Archaeological Prospection, Vol. 2, 217-228 (1995)

From Nanotesla to Picotesla — a NewWindow for Magnetic Prospecting inArchaeology

HELMUT BECKERBayerisches Landesamt fur Denkmalpflege, Department Archaeological Prospectionand Aerial Archaeology, Hofgraben 4 (Postfach 100203), D-80076 Munchen, Germany

ABSTRACT The first use of a picotesla (1 pT = 0.001 nanotesla) sensitivity on the ground is reported formagnetic prospecting in archaeology. The instrument consists of two caesium magnetometers,CS2 (Scintrex, Canada), and a magnetometer processor, MEP720 (Picodas, Canada), which isinterfaced to a subnotebook computer for control and data logging. Time mode sampling every 0.1s allows resampling at 10 cm intervals. Digital image processing techniques are used for thevisualization of data. The system opens a new window for magnetic prospecting in archaeology,especially by detecting archaeological structures with an extremely weak magnetization contrast,caused by biogenic magnetite.

Key words: magnetic prospecting; caesium magnetometer; picotesla (pT); resampling procedure;magnetogram; digital image processing.

Introduction

Magnetic prospecting has been established overthree decades as an important method in ar-chaeological research (Aitken, 1974; Clark, 1990;Scollar et al., 1990). Owing to the variety ofmagnetization processes in archaeological struc-tures, most features of archaeology are imagedabove ground as anomalies of the earth's mag-netic field. Primarily the properties enabling thedectection of archaeological structures has beenexplained by the Le Borgne effect (Le Borgne,1960, 1965). However, this effect cannot apply tomagnetic anomalies caused by non-burnt woo-den posts or palisades, which are in the range ofsome 0.1-0.001 nanotesla (nT) and only de-tectable by high sensitivity caesium magne-tometry (Becker, 1990a,b). Recent investigationsconcerning the magnetic properties of archaeo-logical soils have shown a magnetization processdue to magnetic bacteria, which contain singledomain magnetite crystals acting in the soilforming process by the decomposition of or-ganic material (FaBbinder and Stanjek, 1993;FaBbinder 1994). Under different conditions bio-

CCC 1075-2196/95/040217-12© 1995 by John Wiley & Sons, Ltd.

genic greigite (Fe 3 S 4 ) is formed (FaBbinder andStanjek, 1994), which could be the explanationfor magnetic anomalies within archaeologicalcontexts.

The problem of the visualization of this latentimage had been solved as a combination of highsensitivity and high-resolution magnetics in thefield with digital image processing of data inthe laboratory (Becker, 1983). This techniqueplays an important role in the Bavarian StateAuthority for Monument Protection andArchaeology (Bayerisches LandesamtfurDenkmalpflege, Abteilung Vor-undFrungeschichte), where aerial archaeology iscombined with magnetic prospection on theground on a digital image computer (Becker,1990a). The combined prospection allows thedocumentation of archaeological monumentswithout destruction — even without touchingthe ground. The large area coverage of Bavariais done by aerial photographs with hand-heldcameras through the open window of an aircraft(Cessna 172) taking 20 000 to 40 000 pho-tographs per year during a total of 500 flighthours. Magnetics on the ground concentrates on

Received 27 February 1995Accepted 27 November 1995

21 8 H. Becker

important sites of specific archaeological inter-

est. Only sites with stone structures in theground, e.g. a Roman villa, are measured addi-tionally using resistivity meters (RM15, Geoscan,England). The two caesium magnetometer sys-tems allow the prospection of about 40 ha(400000 m2 ) with 0.5 m sample intervals peryear. The costs per hectare for the instrumentare negligible compared with costs for person-nel. Not using the best instrument available forarchaeological prospection results in a severeloss of information about archaeological fea-tures because of the weak magnetization con-trast, and the inevitable waste of time andmoney.

A new dimension for archaeological prospec-tion was attained recently by an opticallypumped caesium magnetometer with a sensitiv-ity in the picotesla range (1 picotesla (pT) =0.001 nanotesla (nT)). It is the first time thatmagnetic prospecting has been used on theground with picotesla sensitivity.

