Is There a Place for the Proton Magnetometer in Australian Field Archaeology?
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Is There a Place for the Proton Magnetometer in Australian Field Archaeology?
- 56 000 m
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GRAHAM CONNAH, PENELOPE EMMERSON and JOHN STANLEY * The archaeological aspects
Hunter-gatherer sites are not normally thought to provide suitable features for location by magnetic survey techniques (Tite, 1972:43). This paper describes our recent successful use of magnetometers on some archaeological sites in New South Wales, and suggests that these instruments can have a place in Australian prehistoric archaeology. This arises through the ability of a magnetic survey to detect such features as small areas of burnt clay or soil which may result from a camp fire, and magnetic anomalies resulting from enriched soil susceptibility caused by occupation of a site.
Most of the experiments employed a proton precession magnetometer manu- factured by Ceometrics, California, in 1974, and available for a cost of around $A1,600. One of the experiments, however, made use of a caesium vapour magnetometer con- structed by one of us (JS). The greater sensitivity and very greatly increased speed of operation confirms that this instrument can provide a most effective means of archaeological prospecting. It is not, how- ever, readily available commercially and costs around $A7,000. Therefore it was more practical to assess the performance of the more conventional proton precession magnetometer.
Our first experiment, conducted in August 1973, tested the ability of a magnetometer to detect a small hearth (Stanley and Green, 1975). The caesium vapour magnetometer was used to conduct a magnetic survey of a piece of cleared ground near Armidale. Initially there were no significant magnetic
* Department of Prehistory and Archaeology, and Department of Geophysics, University of New England. Ms. received Abril 1975 and revised Sesember. This paper was&originally presented at ANZAAS, Canberra, 1975.
anomalies (FIG. 1A). A pit was then dug, and a fire lit within it and left to burn
56100 I nT A
FIG. 1 Magnetic field profiles measured across the site of a hearth; A-before burning; B-after burning. The fire was lit and left to burn out overnight.
overnight. The firepit was refilled with the earth that had been dug from it and the magnetic survey was repeated. A very significant magnetic anomaly was now found to exist (FIG. 1B). It is thus apparent that, given a circumscribed area where sur- face scatter of stone artefactual material indicates the presence of an open settlement, it should be a practical proposition to locate hearths on such a site.
The next experiments were conducted on prehistoric shell middens in the area of the lower Macleay River. These are clearly observable by means of differential relief and vegetation cover, but shell middens are not always so readily identifiable from sur- face indications and we sought to know whether magnetic prospecting could be used to ascertain the extent of deposit on less easily definable sites.
The linear midden known as Stuarts Point 1, which runs for some 2.5 km in the tidal
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STUARTS POINT I : 1974 : SOUTH SECTION
Connah 75 fTTiilH FIG. 2 A cross-section of the Stuarts Point shell midden showing its composition. A-black humus and
fragmented shell; B-fragmented burnt shell (oyster pi cdoniinates) and much orange and grey ash; C-whole shells and air spaces, very loose, o oyster predominates, c = cockle pre- dominates; %hearths (ash, charcoal, fish bones, finely fragmented shell) ; E-shells and brown sand (cockle predominates) ; F-sand stained black or brown; G-natural white sand, orange at ends of section; H-cemented orange sand.
mud flat area of the lower Macleay River, was chosen for the bulk of our tests. During 1974 a number of test cuts had been made through this site by mining entrepreneurs. At the request of the New South Wales National Parks and Wildlife Service we recut and recorded the section presented by the deepest and longest of these cuts (FIG. 2), and this section and subsequent excavations during 1975 gave control data for our survey. The presence of hearths and burnt material within the midden should be noted. Two magnetic traverses approximately 30 m apart were then made at right angles to the long axis of the midden and as close to the recorded section as the heavy vegetation would allow. Readings were taken at 50 cm intervals along each traverse, and demon- strated quite clearly that the midden had a pronounced magnetic anomaly. Laboratory experiments demonstrated that the anomaly could be interpreted as originating from a slab of material having a magnetic sus- ceptibility greater than the underlying sand. Late in 1974 a further ten traverses were made across the midden, commencing 107 m north of the recorded archaeological section. These traverses were at 25 cm intervals and readings were taken at every 25 cm along each traverse. A typical profile can be seen in FIG. 3A. Susceptibility measurements using a Bison susceptibility bridge were then conducted on the site itself. Samples of material were taken directly from the drawn archaeological section of the midden and from the surface sand at each side of the midden. As TABLE 1 shows, the sus-
nT .56 090
56050 5 10 15 I I 20 25 I 1 3p 3 m 6p
1 nT I I
56070 MIDDENE EDGE^ I 56060 5 lo 15 20 25 30 35
/ h j FIG. 3 A-a magnetic profile of the total field
aiiornaly recorded 5 cm above the midden at Stuarts Point. B-the same profile after filtering by a 30 point box-car integration. Note that the inflection points correspond closely to the edges of the midden.
