Jesse Savage Geo-Surveys
Phone: 0425 892 288
Email: jjsav1@student.monash.edu
Electrical Resistivity
Survey
Collins Settlement - Sorrento
03/12/2012
Electrical Resistivity Survey – Collins Settlement, Sorrento
Electrical Resistivity Survey
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PREFACE
My involvement with this project is through the Monash University work placement program,
and in accordance with Alpha Archaeology. I was required to perform a geophysical survey of the
first settlement site in Sorrento. The data collected would then be used, along with previous survey
knowledge, to locate first-settlers graves with greater precision.
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CONTENTS
INTRODUCTION
3
METHODOLOGY
4
RESULTS
6
DISCUSSION
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APPENDIX A. Target Area Maps
APPENDIX B. Survey Data
APPENDIX C. Inverted Resistivity Profiles
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INTRODUCTION
A Cliffside in Sorrento is the focus of Geophysical and Archaeological examination due to its
early-Australian historical importance. The Nepean Historical Society believes that Sorrento was
home to the first-settlers in 1803. The settlement was under command of Lieutenant Colonel David
Collins and included 300 settlers and convicts. The settlement was forsaken in May 1804 when the
settlers transferred to Tasmania. It is believed that graves belonging to some of the first-settlers
may be found at the location.
In March 2011, Elorane Geo-Surveys conducted a survey at the first-settlement location in
Sorrento. Three grids (1-3) were marked out and were investigated by ground magnetic surveys
(GMS) and ground penetrating radar (GPR).
The GMS was used to identify ferrous materials as well as contrasts in soil structure (due to
ground chemistry variation), which may be a result of excavated and replaced soil. GPR emits
electromagnetic waves (Microwave frequency) and records the reflected waves from ground
features to construct sub-surface images. It is used to identify disturbed soil or remains from a
burial.
The GMS has located several anomalies with those in Grid 3 of great interest due to their
lateral extent and alignment. The GPR anomalies have been identified as correlating with the GMS
anomalies on three accounts for Grid 1 and two accounts for Grid 2. The GMS and GPR surveys are
limited for they are prone to tree root disruption and having to move the equipment over uneven
terrain. This encouraged me to perform an Electrical Resistivity Tomography, or, Electrical
Resistivity (ER) survey.
ER surveys measure change in the grounds electrical resistivity and will relate to variable soil
properties. Electrodes are fixed eliminating uneven terrain as a deterrent; and tree roots do not
affect this survey method, as they do with GMS and GPR.
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METHODOLOGY
ER is a method of geophysical analysis with minimal environmental impact. It is a measurement
of a mediums inability to transmit an electrical current. A medium with a high resistivity has more
difficulty transmitting an electrical current and vice versa. The resistivity of a subsurface body
depends on porosity, hydration and clay content. The result is a 2D profile depicting depth of
stratigraphic anomalies and structures.
This survey was conducted using a SuperSting RI Memory Earth Resistivity Meter. This
instrumentation produces measured apparent resistivity pseudosections, which are converted into
easy to interpret 2D ground cross-sections (inverted resistivity sections) using the Earth Imager 2D
Resistivity Inversion Software.
The ER survey works by setting up a line of electrodes. During the survey an electrical current
will be emitted from the first to the second electrodes (known as transmitters). The four
subsequent electrodes measure the electrical potential of the ground. The difference between the
electrical potential and the current emitted will equate to the ground resistivity. This process is
repeated for each subsequent electrode.
The spacing between each electrode will determine the depth of the profile and the resolution.
Greater spacing between electrodes shows resistivity at greater depths but is compromised with
lower resolution. Conversely, shorter spacing will offer greater resolution, with profiles of
shallower depths. The graves are believed to be approximately 1m wide x 2m long x 2m deep.
Therefore, electrode spacing of 0.5m was chosen to optimize the chance of identifying graves.
Electrodes spaced at 0.5m will ensure that possible graves will not go unseen width-ways and that
a high-resolution profile at 2.7m deep is produced. Each line was produced with 1m intervals to
prevent missing indications of graves length-ways.
The excavation and refilling of ground during burial creates a contrast in the physical structure
of the ground. The replaced soil is much less compacted than the natural soil, allowing for greater
electrical transmission. Therefore, we expect graves to exhibit a lower resistivity than the
surrounding ground.
Grid 2 was believed to be the best location for possible graves, and so the focus of the ER
survey was there. Having two anomalies identified from the GMS and GPR surveys by Elorane, we
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coordinated our ER runs to overlie them. If the ER survey would detect any grave-like structures at
these anomalies, we may improve our likelihood of finding first-settlers graves.
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RESULTS
Technical difficulty during the line 1 survey resulted in corrupted data. However, resistivity in
adjacent lines indicate that line 1 is unlikely to be of significance. Line 2 shows three areas of
lower resistivity, however the dimensions do not match what we would expect from a grave.
Line 3 shows two areas of low resistivity of <60 at 12.5m and 15m from the origin and
approximately 0.5-2m deep (Appendix C). These are overlain by highly resistive ground, <500
(Appendix C).
