22
International Conference on Applied Geophysics
26-27 November 2004, Chiang Mai, Thailand
Feasibility of Gradient Magnetometer Surveys of
Buried Brick Structures at 13th Century (C.E.)
Wiang Kum Kam, Chiang Mai Province, Thailand
S.H. Wood*, L.M. Liberty*, F. S. Singharajwarapan**, T. Bundarnsin** and
E. Rothwell*
* Center for Geophysical Investigation of the Shallow Subsurface, Boise State University, Boise, Idaho, 83725, USA
** Department of Geological Sciences, Faculty of Science, Chiang Mai University, Chiang Mai, 50200, Thailand
ABSTRACT
Excavated brick walls of Wat Pa Dom, at the buried city of Wiang Kum Kam, were surveyed
with a two-sensor cesium magnetometer to evaluate the magnetic-field response of brick
structures. Purpose of study was to examine the feasibility of magnetometer exploration for
buried archaeological structures in this area. Brick walls, 1 to 2 meters high and 0.7 -meter wide,
have 50- to 100-nanotesla anomalies when a sensor is within about 1 meter above the top of the
wall. The estimated volume susceptibility of brick structures is about 0.005, in dimensionless SI
units. Both total field maps from upper and lower sensors detected structures, but the gradient
map obtained by differencing the values gives best resolution to better than 0.1-nanotesla
resolution. While the magnetometer survey can be carried out by two people rapidly in open
ground using a 1-meter lane spacing and continuous data stream along a lane, placing iron
objects or magnets at control points to produce strong point anomalies is recommended to verify
the location system of the data logger. An area of 1,000 square meters was surveyed in 2 hours.
KEYWORDS: Magnetometer Surveys, Wiang Kum Kam
Department of Fine Arts. So, the planned survey
INTRODUCTION
Purpose of this study was to measure the
was compromised, such that only the response of
magnetic-field response of brick structures that
the
have been excavated at Wat Pa Dom in the buried
structures
13 Century city of Wiang Kum Kam (Figures 1
anomaly does not reflect the effect of soil cover
and 2). This information will be useful in planning
that might have had an appreciable magnetic
geophysical exploration of similar archaeological
susceptibility. The brick structures, if buried,
sites in Thailand. Magnetometer surveys have
may have a lesser anomaly because the
been used successfully to explore archaeological
susceptibility contrast would be less. To clarify
sites in other areas of the world (Silliman, 2000;
this,
Davis, 2003). In May, 2003, the temple complex
remanence on both brick and soil samples from
had been partly excavated. It was planned to
the site were determined to estimate the
return later and survey the buried part of the
susceptibility contrast. The original ground
structure. However, in January, 2004, the entire
surface was simulated by stringing a network of
Wat Pa Dom complex was found to have been
nylon fishing line tied to wooden stakes at either
excavated
end, spaced 2 meters apart, and marked every 5
th
under
the
supervision
of
the
excavated
the
was
brick
walls
measured.
magnetic
and
uncovered
Therefore,
susceptibility
and
the
the
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International Conference on Applied Geophysics
26-27 November 2004, Chiang Mai, Thailand
meters. This was done over a 1,000-square meter
them access to the river, thereaft er called
area.
Mæ Sæo, on the northeast side of
The presence of a ruined chedi and piles of
bricks
protruding
through
the
Lamphun. He had stayed there three years
floodplain
when the town flooded in the middle of the
sediments were well known to people living in
rainy season, and the elephants, horses,
this area of the Ping River. The remains of a city
cattle, and buffalo had no place to go. [That
wall and moat were visible in the large area of
place] has been called Chiang Rüa / to the
rice paddy on a 1954 aerial
present day.
photo graph
(Sarasawadee, 2000). The construction of the city
In the rwai set year, s. 648 (1286/87), King
by King Mengrai in 1286-87 is documented in the
Mangrai moved to build Wiang Kum Kam.