ceptibility of the sand forming the environ- ment of the site (it lies on an old coastal dune) is virtually zero, as also is the unburnt shell which makes up the bulk of the midden. The humus layer which had built up over the relatively fertile midden material has the highest susceptibility. The soil and ash from the burnt zone of the section also has an enhanced susceptibility, although it is less than that of the humus.
A single magnetic traverse was also made at Clybucca 3, a shell midden situated at the base of Pleistocene cliffs some 10 km inland from the present coastline. Readings on this traverse were taken at 50 cm inter- vals. Again this midden displayed a pro-
TABLE 1 STUARTS POINT MAGNETIC SUSCEPTIBILITY
Material Magnetic susceptibility x 10-6 c.g.s.
Top humus layer 265 (Mean of six samples)
Burnt zone within midden 127 (Mean of six samples)
Unburnt shell within midden 29 (Mean of four samples)
Stained sand under midden 0 (Mean of five samples)
Natural sand under midden 1 3 (Mean of five samples)
Surface sand east of midden 23 (Mean of five samples)
Surface sand west of midden 42 (Mean of five samples)
nounced magnetic anomaly which could be compared with a section excavated in 1972.
Further traverses have been conducted by one of us (PE) at Bass Point midden. The characteristic fall in magnetic intensity at the southern edge of the midden was apparent but interference from the scatter of modern litter containing magnetic items masked the rest of the anomaly.
The engraved rock shelter at Nobbys Creek in the Clarence Valley was also traversed four times but there are practical problems in using a proton magnetometer in the confined space presented by many rock shelters. The readings did show a tendency to rise as one moved from the back of the rock shelter outwards towards the drip-line but we are doubtful how much we would deduce from this.
Our field tests have shown that some Australian prehistoric deposits are suitable for magnetic survey: hunter gatherers did create structures susceptible to magnetic prospecting measurement. Coastal shell mid- dens (a rather unlikely type of site for such tests) can have a significant magnetic anomaly and it should be possible to detect hearths in an open settlement site. So far as middens are concerned, magnetic survey could be useful for indicating localized centres of human activity within an extensive deposit. In addition, the extent of a midden deposit could be demonstrated in cases where surface indications are absent. The possibility of locating hearth areas in open settlement sites is probably of greater importance. For
instance, the eroded surface character of much of the evidence at Lake Mungo might suggest a proton magnetometer survey as a means of selecting areas of hearths before erosion removes their stratigraphic context.
The geophysical aspects
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Field and interpretation techniques de- veloped in a study of the application of an alkali vapour magnetometer (Stanley, 1975a), have been adapted for use with data recorded using a proton precession instrument.
Two problems arise in the application of magnetic prospecting to an Australian archaeological situation. The magnetic anomaly will typically be very weak and may be camouflaged by the fluctuations which are continually taking place in the Earths magnetic field, or anomalies resulting from the presence of natural magnetic mineral in the soil. Secondly, the magnetic anomaly will usually only cover a small, irregular area.
It is essential to recognize two important facts. The intensity of a magnetic anomaly resulting from an archaeological structure will generally decrease in amplitude with the inverse cube of the distance separating the sensor from the object. It is therefore an advantage to conduct the survey as close as possible to the feature to be located. However, it is well known (Le Borgne, 1955) that magnetic minerals tend to accumulate on the ground surface and these result in magnetic anomalies which must not be confused with those of archaeological significance.
