At the equator: making sense of magnetomer data (PDF Download Available)
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23.69University of Bradford25.23Durham UniversityAbstractWhen undertaking magnetometer surveys in northern Europe or north America it hardly matters whether a vertical vector magnetometer (e.g. fluxgate) or total field instrument (e.g. caesium) is used: the anomalies are fairly similar. In fact even the difference between vertical gradiometers and single sensor instruments is not particularly large. All these instruments can be used alongside each other, and producing a combined data plot often requires not more than some amplitude scaling. However...Discover the world's research14+ million members100+ million publications700k+ research projects
ArchéoSciences33 (suppl.)? (2009)Mémoire du sol, espace des hommes................................................................................................................................................................................................................................................................................................Armin Schmidt, Robin Coningham et Prishanta GunawardhanaAt the equator: making sense ofmagnetometer data................................................................................................................................................................................................................................................................................................AvertissementLe contenu de ce site relève de la législation fran?aise sur la propriété intellectuelle et est la propriété exclusive del&éditeur.Les oeuvres
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l&éditionélectronique ouverte (CNRS, EHESS, UP, UAPV).................................................................................................................................................................................................................................................................................................Référence électroniqueArmin Schmidt, Robin Coningham et Prishanta Gunawardhana, <<?At the equator: making sense of magnetometerdata?>>, ArchéoSciences [En ligne], 33 (suppl.)?|?2009, mis en ligne le 30 octobre 2011, consulté le 04 janvier 2013.URL?: http://archeosciences.revues.org/1815?diteur : Presses universitaires de Renneshttp://archeosciences.revues.orghttp://www.revues.orgDocument accessible en ligne sur : http://archeosciences.revues.org/1815Ce document est le fac-similé de l&édition papier.Tous droits réservés
A?????S???????, revue d’archéométrie, suppl. 33, 2009, p. 345-347When undertaking magnetometer surveys in northern Europe or north America1 it hardly matters whether a verti-cal vector magnetometer (e.g. fl uxgate) or total fi eld instru-ment (e.g. caesium) is used: the anomalies are fairly similar. In fact even the diff erence between vertical gradiometers and single sensor instruments is not particularly large. All these instruments can be used alongside each other, and producing a combined data plot often requires not more than some amplitude scaling. However, when surveys are undertaken closer to the magnetic equator the discrepancies in the data collected with diff erent instruments and in various confi gu-rations create challenges for the overall archaeological inter-pretation of results. As part of the Anuradhapura Hinterland Project in Sri Lanka (the Upper Malwatu Oya Exploration Project, UMOEP (Coningham et al., 2007) three seasons of magnetometer sur two using fl uxgate gradiometers and one with Caesium magnetometers in dual-sensor confi guration.?
e aim of this paper is to review the anomalies expected from diff erent magnetometers close to the magnetic equator and to present theoretical as well as pragmatic solution to the integration of the resulting data.?
e UMOP investigates links between the ancient capi-tal of Sri Lanka, Anuradhapura (4th c. BC to 11th c. AD) and the surrounding settlements, modelling the networks between urban and non-urban communities and the envi-ronment within the plain of Anuradhapura over the course of two millennia. To achieve this, archaeological surveys along random transects were conducted within a 50 km radius from the ancient citadel. Sites identifi ed through pottery scatter or structural remains were then selected for detailed geophysical surveys and auger sampling. Based on results from these investigations small excavation trenches (2 m2 per site) were positioned and geoarchaeological samples taken. ?
ese explorations were underpinned by ethnoarchaeologi-cal studies that helped to formulate a theoretical framework for the archaeological interpretation of results.Many of the geophysically investigated sites either had to be split into several small areas (e.g. individual garden plots or rice fi elds) or were covered in dense vegetation (chena), which made survey-logistsics diffi
cult. Only a small and highly mobile instrument was deemed feasible for these conditions and the Geoscan FM256 fl uxgate gradiome-ter was hence chose for the project. However, the earth’s magnetic fi eld inclination near Anuradhapura is nearly hori-zontal (inclination +1.0°, declination - 2.5° (W) and inten-sity 40516 nT; IGRF-2005), which means that away from magnetic anomalies the vertical fl uxgate sensors measured hardly any signal and the instrument was hence extremely diffi
cult to set up using the manufacturer-recommended pro-cedure2. A pronounced instrument drift was experienced due to high ambient temperatures and even slightly misaligned At the equator: making sense of magnetometer dataArmin S?????? *, Robin C???????? ** and Prishanta G??????????? ***Key words: Single Sensor, Total Field, Gradiometer, Vector Sensor, Sri Lanka* Archaeological Sciences, Division of AGES, University of Bradford, UK. (A.Schmidt@Bradford.ac.uk)** Department of Archaeology, Durham University, UK.*** Department of Archaeology, University of Kelaniya, Colombo, Sri Lanka.1. Or Australia, south America or southern Africa, for that matter.2.