Line 4 and 5 show anomalies that are repeated at 12.5m from the origin (Appendix A). Anomaly
A1 is approximately 1.5m wide and 2m deep (Appendix C). This is consistent with our expected
grave dimensions. The maximum resistivity is 200 at <0.7m deep with lower resistivity (<60)
0.7-2m deep (Appendix C). Anomaly A2 exhibits almost the same resistivity pattern as A1. It shows
a higher (~200) subsurface resistivity with a lower (<60) resistivity down to 2m deep (Appendix
C).
There are two more consecutive anomalies that appear in lines 4 and 5, 14.75m from the origin
(Appendix A). Anomaly B1 is approximately 1m wide and 1.5m deep. Anomaly B2 is very similar in
width, however is slightly shallower depth (~1.3m). Anomaly B1 has a resistivity of <60 from 0.71.5m deep. Anomaly B2 has a resistivity of <60 from 0.7-1.3m deep. Anomalies B1-2 have a
subsurface resistivity of ~200 (Appendix C).
The resistivity patterns of the two anomalies B1-2 correspond with A1-2. Anomalies A1-2 and
B1-2 extend relatively parallel to one another across lines 4 and 5 (Appendix C).
Line 6 shows two areas of low resistivity that roughly align with the anomalies in lines 4 and 5.
However, resistivity patterns do not clearly match those of lines 4 and 5. Lines 7 and 8 show some
lower resistivity though they do not resemble the characteristics of a grave.
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DISCUSSION
When interpreting the inverted resistivity data, we are looking for areas of low resistivity
relative to the surrounding ground. It is important to keep in mind the dimensions of the targets
and their expected orientation.
The ER survey has identified two localities that bare resemblance to graves. These include two
resistivity features that extend through lines 4 and 5 perpendicularly. This is evident from
corresponding resistivity features identified between lines 4 and 5 at 12.5m and 14.75m from the
origin.
It is noteworthy that the A and B anomalies appear to be parallel when compared in lines 4 and
5 considering graves were likely to be dug next to one another. It is significant that these
anomalies are not only parallel, but show similar lateral resistivity changes (moving from one line
to the next).
There is evidence to support two gravesites in Grid 2 despite their locations not complying with
those outlined in Elorane’s survey. The two potential graves are identified at 12.5m and 14.7m
from the origin, intersecting lines 4 and 5. One test hole between lines 4 and 5 for each potential
grave should be adequate in identifying the graves.
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APPENDIX A. Target Area Map
This map of the target area was created to assist in locating the anomalies found in lines 4 and
5. It represents the distance (m) from the origin on the X-axis. The survey lines and their spacing
are shown on the Y-axis. The four corners of the grid mark the pegs placed by Alpha Archaeology
on the first day of the survey. The NW and SW grid corners mark the origin.
Jesse Savage Geo-Surveys
Phone: 0425 892 288
Email: jjsav1@student.monash.edu
Electrical Resistivity Survey – Collins Settlement, Sorrento
Jesse Savage Geo-Surveys
Phone: 0425 892 288
Email: jjsav1@student.monash.edu
APPENDIX B. Survey Data
Appendix B presents the profiles of lines 1-8. It includes the measured apparent resistivity, the
calculated apparent resistivity and the inverted resistivity. The measured apparent resistivity is the
raw data that represents the spatial resistivity distribution. The Earth Imager 2D Resistivity
Inversion Software uses an algorithm to invert these data to produce an inverted resistivity section.
This inverted section represents the internal resistivity distribution. The calculated apparent
resistivity is a quality control for the algorithm used to invert the spatial resistivity distribution.
The greater the similarity between the measured and calculated apparent resistivity, the more
accurate the inverted resistivity section is at representing the internal resistivity distribution. The
algorithm can be manually changed for more accurate inversions; however, the default inversion
formula is sufficient.
Profiles are represented as 2D ground cross-sections running from West to East. The X-axis
represents the distance (m) from the origin. The Y-axis illustrates depth (m) of the section. The
colours represent variation in ground resistivity in Ohms () with numerical values in the legend to
the right of each profile.
Electrical Resistivity Survey – Collins Settlement, Sorrento
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LINE 1
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LINE 2
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LINE 3
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LINE 4
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LINE 5
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LINE 6
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LINE 7
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LINE 8
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APPENDIX C. Inverted Resistivity Profiles
Appendix C contains the 2D resistivity cross-sections for lines 1-8. The X-axis represents the
distance (m) from the origin. The Y-axis illustrates depth (m) of section. The colours represent
variation in ground resistivity in Ohms () with numerical values in the legend to the right of
each profile.
All profiles are aligned and displayed sequentially to assist in comparing resistivity in
adjacent lines.
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LINE 1
LINE 2
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LINE 3
LINE 4
A1
B1
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LINE 5
LINE 6
A2
LINE 6
LINE 6
B2
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LINE 7
LINE 8