible and Ocean were holed soon after, but did not sink immediately. In fact Ocean was seen floating some four hours later. The position of Irresistible was well charted, but that of Ocean was unknown. The two other ships were with- drawn from the area, severely damaged, but afloat.
Later in the campaign three other battleships were sunk; Goliath and Majestic near the European shore at the mouth of the Dardanelles, and Triumph a few miles off Anzac beach some twelve miles north. The positions of the first two were well established, but that of Triumph was not known.
During my trip the previous summer to Bodrum, I met two divers who were carrying out salvage work on the Irresistible and they asked if I could help find Bouvet, Ocean and Triumph. Since this coincided with the delivery of a new boat to Istanbul and the production of our new magnetometer system, it seemed an excellent opportunity to try out the new equipment on some easy targets.
On arrival in the area it was found that the local magnetic gradients were very severe and so instead of running lanes some 300 metres apart, we had to reduce this to about 50 metres, so making the job very much more tedious. Sifice the water was from 40 to 60 metres deep it was never necessary to get the fish any deeper than the natural depth it took up when towed on a 60 metre cable without ballast weights.
Some five days were occupied in location of the three wrecks. We wondered whether the high natural gradients would make identification impossible, but as can be seen by a comparison of figure 2 and figure 4, there is no doubt about the existence of a large wreck when it occurs; figure 4 shows that even when the full scale deflection of the recorder is set at 1000~ (when a non-magnetic background is present an F.S.D. of 1007 is usual) the trace goes off-scale several times.
Figure 4 represents the culmination of three days patient work, some three square nautical miles having been surveyed for Bouvet up to that point.
Final confirmation was afforded by diving.
FIG. 4. Magnetometer record. Detection of Battleship Bouvet.
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The locations of Ocean and Triumph were more nearly known from eye witness reports; moreover, particularly in the latter case, the magnetic gradients in the area were less severe. It is appreciated that these initial tests were carried out on very large (> 10,OOO tons) ships and they bear little relation to archaeological prob- lems except that these tests served to prove the apparatus under sea-going conditions.
3. Akko, Israel (October 1966) A combined Anglo-Israeli expedition was organised for the purpose of survey-
ing the harbour area of Akko. During the period of two weeks the whole area out- side the harbour from + to .) mile to sea-ward was systematically examined.
Where the water was less than 10 metres deep the fish was used unweighted. For depths greater than this, lead weights were added to the cable to take the fish nearer the bottom. An estimated 250 miles of traversing was undertaken. Since the area was near the shore, and comparatively shallow, the use of a plotting system more complicated than visual sighting of buoys was found unnecessary.
This survey gave valuable information regarding logistics for sea-going work, and although the results were not sensational some objects of historical interest were located.
a) Between the end of the old southern breakwater and the Tower of Flies a large wooden ship, between 200 and 300 feet long was located. The exact nature of this wreck has yet to be established. It was built of pine and was copper sheathed and was probably of eighteenth century construction. The actual objects which allowed detection by magnetometer have not been established. The curve which was obtained is shown in figure 5; an anomaly of some 307 was obtained. This wreck is of considerable importance to the Israeli underwater archaeological workers, since its presence will provide them with excellent material for learning
FIG. 5. Magnetometer record. Detection of 18th century wooden wreck at Akko, Israel.
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FIG. 6. Magnetometer iecord. Detection of 150 kilo anchor.
FIG. 7. Magnetometer record. Detection of sunken submarine Shira
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the science of underwater excavation. The wreck itself should also produce interesting details of this type of ship.
b) Three anchors (18th and 19th century) weighing some 150 kilogrammes were found in the area to the west of the town. A typical trace obtained by this smaller type of object is shown in figure 6. It may be noted that the anomaly is still clearly distinguishable from the random background.
c) At least three anomalies were found whose causes were not established before the British team left Israel. Two of these represented perhaps 100 to 200 kilos of iron, whilst the third must have consisted of at least 3 tons of iron. All three sources of anomaly were under the sand and nothing could be distinguished by divers on investigation. It seems possible that the large object might have been an iron cannon but only the use of an air lift would verify this.
d) A non-archaeological diversion wa; provided by a search for the wreck of the 1,000 ton Italian submarine Shira sunk by British planes in 1943. She was found in some 20 fathoms of water 4+ miles from Haifa with her torpedoes still intact (figure 7).
e) An attempt was made to locate a Canaanite wreck believed to be off Ashdod some eighty miles to the south of Akko. The reason for this belief was that on several occasions trawler fishermen had brought up amphora of this early period from this same area, and the location of these trawls was thought to be accurately known. On arrival at Ashdod it was obvious that the location was far from accurately known. However, the suspected area was eventually determined by echo sounder and surveying started in 40 metres of water. Since we were working in this depth. and we were looking for small anomalies, it was necessary to get the fish near the bottom. Various procedures were tried, none of which were entirely satisfactory. We now believe however, that lead piping would afford the best chance of success.
During this trip we also experimented with the position plotting apparatus. This allowed us to see that the principles were sound, but the sea-going engineer- ing was poor-threads seized up, aerials broke or became unsoldered, electric cables were broken because they were incorrectly secured, etc. But the chief snag turned out to be the inability of the buoys, which were meant to hold the sonar transducers above the sea-bed, to withstand the hydrostatic pressure at 40 metres-the poly- styrene or tin cans just collapsed and the transducers sank into the mud. The experience gained however, was very well worthwhile and we are now sure that we can produce a properly engineered system.
Aitken, M. J., 1958, Magnetic Prospecting, Archaeometry 1, 24. Black, G. A. and Johnston, R. B., 1962, A test of Magnetometry as an aid to Archaeology.
Hall, E. T., 1962, Detector Heads for Proton Magnetometers, Archaeomefry 5, 139. Waters, G. S. and Francis, P.D., 1958, A Nuclear Magnetometer, J . Sci. lnsr. 35, 88.
Amer. Antiquity 28, No. 2.