A set-up in horizontal orientation was attempted but maintaining an exact horizontal alignment while rotating through 360° was impossible. It might be worth constructing a non-magnetic rig for this purpose.
346 Armin SCHMIDT, Robin CONINGHAM, Prishanta GUNAWARDHANAA?????S???????, revue d’archéométrie, suppl. 33, 2009, p. 345-347sensors (heading errors) resulted in striped data. It was hence necessary to balance and align the instrument frequently, at least every two grids. ?
is facilitated the collection of consistent data with low noise levels that allowed to discern archaeological features against a geological background. A relatively high spatial resolution (0.25 m × 0.5 m) was essen-tial to identify archaeological anomalies.In order to eliminate the time-consuming set-up proce-dure, a two-sensor Caesium magnetometer was used for the third fi eld season (Geometrics G-858). ?
e two sensors were mounted 0.5 m horizontally apart to accommodate the necessary cross-line separation. It had been established earlier that such duo-sensor confi guration is only minimally infl uence by diurnal variations over the length of a survey line (Becker, 1995; Tabbagh, 2003) and the time-saving over deployment in vertical gradiometer mode (i. e. not requiring two traverses for the same spatial resolution) was deemed signifi cant. It is known that the ferrous content of this particular instrument’s console requires a separation to the sensors of about one metre to avoid noise, and it is hence frequently deployed along a long beam (manufac-turer’s recommendation), on a handheld wooden frame (Fassbinder & Gorka 2009) or mounted on a cart (Becker, 1995). However, the vegetation on the investigated sites and sensitivity to even slight heading errors made it necessary to obtain very fi rm control over the location of the sensor array during the survey while maintaining high manoeuvrability. ?
e instrument was hence deploye the front one fi rmly holding the sensor array and maintaining a steady pace and the rear operator carrying the console and inserting markers as directed by the front operator (Fig. 1). ?
e two operators had to be non-magnetic and well-trained and this resource implication was nearly as restrictive as the time that was required to set up the fl uxgate gradiometer in previous seasons.As a result of the use of diff erent instruments in contras-ting confi gurations for the three fi eld seasons the compari-son of data became diffi
cult across the sites surveyed. Both instruments were also tested over the same anomaly and their output was found to look very diff erent (Fig. 2).Land-based magnetometer surveys are usually described by magnetostatic theory and results can therefore be mathe-matically converted between instrument types (e.g. between vector and total fi eld instruments) and sensor confi gurations (e.g. between gradiometer and single sensor) (Blakely, 1996; Tabbagh et al., 1997). However, the required frequency-domain processing is known to enhance noise levels and cannot reconstruct static off set values. To overcome these limitations processing schemes in the space – domain are evaluated for their eff ectiveness. ?
e background removal eff ect typically associated with a gradiometer can be pro-duced by applying a high-pass fi lter to single-sensor data. Nevertheless, the diff erence in anomaly-shape between total fi eld and vector sensors remains signifi cant at this low magnetic latitude. Data from a dual-sensor system can be Figure 1: Geometrics G-858 in dual-sensor confi guration with two operators.Figure 2: Magnetic anomaly (see outline) at site F102, south of Anuradhapura, Sri Lanka. (a) fl uxgate gradiome-ter FM256, (b) Caesium total fi eld sensors G-858 in dual-sensor confi guration and (c) total fi eld data after high-pass fi ltering.
? e scale and data range is the same for all data plots (see (b).