Magnetic prospecting system withcaesium magnetometers, V101

For more than 10 years a magnetic prospectingsystem has been used in Bavaria and elsewhereconsisting of caesium magnetometers, V101(Varian and Scintrex, Canada), on a wheeleddevice, optoelectronic positioning, and datarecording on hand-held computers (Figure laand b). Over sites with rough surfaces, such asTroy, the sensor configuration can be carried; inthis case the positioning is managed by a fish-erman rope which is pulled from a wheel withan optoelectronic slot disk producing 2 cmpulses. The system allows the coverage of 1 haat 0.5 m intervals (40 000 readings) in one ortwo days. The magnetometers have a sensitivityof 0.1 (±0.05) nanotesla (nT) at 10 Hz cycles.The sensors can be freely configured as a gra-diometer magnetometer (vertical configurationof two sensors 0.3 and 1.8 m above ground) or avariometer magnetometer (one sensor at afixed-base position and the moving sensor at 0.3m above ground). For near-surface structuresthese configurations differ in amplitude by a

Figure 1. Magnetic prospecting system on a non magneticcart in gradiometer mode (0.3 and 1.8 m) of caesium sen-sors V101 (Varian and Scintrex) or CS2 (Scintrex). Distancetriggers are given by the rotation of the wheels via anoptoelectronic device with 2 cm resolution. (b) Aluminiumbox with magnetometer processor MEP720 (Picodas), inter-face electronics, power supply and subnotebook computer,which is connected to the sensor unit by a 50-100 m cableset.

factor of 1.8 (Becker, 1990a). Normally the verti-cal gradiometer mode over 1.5 m on the cart isused for easy field operation, because there isonly one cable set between the sensors and the

The Picotesla for Magnetic Prospecting 219

readout–data storage unit. Only when higher and insufficient spatial resolution. In particular,sensitivity is needed for deeper structures or for sites consisting of small features, such as post-easier operation by carrying the sensor, the van- holes with very low contrast in susceptibility, orometer mode is preferred. deeply buried structures showed these limita-

A vertical sensor separation of 0.5 m, which is tions.used for most fluxgate gradiometers, reducesthe amplitude of the magnetic anomaly by afactor of five. Compared with the sensitivity ofthe V101 caesium magnetometer (±0.05 nT) for Picotesla magnetometerSystema total field, the sensitivity of fluxgate gra- MEP720/CS2diometers for the vertical component of themagnetic field without digital averaging is about10 to 20 times less. Therefore almost all archaeo-logical structures with biogenic magnetite arenot detectable by fluxgate gradiometers.

Although the V101 caesium magnetometergives an output of data every 0.1 s, only discretevalues were sampled every 0.5 m, because ofinsufficient speed and capacity of the firsthand-held computers. A 20-m grid with 0.5 by0.5 m samples (1600 data points) was first loggedin the core of the computer and then written ona microcassette. Sample intervals of 0.25 mcaused a severe loss in speed of operation. Fastoperation of the sensors in zigzag mode resultsin a shift of linear anomalies over adjacent lines.This shift effect changes a linear structure to azigzag feature even after perfect positioning onthe line (Figure 2b). The reason for this phaseshift lies in the windowing technique of V101:the determination of the Lamor frequency for asensitivity of ± 0.05 nT in 0.1 s is only achievedby the determination of a ± 0.035 Hz signal outof 300 MHz. Unfortunately this phase shift cah-not be corrected easily for V101 systems, be-cause it is non-linear and amplitude dependant.This problem could be avoided in parallel mode,but this means a double speed reduction, whichis never acceptable; especially for the large areasinvolved and the limited time available for fieldwork

Nevertheless many projects, such as Assur inIraq (Becker, 1991), Munbaqa in Syria (Beckerand Jansen, 1994), Troy in Turkey (Becker et al.,1993; Becker and Jansen, 1994) (Figure 2a) andvarious sites in Bavaria (Becker 1990a) (Figure3a), have demonstrated the archaeological im-pact of this technique. However, the possibili-ties for archaeological prospection were stilllimited by the sensitivity of the magnetometers