Chiang Mai Chronicle (Wyatt and Wichienkeeo,
He built a moat around the city on all four
1998). The significance of this area began to be
sides, channeling the flowing waters of the
appreciated in the early 1980s, but considerable
Mæ Raming [another name for the Ping
housing development had already encroached
River]. He built a palisade on all four sides
over the northern part of the buried city.
of the city, and had a great many dwellings
Excavation and partial restoration of the many
and buildings constructed. King Mangrai
individual
under
built his extensive royal dwelling, palace
supervision of the Fine Arts Department. Thirty-
and hall(s), / spreading all around that
four historic sites within the complex have been
site; and it has been called the New Village
excavated and partially restored as a major
(Ban Mai) to the present day. Cao Mangrai
tourist attraction (Harbottle-Johnson, 2002).
ordered men to dig a pond by his palace
temples
began
in
1983
window so that he could watch the work,
HISTORICAL BACKGROUND
and it has been called the Tang (“Window”)
It is interesting to quote from the Chiang Mai
Pond to the present day. Cao Mangrai had
Chronicle on the construction of the city.
the Kum Kam Market established / for
Buddhist chronicles of Southeast Asia were
public commerce”.
copied and recopied as palm leaf manuscripts
A topographically low corridor and water
and
course, now used as an irrigation canal, circles
preserved in monasteries (Wyatt, 1976). Wyatt
around the north end of Wiang Kum Kam (Figures
and Wichienkeeo (1998) searched out and
1 and 2). It has been proposed that the corridor is
translated to English the most complete version
an abandoned channel of the Ping River
of the Chiang Mai Chronicle, which they believe
(Velochovsky and others, 1987; Pitrakul and
was copied from earlier works and included
Uttamo, 1987) that at one time extended for 30
historical material up to 1827:
kilometers to the south and fronted the east side
and
handed
down
through
the
ages
“Two years later, in the ka met year, s.
645 (1283/84), King Mangrai bestowed the
of Lamphun in the time of King Mangrai (Figure
1).
town and villages of Lamphun upon Khun
King Mangrai resided at Wiang Kum Kam for
Fa as lord of Lamphun, and moved to build
5 years and then constructed and moved to the
a [new] city, /fº 2.05/ declaring that he
walled and moated city of Chiang Mai in 1296.
would construct a fortified city with a great
The city of Wiang Kum Kam apparently continued
many households. King Mangrai had the
until the end of the Mangrai Dynasty in 1558.
river dug to pass by [some] villages to give
During the subsequent Burmese occupation of
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International Conference on Applied Geophysics
26-27 November 2004, Chiang Mai, Thailand
northern Thailand, there is no record of the city.
the result of just one flood, with cross-bedded
At some time between 1558 and 1831 the city was
sand indicating flowage from the north and
buried by 1.5 to 1.8 meters of river sediment.
northwest. This may have been fr om the
Velechovsky
present
devastating flood of 1831. However, they suggest
sedimentological studies that indicate burial was
that the city may have been abandoned earlier.
and
others
(1987)
Figure 1.
Map showing Chiang Mai, Wiang Kum Kam, Lamphun, and the former course of the Ping River
(with an inset map of Thailand).
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International Conference on Applied Geophysics
26-27 November 2004, Chiang Mai, Thailand
Figure 2.
1954 air photo of the Wiang Kum Kam area (from Ongsakul, 2000).
METHOD
considerable
effort
of
clearing
brush.
The
Due to time limitations, unexcavated areas
purpose of this preliminary study was to
were not surveyed for buried ancient brick
demonstrate the feasibility of a magnetometer
structures. The many houses, roads, fences, and
survey before investing time in exploration.
power lines, as well as dense brush in the area,
A Geometrics G-858 cesium magnetometer
made a 1-meter grid difficult to survey without
with two sensors mounted on a vertical staff was
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International Conference on Applied Geophysics
26-27 November 2004, Chiang Mai, Thailand
used. The two sensors were spaced 0.5 meter
nanotesla precision. The MagMapper console,
apart (Figure 3). Measured total field from each
the data acquisition system, and the battery
sensor was simultaneously logged on the
power supply were carried on a front pack by the
MagMapper console every 0.5 second to 0.02-
operator.
Figure 3.