The second consideration is the distance selected between magnetic measurements. To recognize the presence of an anomaly, at least several measurements must be re- corded within its influence. The weaker the magnetization the closer the spatial dimen- sion of the anomaly will approach that of the object producing it. Hence we can specify that the maximum allowable distance be- tween magnetic measurements must be several times less than the dimension of the body being sought. In practice, if we are hoping to locate an object 1 m wide (for example a fire hearth) sampling at least every 0.1 m is necessary to adequately define the anomaly. Herein lies the limiting restriction on the use of a proton precession
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magnetometer. A proton precession mag- netometer resolving 1 nT can be operated no faster than about 10 measurements per minute. By comparison, the alkali vapour magnetometer can record 3000 such measurements in the same time. None the less, surveying at 10 measurements per minute is still perfectly practical for mapping an archaeological site.
Referring to FIG. 3, the most obvious characteristic of the upper profile is the magnetic signature (Stanley, 1975b) associated with the midden. This jagged section of the profile is only indirectly associated with the midden. The added fertility of the midden material has resulted in increased vegetable humus accumulation on the ground surface. As seen in TABLE 1, this layer was the most magnetic of those sampled and it was also the closest to the magnetometer sensor. Consequently, a method was sought for filtering the anomaly due to the humus from that due to the more bulky but less magnetic midden.
The most obvious approach was to raise the height of the sensor above the ground and by virtue of the inverse cube law des- cribed, diminish the surface humus anomaly with respect to that of the midden. The disadvantage of such a strategy is that the magnitude of the already weak anomaly is
STUARTS POINT N.S.W. N
further reduced to perhaps an immeasurably small value.
The method adopted involved recording the magnetic profile right at ground level where the anomaly was greatest. The data were then digitally filtered with a pocket calculator. The filtering used was simply a running average calculated along the profile. FIG. 3B shows a 30 point running average profile and 3A the unfiltered total field data. The filtered curve is relatively free of the high frequency component and it very closely resembles a curve recorded in the laboratory using a model of the midden. Note that the curve over the midden is clearly pronounced and that the inflection points correspond closely to the edges of the midden.
If a large number of filtered parallel traverses are traced side by side we pro- duce a pseudo-perspective image of the anomaly (FIG. 4). The presence of the midden is emphasized and susceptibility changes within it are apparent. The field data for this image took the three of us a day to record using a proton precession mag- netometer. Filtering was done in a day by one person (JS) and the actual tracing of the profiles took little over an hour. Clarity of the image was improved by interpolating between traverses. Magnetic inhomogeneities within the midden (principally burnt areas)
35 30 25 20 15 10 5 0 Metre FIG. 4 A pseudo-perspective image of the magnetic field associated with the Stuarts Point midden.
The image was composed by direct tracing of filtered parallel profiles traversing the midden.
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FIG. 5 An isometric image of the magnetic field recorded over the site of a hearth.
can be identified from the isometric image provided they cover an area greater than two square metres. A more detailed survey would be capable of isolating even smaller areas of inhomogeneity.
Another example of the isometric enhance- ment technique is an image of the magnetic anomaly associated with the simulated hearth previously described (FIG. 5 ) . These data were recorded with the alkali vapour magnetometer and each profile contains approximately 3000 measurements. Record- ing of the data took one hour and drafting of the isometric took two hours. Had the proton precession magnetometer been used, satisfactory data (say 30 measurements per traverse) would have taken about a day to record. Note that with this image, anomalies of amplitude just a few nT and covering an area less than 0.1 m2 are clearly visible. The eye completely fails to register the random noise variations which are in fact present in each traverse.
With the data processing techniques des- cribed, the proton precession magnetometer is capable of mapping disturbances originat- ing from prior aboriginal habitation. The spatial resolution that is achieved is directly related to the grid spacing. Although a highly detailed survey is perhaps tedious to perform, the processing of the data can be straight- forward, rapid, and can be performed on site.
LE BORGNE, E. 1955. Susceptible magnetique anormale. Annls. Gtophys., 11 , 399.
STANLEY, J. M. 1975a. An alkali vapour mag- netometer and its applications. Unpublished Phd...