In collaboration with Picodas (Canada), a newmagnetometer system has been developed con-sisting of CS2 sensors (Scintrex) and MEP720magnetometer processors (Picodas) with a sen-sitivity of ± 0.001 nT and again at 10 Hz cycles.Built-in filters allow complete cancellation of, 50and 16 2/3 Hz signals, which opens the possi-bility for operation directly under power linesor near electric railways. MAGGRAD software (Pi-codas), originally designed for aeromagnetics,has been especially modified for the purpose ofarchaeological prospection on the ground. Thesensors and sensor electronics are interfaced bya cable via the magnetometer processor with asubnotebook computer which controls theprocess and storage of data. Data from the twomagnetometers, with gradiometer or variometerconfiguration of the sensors 0.3 m (1.8 m) aboveground, are sampled in time mode (every 0.1 s)on separate channels. Other channels are usedfor distance positioning, status of the magne-tometer, time, etc., which allows resampling upto 0.1 m intervals. The system is powered by a12 V car battery the whole day. For field opera-tion, two people are necessary: one push/pullsthe cart with the sensors in a 20 m grid, theother controls the magnetometers and datastorage under MAGRAD (Picodas). A graphic dis-play of all on-line values and an acoustic warn-ing in the case of magnetometer problems are agreat help in controlling the sampling process inthe field.

GRIDPLOT software (Picodas) gives a graphicplot of a 20 m grid on the screen of the subnote-book computer used in the field. REPLOT soft-ware (Picodas) outputs the data (line, distance,magnetometer 1, magnetometer 2, magnetome-ter gradient, magnetometer status, ext. status,time, etc.) for every 0.1 s in ASCII format, which

22 0 H. Becker

defines the base for the resampling programme(RESAM). One of the important steps in the re-sampling procedure is the shift correction, whichis necessary because of the fast operation of thesystem in zigzag mode. Time mode samplingevery 0.1 s allows a speed dependent shift cor-rection, which results in a rather 'sharp' imageof magnetic alignments. This procedure wouldhave not been applicable for the previous V101system because of a non-linear phase shift,which is dependent on the amplitude of thesignal (Figure 2).

GEOPLOT software (Geoscan, England) can beused after resampling for preliminary data pro-cessing directly in the field. Final data evalua-tion and visualization is made by digital imageprocessing in the laboratory.

Digital image and graphical dataprocessing

The identification of extremely weak magneticanomalies of archaeological structures, such aspalisades or single post-holes, is achieved onlyby digital image processing (Becker, 1983). Theidea of this technique, which can be applied toall two-dimensional geophysical data, is verysimple. The measured point in the field is con-sidered as a pixel (picture element) and themeasured value is transformed to a grey valuebetween 0 (black) and 255 (white). The rathercoarse raster structure of the image may beeliminated by bilinear interpolation or medianfiltering. Several techniques of contrast enhance-

TROIA - ILION

The Picotesla for Magnetic Prospecting 221

Figure 2. Magnetic prospecting in Troy 1992-1994 using caesium magnetometer V101 (0.1 nT) at 0.5 by 0.5 m intervals (9ha, 1992 + 1993 on the right-hand side) and using the CS2-MEP720 system (0.005 nT) with 0.5 by 0.25 m intervals (5 ha1994 on the left-hand side); both in variometer mode; sensor height above ground 0.3 m; magnetogram in digital imageprocessing technique; dynamics -10.0 to +15.5 nT in 256 greyscale (black to white), 20 m grid. (b) Magnetic prospecting inTroy 1993-1994. Detail of Figure 2a, which shows the 'sharper' positioning of the CS2-MEP720 system (left side) afterspeed-dependent shift correction. Technical data for V101 (right-hand side) and CS2-MEP720 same as Figure 2a.

ment and false colour transformation help inidentifying archaeological structures. However,magnetic alignments of palisades or narrow wallditches of houses are identifiable only in grey-shading images. False colour results in slicing oflinear features, which disturbs their context.

