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ABSTRACT
Recent studies of known lightning
impacts and laboratory testing of samples from a suspected lightning-induced
anomaly appearing in magnetic survey data allow us to characterize
these formerly enigmatic responses. Remanent magnetization associated
with the current path of a lightning discharge produces strong anomalies
that can be recognized in magnetic survey data, and can be positively
identified using laboratory methods.
LIGHTNING-INDUCED MAGNETIC ANOMALIES
ON ARCHAEOLOGICAL SITES
Geoffrey Jones and David L. Maki
Archaeo-Physics, LLC
Key Words:
Magnetic; gradiometer; lightning; remanence; lightning-induced remanent
magnetism (LIRM)
INTRODUCTION
Lightning-induced remanent magnetism
(LIRM) is a phenomenon that appears to be commonly encountered on
archaeological sites, but rarely recognized in magnetic survey data.
Strong bipolar anomalies of linear, radial, or dendritic form appearing
in magnetic data plots have sometimes been interpreted as ferrous
metal or igneous intrusions. A number of these enigmatic anomalies
have appeared in magnetic field gradient surveys conducted by the
authors as well as by other investigators. Several of these were
investigated by coring and hand excavation, but no apparent sources
for these anomalies could be found. The lack of visible anomaly
sources and polarities not aligned to the geomagnetic field (either
past or present) suggested lightning strikes as a possible cause
of magnetization. Fortunately, soil samples from one of 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-consolidated lumps that could be subjected
to laboratory analysis.
Figure 1. Magnetic survey data from the 3030
Winchester Site. A Lightning induced anomaly appears
at N20/E20. Smaller circular anomalies in the southern portion
of the data plot are caused by Prehistoric hearth features
(Jones, 2001; Munson, 2002).
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LABORATORY TEST RESULTS
Consolidated soil samples from the
suspected LIRM anomaly were analyzed using paleomagnetic techniques.
The orientations of the samples were not recorded as they were collected.
However, this was not a factor, as the laboratory analysis did not
measure inclination or declination, but rather focused on intensity
of magnetization and the magnetic mineral concentration, grain size,
and mineralogy of the magnetized soil. The analysis used criteria
for identifying LIRM previously presented by Dunlop et al. (1984)
and Verrier and Rochette (1999, 2002). A complete discussion of
this rather involved topic is the subject of a separate article
(Maki, in press), however the results will be briefly summarized
below.
The laboratory analysis determined
that samples collected from near the center of the LIRM anomaly
possessed an unusually intense remanent magnetization. The natural
remanent magnetization (NRM) of these samples was more than 20%
larger than a laboratory imparted saturation isothermal remanent
magnetization (SIRM). The ratio of NRM to the magnetization induced
in the presence of the earth's magnetic field (Königsberger
ratio, Qn) was greater than 10. Such a large Qn ratio cannot be
explained by other more common sources of remanence such as thermoremanent
magnetization (TRM) or chemical remanent magnetization (CRM). Alternating
field (AF) demagnetization of the LIRM sample resulted in a relatively
hard curve shape, indicating that high coercivity minerals had been
magnetized. Finally, the ratio known as REM' should peak at greater
than 0.1 in materials magnetized by high-field isothermal processes
(Verrier and Rochette, 2002; Gattacceca and Rochette, in press).
REM' is the derivative of the AF demagnetization curve (dNRM/dAF)
normalized by the derivative of the AF demagnetization curve of
a sample given a laboratory imparted SIRM (dSIRM/dAF). Soil samples
from the suspected LIRM possessed peak REM' values of approximately
1.5, well in excess of the 0.1 criteria for LIRM acquisition suggested
by Verrier and Rochette (2002).
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Figure 2. Suspected LIRM anomaly from site AR-03-04-06-838
CNF, in Coconino National Forest, Arizona. Blank values were
recorded where the gradient exceeded the instrument's dynamic
range of +/- 2047.5 nT. Excavation revealed no visible anomaly
source (Somers et al., n.d.).
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In addition to the above mentioned
measures of magnetic remanence, several tests were conducted in
an effort to determine whether the magnetic mineral concentration,
grain size, or mineralogy from near the center of the LIRM anomaly
was different than that found in samples obtained from surrounding
undisturbed soil profiles. These tests included measurement of magnetic
susceptibility, anhysteretic remanent magnetization (ARM), and the
S-Ratio (a complete description of these environmental magnetism
parameters can be found in Evans and Heller (2003)). No significant
variation in any of these parameters was observed. The lack of a
susceptibility contrast shows that induced magnetization could not
have been responsible for the observed anomaly. The lack of contrast
in magnetic mineral concentration, grain size, or mineralogy indicates
that thermally induced mineralogical transformations did not occur
in the vicinity of the lightning strike. This supports the previously
discussed interpretation that TRM was not a factor in the observed
signal response, as temperatures above the Curie temperature of
most magnetic minerals would also result in some high-temperature
mineralogical transformations. These findings are consistent with
Cox (1961) and Sakai et al. (1998), who conclude that TRM is, at
best, a relatively minor component of LIRM anomalies.
