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Electrical Resistivity Survey

Collins Settlement - Sorrento

03/12/2012

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.

CONTENTSINTRODUCTION 3METHODOLOGY4RESULTS6DISCUSSION7APPENDIX A. Target Area MapsAPPENDIX B. Survey DataAPPENDIX C. Inverted Resistivity Profiles

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.

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 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.

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