Lightning-induced magnetic anomalies on archaeological sites

Archaeological ProspectionArchaeol. Prospect. 12, 191–197 (2005)Published online 21 July 2005 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/arp.257

Lightning-inducedMagneticAnomaliesonArchaeologicalSites

GEOFFREYJONES* ANDDAVID L.MAKI

Archaeo-Physics, LLC,Minneapolis,MN55414, USA

ABSTRACT Recent studies of known lightning impacts and laboratory testing of samples from a suspectedlightning-inducedanomalyappearinginmagnetic surveydataallowustocharacterizetheseformerlyenigmatic responses. Remanent magnetization associated with the current path of a lightningdischarge produces strong anomalies that can be recognized in magnetic survey data, and can beidentifiedpositivelyusinglaboratorymethods.Copyright�2005 JohnWiley&Sons,Ltd.

Key words: magnetic; gradiometer; lightning; remanence; lightning-induced remanent magnetism(LIRM)

Introduction

Lightning-induced remanent magnetism (LIRM)is a phenomenon that appears to be commonlyencountered on archaeological sites, but rarelyrecognized in magnetic survey data. Strongbipolar anomalies of linear, radial or dendriticform appearing in magnetic data plots havesometimes been interpreted as ferrous metalor igneous intrusions. A number of these enig-matic anomalies have appeared in magneticfield gradient surveys conducted by the authorsas well as by other investigators. Several ofthese were investigated by coring and handexcavation, but no apparent sources for theseanomalies could be found. The lack of visibleanomaly sources and polarities not aligned to thegeomagnetic field (either past or present) sug-gested lightning strikes as a possible cause ofmagnetization. Fortunately, soil samples fromoneof these (Figure 1), the 30-30 Winchester Site(48CA3030), located in northeastern Wyoming,were retained by the archaeological investigator,

and were found to contain still-consolidatedlumps that could be subjected to laboratoryanalysis.

Laboratory test results

Consolidated soil samples from the suspectedLIRM anomaly were analysed using palaeomag-netic techniques. The orientations of the sampleswere not recorded as they were collected. How-ever, this was not a factor, as the laboratoryanalysis did not measure inclination or declina-tion, but rather focused on intensity of magneti-zation and the magnetic mineral concentration,grain size and mineralogy of the magnetized soil.The analysis used criteria for identifying LIRMpreviously presented by Dunlop et al. (1984) andVerrier and Rochette (1999, 2002). A completediscussion of this rather involved topic is thesubject of a separate article (Maki, 2005), how-ever, the results will be briefly summarizedbelow.

The laboratory analysis determined thatsamples collected from near the centre of theLIRM anomaly possessed an unusually intenseremanent magnetization. The natural remanent

Copyright # 2005 John Wiley & Sons, Ltd. Received 1 July 2004Accepted 8 December 2004

* Correspondence to: G. Jones, Archaeo-Physics, LLC, 13135th Street SE suite 315, Minneapolis, MN 55414, USA.E-mail: [email protected]

magnetization (NRM) of these samples was morethan 20% larger than a laboratory impartedsaturation isothermal remanent magnetization(SIRM). The ratio of NRM to the magnetizationinduced in the presence of the earth’s magneticfield (Konigsberger ratio, Qn) was greater than10. Such a large Qn ratio cannot be explained byother more common sources of remanence suchas thermoremanent magnetization (TRM) or che-mical remanent magnetization (CRM). Alternat-ing field (AF) demagnetization of the LIRMsample resulted in a relatively hard curve shape,indicating that high coercivity minerals had beenmagnetized. Finally, the ratio known as REM0

should peak at greater than 0.1 in materialsmagnetized by high-field isothermal processes(Verrier and Rochette, 2002; Gattacceca andRochette, 2004). The REM0 is the derivative ofthe AF demagnetization curve (dNRM/dAF)

normalized by the derivative of the AF demag-netization curve of a sample given a laboratoryimparted SIRM (dSIRM/dAF). Soil samplesfrom the suspected LIRM possessed peak REM0

values of approximately 1.5, well in excess of the0.1 criteria for LIRM acquisition suggested byVerrier and Rochette (2002).

In addition to the above mentioned measuresof magnetic remanence, several tests were con-ducted in an effort to determine whether themagnetic mineral concentration, grain size ormineralogy from near the centre of the LIRManomaly were different from that found in sam-ples obtained from surrounding undisturbed soilprofiles. These tests included measurement ofmagnetic susceptibility, anhysteretic remanentmagnetization (ARM) and the S-Ratio (a com-plete description of these environmental magnet-ism parameters can be found in Evans and Heller

Figure1. Magnetic surveydata fromthe 3030WinchesterSite.AlighteninginducedanomalyappearsatN20/E20.Smallercircularanomalies in the southern portion of the data plot are caused by prehistoric hearth features (Jones, 2001; Munson, 2002; http://www.archaeophysics.com/3030/, accessed 23 April 2004).

192 G. Jones and D. L. Maki

Copyright # 2005 John Wiley & Sons, Ltd. Archaeol. Prospect. 12, 191–197 (2005)

(2003)). No significant variation in any ofthese parameters was observed. The lack of asusceptibility contrast shows that induced mag-netization could not have been responsible for theobserved anomaly. The lack of contrastin magnetic mineral concentration, grain size ormineralogy indicates that thermally inducedmineralogical transformations did not occur inthe vicinity of the lightning strike. This supportsthe previously discussed interpretation that TRMwas not a factor in the observed signal response,as temperatures above the Curie temperature ofmost magnetic minerals would also result insome high-temperature mineralogical transfor-mations. These findings are consistent with Cox(1961) and Sakai et al. (1998), who conclude thatTRM is, at best, a relatively minor component ofLIRM anomalies.