Local anomalies are ideally visible after acombination of median and contour filtering.Edge detecting or contour filtering transformsthe grey shading into a drawing; after the sub-traction of these contours from the median fil-tered image even single posts become de-

tectable (Figure 3c). However, the application ofany filtering must be carefully adjusted to thewavelengths of the archaeological features andmust be controlled on the image monitor so asnot to lose the important information.

Software routines for the transformation ofthe highly dynamic picotesla data into the 256grey scale for final digital image processing arenot yet finished. At the present time only lineartransformations in the window technique ornon-linear arctan functions are available.

Digital image processing is also used for recti-

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Figure 3. Neolithic ring ditch site near Schmiedorf in Lower Bavaria (5th millenium Bc), a neolithic ditch (6th millenium ac)and a Hallstatt period rectangular enclosure (1st millenium so). Magnetic prospecting in 1986 with caesium magnetometerV101, gradiometer configuration (0.3 and 1.8 m), sensitivity 0.1 nT, raster 0.5 by 0.5 m. section of the magnetogram as digitalimage, dynamics -1.5 to +3.5 nT in 256 grey-scale (white to black), raw data, 20 m grid. (b) Magnetic prospecting in 1994with caesium magnetometer CS2-MEP720 in variometer mode (one sensor at fixed base, the moving . sensor 0.3 m aboveground), sensitivity 0.001 nT (1 pT), raster 0.5 to 0.25 m, same section of the magnetogram as digital image (Figure 3a),dynamics -3.0 to +6.5 nT in 256 grey-scale (white to black), raw data. 20 m grid. (c) Same as Figure 3b, but subtraction ofthe contour filter from the median filtered image, which results in the traces of some individual posts of the wooden palisade,dynamics -1.00 to 1.55 nT (10 pT per grey value from white to black).

Figure 4. Computer system for digital image and graphic-cartographic data processing at the Bayerisches Landessamt far Denkmalpflege in Munich (stage 9/94).

224 H. Becker

fication of oblique aerial photos and the compi-lation of aerial archaeology with ground geo-physics (Becker, 1990a). Any archaeological in-terpretation is done directly on the screen of thedigital image computer in a graphic overlay,which can be defined in several colours. Thearchaeological features traced are stored as po-lynoms and transformed to a graphic-carto-graphic computer system for final processingand plotting of the plan (Becker, 1990c) (Figure5).

Comparison between the nanotesla andthe picotesla systems

First tests of the Picotesla system in Turkey (e.g.Troy, Figure 2a and b) and Bavaria (e.g.Schmiedorf, Figures 3b, 3c and 5a) in 1994 showsomewhat optimal results in archaeologicalprospection. In Troy the high sensitivity of thepicotesla system could not be used because ofthe extraordinarily high content of geologicalmagnetite in the cultural debris, but the instru-ment was even faster in the field than the previ-ous V101 system. In 1992 and 1993 about 4 haeach (160 000 samples at 0.5 m intervals) weremeasured with that V101 system over 10 days.In the same time in 1994 the MEP720-CS2 sys-tem allowed a coverage of 6 ha at an evenhigher spatial resolution (600 000 samples at 0.5by 0.2 m intervals). The sample rate had beenreduced from 0.1 to 0.2 s because of storageproblems of the pre-processed ASCII data. Withthe picotesla system there seems to be hardlyany non-linear phase-shift in the zigzag-modesampled data. The speed dependent resamplingprocedure results in an almost sharp image,even after fast sampling in the field (Figure 2aand b).