Photograph of magnetometer survey of Wat Pa Dom, showing fishing leader lanes, strung every 2
meters
For horizontal control, nylon fishing
program allows the operator to interrupt the data
lines were strung tightly parallel to each other.
stream if he encounters slow or difficult walking
They were spaced 2 meters apart at about what
and climbing between the 5-meter marks, or
had been the ground level prior to excavation.
whatever mark frequency is designated.
The upper sensor was held at this level, which
The spatial outlines of all the brick structures
would have been at ground level prior to
was then surveyed and mapped with a Sokkia-
excavation. The lower sensor would have been at
Set-3 total-station-surveying instrument, noting
a level 0.5 meter below the ground level prior to
the
excavation. Lines were then marked with tape
structures
every 5 meters. The instrument operator
magnetometer lanes.
heights
and
and
dimensions
the
of
endpoints
the
of
brick
the
designated each line in the input keyboard on the
Advantage of a cesium magnetometer is that
magnetometer console and then marked, by
two-sensor data are continuously and rapidly
keying the consol with a mark, every 5 meters
acquired. This permits the operator to cover an
along the traverse. The instrument takes a
area at a normal walking pace. Total field is
continuous stream of data as the operator walks
measured at two heights so that the difference
the line at a normal pace. The acquisition
between the sensors is a measure of the vertical
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International Conference on Applied Geophysics
26-27 November 2004, Chiang Mai, Thailand
gradient of the total field. The differenced
measurements are not affected by the di urnal
RESULTS
variations in the field. This eliminates the need
Maps showing the total field measurements
for a base station measurement if the operator is
of individual sensors define the larger brick
only interested in the gradient. For this survey,
structures, but the gradient map shows more
however, a second instrument, a Geometrics
detail. (Figure 4). The wall on the northeast side
Proton Precession instrument, was periodically
of the map is quite well defined by the lower
read during the survey at a base station, but
sensor. For reasons not understood, the upper
since the diurnal variation in the field was slight,
sensor defines the wall along the northwest side
it was not significant enough to be evaluated. An
better than the lower sensor does. This possibly
area of 1,000 square meters was covered in 2
because the top sensor was closer to the
hours. Twenty lanes, each one 50 meters long,
polarized top of the wall.
were surveyed.
Figure 4.
Map of brick structures at Wat Pa Dom. and maps of total field of top and bottom sensors, and the
vertical gradient (top – bottom). Total field is about 44,000 nanoteslas , and H signifies high values of
total field, and L signifies low values. Ridge like anomalies correspond to walls. Circular anomalies
are high remnant brick structure, and some may be buried iron objects.
In this region the magnetic field is about
44,000 nanoteslas, has a downward inclination of
meter from the top of the wall, the anomaly is
typically 100 nanoteslas.
20° north, and zero declination. The brick walls
The gradient map, which results from the top
are mostly oriented 298° azimuth (Figure 4). On
sensor minus the lower sensor readings, does
the total field maps the greatest anomalies are
show most of the major brick structur es with
the brick walls, which are about 2 meters high
walls thicker than 0.7 meter and 1 meter high
and 1 meter at the base. They taper to 0.5 meter
(Figure 4). Thinner-walled structures do not show
at the top. Where the lower sensor is about 0.7
up clearly even on the gradient map. Very strong
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International Conference on Applied Geophysics
26-27 November 2004, Chiang Mai, Thailand
circular anomalies on the maps may be from
the brick walls adds to the Earth’s field on the
buried iron objects.
west side of walls, but on the east side of walls,
A perfect match of all structures with the
the field subtracts from the Earth’s field (Figure
anomalies could not be made. Part of the
5). This produces a complex map of the induced
matching difficulty arises from the nature of the
field, but these pairs of high values, H, and low
induced magnetization. Because the Earth’s field
values, L, on the map of Figure 4 do associate
is inclined 20 degrees to the north and the walls
correctly with the wall positions (Figure 4).
are oriented 298º azimuth, the field induced by
Figure 5.
Expected shape of anomaly of a brick wall, with Earth’s field inclination of 20 degrees. These
shapes appear in the total field maps as ridge-like anomalies, with a high (H) on the west side, and
a low (L) value on the east side.