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Figure 3. Suspected LIRM anomalies from the Elusive Porcupine
site (32RB300), southeastern Montana (Jones, 2003).
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DISCUSSION
Recent studies have mapped known
lightning-induced anomalies in soils (Verrier and Rochette, 1999,
2002; Sakai et al., 1998). Although these maps are schematic in
nature, based on laboratory analysis of collected samples, they
are consistent with the models proposed in this article. While these
studies are mainly concerned with magnetization directly associated
with the point of impact, earlier studies (Cox, 1961; Graham, 1961)
documented horizontal paths of lightning conduction in the near
surface of bedrock outcrops. Bevan (1995) calculates that the anomaly
resulting from a vertical current path would be bipolar with the
maxima of the poles in east-west alignment. Vertical current paths
have been observed on sandy seashores, and may be expected where
more conductive materials occur beneath the surface.
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Figure 4. Two suspected linear LIRM anomalies appear at N55/E15
and N23/E83 in this data plot from the Odessa
Yates Site (34BV100) in western Oklahoma. These anomalies
have no correlates in resistance data and extensive coring
revealed no visible anomaly source. The source of the magnetic
high at N18/E67 is known to be a Prehistoric pit house (Brosowske
et al., 2000).
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Several suspected LIRM anomalies
appear in Figures 1-5. Three of these data sets (Figures 1, 3, and
4) were collected by the authors on sites on the Great Plains of
the United States. The majority of the suspected LIRM anomalies
observed by the authors (typically several each year) have occurred
in this region. This may represent a regional bias in our work,
although the American Plains do experience frequent thunderstorms.
Figures 2 and 5 illustrate examples collected by other investigators
in the Southwest and Midwest (respectively) of the United States.
These anomalies are either linear
or radial in form, the radial anomalies having a number of linear
components radiating from a common center. Each linear component
is bipolar, with a polarity that is perpendicular to the long axis
of the anomaly. As lightning often strikes trees, it is tempting
to imagine that anomalies of radial pattern result from the conduction
of current by tree roots, although there is no evidence of trees
being associated with the examples given here. Other examples showing
a simple linear anomaly are presumably the result of the conducted
current following a single path of low resistance.
Over 90% of lightning strikes worldwide
are negatively charged. The polarity of the lightning discharge
is expressed by the configuration of positive and negative components
of the LIRM field. IRM associated with a current path will be concentric,
as illustrated in Figure 6. The direction of magnetic field follows
the "right hand rule." The observed gradient anomaly would
be positive where the direction of magnetization adds to the geomagnetic
field, and negative where it subtracts. All of the radial anomalies
illustrated here (Figures 1, 2, 5, and one of the anomalies in Figure
3) appear to result from negative discharges, with the current flow
toward the point of impact. Where isolated linear anomalies occur
(Figure 4), the polarity of the discharge may not be apparent if
the point of impact is not known.
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Figure 6. Model showing the direction
of magnetization of earth surrounding a lightning discharge
path.
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Anomaly strengths will depend on a number of factors
that have been studied extensively by lightning researchers, but
are of little interest in an archaeological context. Observed magnetic
gradient anomalies have ranged from beyond the maximum range of
the instrument (+/-2047.5 nT) to a few tenths of a nT, only faintly
perceptible in even the quietest data set. We suggest that lightning-induced
anomalies may initially display a very high remanence, but may diminish
with time. As individual sediment grains are re-oriented by turbation
processes, vector subtraction of increasingly randomised magnetic
moments will eventually reduce the net remanence to zero. Unlike
lightning-induced anomalies, those due to thermal features can remain
detectable indefinitely because of enhanced magnetic susceptibility.
CONCLUSION
Although the examples presented here are of North American
provenance, Lightning and LIRM occur throughout the world. Presumably
LIRM effects may be observed in magnetic surveys elsewhere. It is
the purpose of this report to familiarize archaeological geophysicists
with the characteristics of LIRM anomalies, so that they may be
identified and differentiated from anomalies originating in the
archaeological record. Although laboratory testing of soil samples
may be required to conclusively identify LIRM as an anomaly source,
LIRM may be suspected based on survey and excavation data. The strength
and geometry of magnetic anomalies may suggest LIRM; upon excavation,
a lack of a visible anomaly source (or the presence of fulgurites)
would tend to support this identification. Ongoing lightning research
by geophysicists and atmospheric scientists will undoubtedly contribute
to a better understanding of the expression of LIRM in magnetic
survey data.
ACKNOWLEDGMENTS
The authors would like to thank Kenneth L. Kvamme and
Lewis Somers for contributing data sets used as examples, and Pierre
Rochette, Violaine Verrier, and Bruce Bevan for their invaluable
insights and assistance in developing our understanding of this
phenomenon.
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