A?????S???????, revue d’archéométrie, suppl. 33, 2009, p. 345-347At the equator: making sense of magnetometer data
347treated as if produced by a horizontal gradiometer (Fassbinder & Gorka, 2009).During a project diff erent magnetometers may be used for various practical reasons. However, their particular pro-perties have to be considered carefully when interpreting results. ?
is is especially important at the magnetic equa-tor, where data from total fi eld and vector sensors are very diff erent. A thorough understanding of these geophysical relationships is particularly relevant for the interpretation of archaeological anomalies and some processing steps are available to aid in this process.ReferencesBECKER, H., 1995. From Nanotesla to Picotesla-a New Window for Magnetic Prospecting in Archaeology. Archaeological Prospection 2(4): 217-228.BLAKELY, R. J., 1996. Potential ?
eory in Gravity and Magnetic Applications. Cambridge: Cambridge University Press.CONINGHAM, R., GUNAWARDHANA, P., MANUEL, M., ADIKARI, G., KATUGAMPOLA, M., YOUNG, R., SCHMIDT, A., KRISHNAN, K., SIMPSON, I., MCDONNELL, G., BATT, C., 2007. ?
e state of theocracy: defi ning an early medieval hinterland in Sri Lanka. Antiquity 81(313): 699-719.FASSBINDER, J. W. E., GORKA, T. H., 2009. Beneath the Desert Soil – Archaeological Prospecting with a Caesium Magnetometer. In M. Reindel and G. A. Wagner (eds) New Technologies for Archaeology: 49-69. Berlin, Heidelberg: Springer.TABBAGH, A., DESVIGNES, G., DABAS, M., 1997. Processing of Z Gradiometer Magnetic Data Using Linear Transforms and Analytical Signal. Archaeological Prospection 4: 1-14.TABBAGH, J., 2003. Total fi eld magnetic prospection: are vertical gradiometer measurements preferable to single sensor survey? Archaeological Prospection 10(2): 75-81.
CitationsCitations3ReferencesReferences9With the help of &archaeological knowledge& and the excavation report of a similar type of building with two phases ( M?slein, 2002), it was possible to interpret the magnetogram ( Figure 11). Magnetic prospection near the geomagnetic equator Although there are numerous case studies of magnetic prospection for archaeological sites in the northern hemisphere, only rare papers report on sites located near the geomagnetic equator ( Tite, 1966;Fassbinder and Becker, 1999;Magnavita and Schleifer, 2004;Schmidt et al., 2009;Fassbinder and Gorka, 2011). There are probably two reasons for this. ABSTRACT: Magnetic prospection was applied for the first time to archaeology in 1956 (Belshé, 1957; Aitken, 1958), and over the years since then, it has become one of the most important archaeological methods for the detection and mapping of buried remains at large archaeological sites (Aitken, 1974; Scollar et al., 1990; Clark, 1996; Neubauer et al., ; Benech, 2005; David et al., 2008). Magnetic detection methods are extremely sensitive in the characterization and analysis of iron oxides, much more so than any other form of chemical analysis. Therefore, given a full understanding of the nature of magnetic properties, many details of soil layers and buried archaeological structures can be discovered, visualized, and interpreted only by the “magnetic eye” (Schleifer et al., 2003; Fr?hlich et al., 2003; Schleifer, 2004). A complete archaeological interpretation prior to excavation must consider all available archaeological background information as well however, many more crucial details can be derived through a comprehensive soil magnetic analysis, and many new archaeological questions arise from such geophysical prospecting results. For a long time, archaeologists held the firm conviction that geophysical prospecting results on their own would be only of limited use in the resolution of archaeological problems. Today, it has become commonplace that the initiation of a modern archaeological excavation must be preceded by some kind of geophysical prospecting (Schmidt, 2002; Fassbinder, 2007; Aspinal et al., 2008; Fassbinder, 2015a).