The advantages of the new picotesla systembecome even more evident in the case of theneolithic site near Schmiedorf in Lower Bavaria,which is possibly a sun temple from the fifthmillenium BC (Figure 3a-c). Soil marks in aerialphotographs and a sondage in 1986 have shownthat the site is directly under the plough. Themagnetization, especially of the double pal-isades, is due to biogenic magnetite only, which

causes anomalies in the range of several 0.01 to0.3 nT. The site had been surveyed first in 1986with caesium magnetometer VI01 (0.1 nT sensi-tivity) with 0.5 by 0.5 m line-spacing/intervalsin gradiometer mode, when an almost completeplan of the three ring ditches with the doublepalisade in the interior could be establishedexclusively on the basis of magnetic prospect-ing. The ring ditch site, of 80 m diameter, issurrounded by a large settlement which is alsoenclosed in a double ditch system 250 m indiameter. There is also a simple ring ditch of 40m diameter, which may be of the same neolithicage. The neolithic temple site is overlain by aLate Iron Age (Hallstatt period) squared enclo-sure (Becker, 1990b).

The centre of the three-ring ditch enclosurewas remeasured in 1994 using the CS2-MEP720magnetometer system, with 0.001 nT sensitivityin variometer mode and 0.5 by 0.25 m rasterafter resampling with almost the same surfaceconditions of the corn field. A comparisonbetween the surveys in 1986 and 1994 demon-strates clearly the amount of erosion during 8years. Only the deep middle (D2) and inner ringditch (D3) seem not to be affected: also, in themodel calculation, one can find only small dif-ferences between the anomalies of 2.8-m- or2.6-m-deep ditches. However, the anomaly ofthe outer ring ditch (Dl) and the Iron Age ditch(Hal) has been reduced since 1986 to half am-plitude. The anomalies of the interior doublepalisade are no longer detectable in this profilepresentation (Figure 6).

In consideration of the fact that at least 10-20cm of the surface soil has been removed due todeep ploughing and erosion by wind and rainin this period, the clearer image of the interiordouble palisades is due only to the higher sensi-tivity and higher spatial resolution of the newinstrument (Figures 3a and 6). This structurewas almost invisible even in the open trench ofa sondage, which was excavated in 1986 by anarchaeology team from Edinburgh University.For the picotesla magnetometry a content ofbiogenic magnetite of less than 10 -6 per centmay be sufficient, which causes no change at allin colour of the soils. This means that there maybe a wide range of archaeological structures

The Picotesla for Magnetic Prospecting 225

detectable only by magnetic prospection. Thedynamic range of the new magnetogram in thedigital image technique ranges from -1.00 to+1.55 nT, with 256 grey values, which makes10 Picotesla anomalies visible. After the applica-tion of gradient filtering to the median filtereddata and the subtraction of the two filtered datasets, some anomalies of individual posts of thepalisades are detected (Figure 3c).

Single post-holes also became visible in the

neolithic settlement near Riekofen-Regensburg,where the complete plan of some neolithichouses with all interior posts and wall ditchesbecame identifiable only by the magnetogram(Figure 5a and b).

Future developments

Further developments of the picotesla system

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Figure 5. Neolithic settlement near Riekofen-Regensburg (section). Magnetogram of caesium magnetometer CS2-MEP720,gradiometer mode (0.3 and 1.8 m), sensitivity 5 pT, raster 0.5 by 0.25 m, dynamics -3.0 to +3.4 nT in 256 grey-scale (40 pTper grey-value from white to black), subtraction of median-contour filtered images, 20 m grid. (b) Plan (detail) of the neolithicsettlement near Riekofen-Regensburg with the complete layout of neolithic long-houses on the basis of the magnetogram(Figure 5a) after graphical data processing.

The Picotesla for Magnetic Prospecting 227

m

m

Figure 6. Comparison of caesium magnetometer V101 (1986) and CS2-MEP720 (1994) on a south-north profile crossingthe ditches (D1, D2, D3) and palisades (P1, P2) of the neolithic ring ditch site, a neolithic ditch (N1) and the Hallstatt periodrectangular enclosure (Hal) near Schmiedorf (for location see Figure 3a). (a) Original data of V101 in gradiometer mode (0.3and 1.8 m, solid line) and calculated values for variometer mode (dashed line). (b) Original data of CS2-MEP720 survey invariometer mode at 0.3 m above ground (solid line) and calculated data of V101 in variometer mode for the same hight aboveground (dashed line) showing the effect of erosion after 8 years.