Because of the complexity of the map, it is
objects, such as rebar, iron pipe, or strong
not certain that the survey recorded by the
magnets should be placed at key survey points to
MagMap system of the magnetometer is free of
verify that the survey system recorded in the
location errors. Errors could occur if the lane
magnetometer data logger actually matched the
numbering skipped lanes or if 5-meter marks
ground survey. For example, a 1-kilogram iron
were incorrectly entered by the operator walking
object should show about a 60-nanotesla anomaly
the survey. These possible errors in bookkeeping
at a distance of 1 meter, a 7-nanotesla anomaly
could occur by the operators keying the MagMap
at a 2-meter distance, and probably is not
console. Location of the actual measurement
detectable at a distance of 3 meters from the
points is the difficult part of this kind of survey,
magnetometer (Breiner, 1999).
especially when measurement is on a 1-meter
The 100-nanotesla anomaly typical of most of
grid or lane spacing, such as is done in most
the larger brick walls can be used to roughly
archaeological surveys. For future surveys, iron
estimate the volume susceptibility of the brick
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International Conference on Applied Geophysics
26-27 November 2004, Chiang Mai, Thailand
structures by using a formula from Breiner (1999)
magnetization was found in soils at the 0.1-
and Zietz and Andreasen (1967). Consider a long
nanotesla detection limit, but in the bricks
prism, 2 meters high and 0.7 meter wide. Depth
several
to center, when the sensor is 0.7 meter above the
detected,
top, is 0.7 + 1.0 = 1.7 meters. The formula to
magnetization. Samples of soil clumps and old
roughly estimate the maximum anomaly for a
fired bricks from the site are being analyzed for
long prism with a uniform cross-sectional area
both volume susceptibility and strength of
is:
remnant magnetization, but those measurements
nanoteslas
both
as
of
magnetization
induced
and
were
remnant
are not yet available.
F = k H (1.0 A)/ r 2
CONCLUSIONS
where:
Buried brick structures, having walls over 0.5
F = total field anomaly
m thick and at least 1 meter high, should be
k = volume susceptibility, dimensionless, in
detectable at a burial depth of 2 meters or
SI units; however, if given in cgs units, it
shallower. Massive walls and structures, 1 meter
must be multiplied by 4 p (see Thompson
thick and over 1 meter high should be detectable
and Oldfield, 1986, p. 22)
by a magnetometer with 1-nanotesla precision at
H = Earth’s total field in nanoTesla units.
greater depths. Given a typical anomaly of 100
A = cross-section area of a prism in square
nanoteslas
meter units
for
brick
walls,
the
volume
susceptibility of brick st ructures is about 0.005
r = distance or depth to center of prism in
meters
SI units. Walls 0.5 meter thick and 1 meter high
and buried 1 meter should have an anomaly of 10
2
k = F r / (H A)
to 50 nanoteslas. Both total-field and gradient
2
2
= 100 nT x (1.7) / (44,000 nT x 1.4 m )
instruments with 0.1-nanotesla precision should
k = 4.7 x 10 -3 (dimensionless)
be useful in surveys for brick structures of the
type built in the AD 11th -17 th Centuries in
This value is similar to direct measurements
of the average volume susceptibility of 21 bricks
th
northern Thailand.
A 1-meter or 2-meter lane grid, with marks
from a 17 Century wall in Belgium, where k =
every 5 meters, should work at archaeological
3.4 x 10-3 , but it varied from about 1.3 to 9 x 10-3
sites. However, it is necessary that the grid
(Hus and others, 2003). Hus and others inferred
recorded by the magnetometer corresponds to
from susceptibility-temperature data that the
the ground survey. For this, iron stakes or small
main magnetic mineral in the bricks was
strong magnets, should be placed at key points in
magnetite, Fe 3O4.
the grid to verify that the recorded magnetic data
Since
some
soils
and
sediments
have
are correctly located.
susceptibilities of 10 - 3, it is possible that the
contrast between brick structures and soil is not
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International Conference on Applied Geophysics
26-27 November 2004, Chiang Mai, Thailand
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