The great success of magnetic prospection in general is due to the fact that almost all soils of the world show an enhancement of magnetic minerals such as maghemite or magnetite in the topsoil (Le Borgne, ). Except for very rare situations, mostly on sites with dammed-up water and consistent soil wetness, there exist no limiting geological factors precluding the application of magnetic prospecting. Enrichment of these minerals in archaeological soil layers – especially in fireplaces, but also in ditches, pits, or postholes – is caused by the formation of these minerals either by natural or anthropogenic fires, varied pedogenic processes (Taylor et al., 1987), or magnetotactic soil bacteria (Fassbinder et al., 1990; Stanjek et al., 1994). The use of fire, however, plays the major role in the enhancement of magnetic minerals in soils, since this occurs on nearly all sites from the Paleolithic to modern times. Full-text · Chapter · Oct 2016 · Journal of Archaeological Science nT (10 ?9 T and 0 to ±90 respectively)) and hence play a great role in the archaeological interpretation of the data. Although there are numerous case studies from magnetic prospection of archaeological sites in the northern hemisphere, only rare papers report from sites close to the geomagnetic equator (Tite, 1966; Fassbinder and Becker, 1999; Magnavita and Schleifer, 2005; Schmidt et al., 2009; Fassbinder and Gorka, 2011; Welham et al., 2014). The reason is probably twofold: firstly, geophysics is well established as a prospecting method for archaeological fieldwork in Europe, Russia and North America, as well as in China and Japan, but poorly in the countries at the equatorial latitudes. ABSTRACT: Geophysical science offers a large range of methods that have been adapted for the detection of archaeological structures beneath the surface. Magnetometry is among others the most successful, and with respect to large survey areas, the most widely-used scientific toolkit that is used by archaeologists. New developments in instrument techniques and real-time GPS enlarged the role of magnetometer prospecting for archaeological science considerably and allow even the detailed prospecting and analysis of landscapes. An integral part of this method however should be the archaeological interpretation of the geophysical result. In this paper the rock magnetic and soil magnetic background will be discussed, which is necessary for a comprehensive understanding of survey results. The diversity of the magnetic anomalies is exemplified on the basis of selected survey results, and explains the role of magnetic prospecting in the field of archaeological science. Full-text · Article · Apr 2015 nT (10 ?9 T and 0 to ±90 respectively)) and hence play a great role in the archaeological interpretation of the data. Although there are numerous case studies from magnetic prospection of archaeological sites in the northern hemisphere, only rare papers report from sites close to the geomagnetic equator ( Tite, 1966;Fassbinder and Becker, 1999;Magnavita and Schleifer, 2005;Schmidt et al., 2009;Fassbinder and Gorka, 2011;Welham et al., 2014). The reason is probably twofold: firstly, geophysics is well established as a prospecting method for archaeological fieldwork in Europe, Russia and North America, as well as in China and Japan, but poorly in the countries at the equatorial latitudes. ABSTRACT: Geophysical science offers a large range of methods that have been adapted for the detection of archaeological structures beneath the surface. Magnetometry is among others the most successful, and with respect to large survey areas, the most widely-used scientific toolkit that is used by archaeologists. New developments in instrument techniques and real-time GPS enlarged the role of magnetometer prospecting for archaeological science considerably and allow even the detailed prospecting and analysis of landscapes. An integral part of this method however should be the archaeological interpretation of the geophysical result. In this paper the rock magnetic and soil magnetic background will be discussed, which is necessary for a comprehensive understanding of survey results. The diversity of the magnetic anomalies is exemplified on the basis of selected survey results, and explains the role of magnetic prospecting in the field of archaeological science. Full-text · Article · Feb 2015 ProjectProject[...]ArticleAugust 2015 · Antiquity · Impact Factor: 1.43ChapterJanuary 2009Recent surveys of archaeological sites in northern Sri Lanka have highlighted a high level of destruction, in particular at Buddhist monastic sites. Whilst this includes damage resulting from the expansion of agricultural fields and villages or the quarrying of ancient stone slabs and pillars for building materials, most image houses and stupas have been targeted for their Buddhist relics.... ArticleMarch 2013 · Antiquity · Impact Factor: 1.43The domed stupas are among the most distinctive of South Asia&#x27;s religious monuments and have been shown to be sensitive indicators for their society. Since arguments for economic and political change depend on accurate dating, and since the stupas are largely composed of brick, the authors here assess the potential for dating building sequences by applying optically stimulated luminescence to... ArticleAugust 2015 · Antiquity · Impact Factor: 1.43Archaeology aims at imagining past societies, using physical data together with, if available, historical documentation. But this imaginative process is bound by factors widely discussed in social epistemology, including unequal social relations among researchers. Such unequal geopolitics in knowledge has been explored by the present author and others (Goonatilake , ;... Data provided are for informational purposes only. Although carefully collected, accuracy cannot be guaranteed. Publisher conditions are provided by RoMEO. Differing provisions from the publisher&#x27;s actual policy or licence agreement may be applicable.This publication is from a journal that may support self archiving.Last Updated: 11 Jul 17}