THE USE OF THE PROTON MAGNETOMETER IN UNDERWATER ARCHAEOLOGY
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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.
34 A H C H A E O M E T R Y
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.
36 A R C H A E O M E T R Y
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.
A R C H A E O M E T R Y 37
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
38 A R C H A E O M E T R Y
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 lum...