MEP720-CS2 are planned by using an array offour sensors rather than a pair for higher spacialresolution and time reduction in the field, whichis necessary because of the limitations due tointensive agricultural use. Line spacing of 0.25m would be no problem for detection, even ofthe small anomalies of wooden posts, whichform the majority of archaeological structures incentral Europe. The MEP730-760 magnetometer

processors (Picodas) for three to six opticallypumped sensors are already available for fielduse on the ground. On the other hand, specialsoftware routines for the digital image process-ing of the data should be developed, allowing ahigher dynamic range for better identification ofweak anomalies. The limits of magnetics on theground for archaeological prospection have nowbeen reached with the physical limits of the

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method itself. The high sensitivity and spatialresolution of the picotesla magnetometer willcover almost the whole range of the wood–eartharchaeology in humid areas with biogenic mag-netite which documents most of the archaeolog-ical heritage of central Europe.

References

Aitken, M. J. (1974). Physics and Archaeology. Oxford:Clarendon Press.

Becker, H. (1983). Aufbau einer Anlage zur digitalenVerarbeitung von archaologischen Luftbildern undProspektionsmessungen. Arch. Jahr Bayern 1983,201-203.

Becker, H. (1990a). Combination of aerial-photogra-phy with ground magnetics in digital image pro-cessing technique. In Aerial Photography and Geo-physical Prospection in Archaeology. Proceedings ofthe 2nd International Symposium, Brussels, 1986,Vol. 2, 25-35. Brussels: C.I.R.A.-I.C.L.

Becker, H. (1990b). Mittelneolithische Kreisgrabenan-lagen in Niederbayern und ihre Interpretation aufGrund von Luftbilderm und Bodenmagnetik. Vortr.d. 8. Niederbayer. Archkologentages 8, 139-176.

Becker, H. (1990c). Digitale Bildverarbeitung undgraphische Datenverarbeitung in der archao-logischen Prospektion. Archaologische Informa-tionen. 13, 179-186.

Becker, H. (1991). Zur magnetischen Prospektion inAssur. Testmessung 1989. MDOG. 123, 123-131,1991.

H. Becker

Becker, H. (1994). From nanotesla to picotesla — anew window in magnetic prospecting. Report onAchaeometry '94, Ankara.

Becker, H., FaBbinder, J. W. E. and Jansen, H. G.(1993). Magnetische Prospektion in der Untersied-lung von Troia 1992. Studia Troica 3, 117-134.

Becker, H. and Jansen, H. G. (1994). MagnetischeProspektion 1993 der Unterstadt von Troia undIlion. Studia Troica 4, 105-114.

Clark, A. (1990). Seeing Beneath the Soil — ProspectingMethods in Archaeology. London: Batsford.

FaBbinder, J. W. E. (1994). Die magnetischen Eigen-schaften und die Genese ferrimagnetischer Min-erale in Boden im Hinblick auf die magnetischeProspektion archaologischer Bodendenkmaler.Buch am Erlbach.

FaBbinder, J. W. E. and Stanjek, H. (1993). Occurrenceof bacterial magnetite in soils from archaeologicalsites. Archaeologia Polonia 31, 117-128.

FaBbinder, J. W. E. and Stanjek, H. (1994). Magneticproperties of biogenic soil greigite (Fe 3 S 4 ). Geo-physical Research Letters 21, 2349-2352.

Le Borgne, E. (1960). Influence du feu sur lesproperites magnetiques du sol et sur celles duschiste et du granite. Ann. Geophys. 16, 159-196.

Le Borgne. E. (1965). Les proprietes magnetiques dusot. Application a la prospection des SitesArcheologiques. Archaeo-Physika 1, 1-20.

Scollar, I., Tabbagh, A., Hesse, A. and Herzog, I.(1990). Archaeological Prospecting and Remote Sens-ing. Cambridge: Cambridge University Press.

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