Discussion

Recent studies have mapped known lightning-induced anomalies in soils (Sakai et al., 1998;

Verrier and Rochette, 1999, 2002). Although thesemaps are schematic in nature, based on labora-tory analysis of collected samples, they are con-sistent with the models proposed in this article.Whereas these studies are mainly concernedwith magnetization directly associated with thepoint of impact, earlier studies (Cox, 1961;Graham, 1961) documented horizontal paths oflightning conduction in the near surface of bed-rock outcrops. Bevan (1995) calculates that theanomaly resulting from a vertical current pathwould be bipolar with the maxima of the poles ineast–west alignment. Vertical current paths havebeen observed on sandy seashores, and may beexpected where more conductive materials occurbeneath the surface.

Several suspected LIRM anomalies appear inFigures 1–5. Three of these data sets (Figures 1, 3and 4) were collected by the authors on sites onthe Great Plains of the USA. The majority ofthe suspected LIRM anomalies observed by theauthors (typically several each year) haveoccurred in this region. This may represent aregional bias in our work, although the American

Figure 2. Suspected LIRManomaly fromsiteAR-03-04-06-838CNF, in CoconinoNational Forest,Arizona.Blankvalueswerere-corded where the gradient exceeded the instrument’s dynamic range of � 2047.5 nT. Excavation revealed no visible anomalysource (Somerset al., in press).

Lightning-induced Magnetic Anomalies 193


Figure 3. Suspected LIRManomalies from the Elusive Porcupine site (32RB300), southeasternMontana (Jones, 2003).

Figure 4. Two suspected linear LIRM anomalies appear at N55/E15 and N23/E83 in this data plot from the OdessaYates Site(34BV100) inwestern Oklahoma.Theseanomalieshaveno correlates in resistance data andextensive coringrevealednovisibleanomaly source.The source of themagnetic highat N18/E67 is known to bea prehistoric pit house (Brosowske et al., 2000).



Plains do experience frequent thunderstorms.Figures 2 and 5 illustrate examples collected byother investigators in the southwest and mid-west (respectively) of the USA.

These anomalies are either linear or radial inform, the radial anomalies having a number oflinear components radiating from a commoncentre. Each linear component is bipolar, with apolarity that is perpendicular to the long axis ofthe anomaly. As lightning often strikes trees, itis tempting to imagine that anomalies of radialpattern result from the conduction of currentby tree roots, although there is no evidence oftrees being associated with the examples givenhere. Other examples showing a simple linearanomaly are presumably the result of the con-ducted current following a single path of lowresistance.

Over 90% of lightning strikes worldwide arenegatively charged. The polarity of the light-ning discharge is expressed by the configura-tion of positive and negative components ofthe LIRM field. The IRM associated with acurrent path will be concentric, as illustratedin Figure 6. The direction of magnetic fieldfollows the ‘right hand rule’. The observedgradient anomaly would be positive where the

direction of magnetization adds to the geomag-netic field, and negative where it subtracts.All of the radial anomalies illustrated here(Figures 1, 2, 5, and one of the anomalies inFigure 3) appear to result from negative dis-charges, with the current flow toward the pointof impact. Where isolated linear anomaliesoccur (Figure 4), the polarity of the dischargemay not be apparent if the point of impact is notknown.

Anomaly strengths will depend on a numberof factors that have been studied extensively by

Figure 5. Suspected LIRManomaly fromtheGreat Hopwell Road,Ohio (SimpsonandKvamme,2001; http://www.cast.uark.edu/nadag/projects database/Simpson1/Simpson1.htm, accessed 23 April 2004).

Figure 6. Model showing the direction of magnetization ofearth surroundinga lighteningdischargepath.



lightning researchers, but are of little interest inan archaeological context. Observed magneticgradient anomalies have ranged from beyondthe maximum range of the instrument(� 2047.5 nT) to a few tenths of a nT, onlyfaintly perceptible in even the quietest dataset. We suggest that lightning-induced anoma-lies may initially display a very high rema-nence, but may diminish with time. Asindividual sediment grains are reoriented byturbation processes, vector subtraction ofincreasingly randomized magnetic momentswill eventually reduce the net remanence tozero. Unlike lightning-induced anomalies,those due to thermal features can remaindetectable indefinitely because of enhancedmagnetic susceptibility.

Conclusion

Although the examples presented here are ofNorth American provenance, lightning andLIRM occur throughout the world. PresumablyLIRM effects may be observed in magneticsurveys elsewhere. It is the purpose of thisreport to familiarize archaeological geophysi-cists with the characteristics of LIRM anoma-lies, so that they may be identified anddifferentiated from anomalies originating inthe archaeological record. Although laboratorytesting of soil samples may be required to con-clusively identify LIRM as an anomaly source,LIRM may be suspected based on survey andexcavation data. The strength and geometry ofmagnetic anomalies may suggest LIRM; uponexcavation, a lack of a visible anomaly source(or the presence of fulgurites) would tend tosupport this identification. Ongoing lightningresearch by geophysicists and atmosphericscientists will undoubtedly contribute to a bet-ter understanding of the expression of LIRM inmagnetic survey data.

Acknowledgements

The authors would like to thank Kenneth L.Kvamme and Lewis Somers for contributingdata sets used as examples, and Pierre Rochette,

Violaine Verrier and Bruce Bevan for their in-valuable insights and assistance in developingour understanding of this phenomenon.

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Lightning-induced magnetic anomalies on archaeological sites

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