Tag Archives: Geophysical Techniques

The Magnetometer and its use in Underwater Archaeology

By Alex Kilpa Maritime Archaeology Masters Student

Introduction

So you’ve done all your homework, you’ve consulted all the relevant historical documentation and you know its approximate location, but where exactly is that illusive shipwreck? This is a typical scenario that maritime archaeologists are confronted with when trying to locate cultural materials deposited in an underwater environment. One remote sensing device that can assist in detecting shipwrecks and other cultural materials is the magnetometer. In essence, a magnetometer is an instrument that measures “magnetic force field intensity and direction”. This is done at the sensor data collection point where the measurements are taken. (Hine 1968:125; Ripka 2001:xvii).

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Executive summary of my directed study project

I have just concluded my directed study project which was focused on researching a new methodology for the indirect detection of unmarked burial sites using ground penetrating radar. Being responsible for researching and writing up a larger sized project, and drawing on various sources for literature including interstate collections has been a valuable learning experience. However the most rewarding experience has been the opportunity to be involved in undertaking research with important implications for locating the burial location of a significant Indigenous historical figure.

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ARCH8307 “Introductory Archaeological Geophysics” 2010



Thank you to all of my intrepid students who finish up their ARCH8307 “Introductory Archaeological Geophysics” topic this afternoon by presenting the data they have collected from the historic Meadows Wesleyan cemetery in the Adelaide Hills. The students, split in two groups entitled “The A Team” and “The Sextons”, collected, processed and interpreted ground penetrating radar, electromagnetic induction and magnetometer data to try to locate the foundations of the former church and some of the more than 50 unmarked burials know the exist within this cemetery. Students also were fortunate to be able to assist Flinders PhD candidate Martin Wimmer by searching for an air raid shelter in Souter Park, Goodwood. The investigations are still a bit inconclusive, but our preliminary interpretation is that a feature did exist on this site however any material used for it’s construction has now been removed.

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Ground Penetrating Radar

The Flinders University Archaeology Department ran a masterclass a little while back on Ground Penetrating Radar (GPR) presented by Prof. Larry Conyers. I signed up for this course because I like the idea of being able to see below the earth’s surface without having to dig, that as archaeologists we are getting to the point of being able to examine some aspects of a site without destroying it in the process.


So when we are looking at GPR data, what are we actually looking at?
* the path of an electromagnetic wave as it passes through the earth.
* reflections: the waves are transmitted, hit something, partially bounce back and are collected by the receiver.
* porosity and permeability of the earth – water can both help and hinder the transmission of the waves. In some cases, water is trapped underground by substances such as wood so they show up better. In other cases, water might flow between cobblestones or pavers to create very differentiated results, or alternatively it does not flow under asphalt, so the stratigraphy below is both slower to change and more intact.
* dislocations in stratigraphy – bones might not show up, but perhaps the edges of the grave will.
* history of the environment, for example old river channels that became lakes, or old ocean floors, old field furrows. The history of the environment is important information to archaeologists; environment in many ways dictates activity.

The first day was all theory – an introduction to different aspects of the science, maths and software involved. On the second day we headed up into the hills to an old graveyard to survey. A couple of the participants brought their own GPRs with them and we spent a large part of the cold, rainy day collecting data on top of a windswept hill, dodging the odd patch of hail.


The third day (sunny of course) was back in the lab looking at what we had collected, trying out different angles and piecing things together. It was a great intro to a very complex survey method. I’m interested to see more applications.

The Geophysical Detection of Historic Graves Seminar

On the 26th of March, 2009, Adjunct Associate Lecturer Ian Moffat gave a seminar entitled the Geophysical Detection of Historic Graves. The seminar room was filled with forty-one (41) fellow academics and students from the Department of Archaeology.

The seminar focused on the effects of geology on current grave detecting methods such as the magnetometer and the ground penetrating radar. There are three divisions of burials or graves; the Indigenous, the Clandestine, and the Historic. Ian’s case studies were focused on the latter division which included the Selheim Cemetery (North Queensland), the Pioneer Park Cemetery (Adelaide), the Encounter Bay Cemetery, and the Meadows Cemetery (Adelaide Hills).

The Selheim Cemetery was the most successful of the four case studies with the rediscovery of all thirty previously ‘lost’ burials. Both the Pioneer Park and Meadows Cemeteries case studies produced good quality data but the amount of time used was a minor setback. Geology and its effect on burial data analysis was profound in both the Encounter Bay and Meadows Cemeteries. The heavy vegetation in Encounter Bay and the sandy conditions at Meadows Cemetery have deterred the geophysical survey process.

Some of Ian’s conclusions at the end of the seminar were:

  • the magnetometer and EMI methods are greatly affected by the presence of surface metal
  • decrease in line spacing of GPR data leads to substantial increase in the ability to detect burials effectively
  • GPR is more effective in sandy locations than in clay (contrary to previous studies)
  • the greater the number of trees on the site, the slower and less effective are the geophysical methods

Even though I do not have an archaeology background, Ian’s presentation was very organised and was not too difficult to comprehend. His research and conclusions were well-received with various questions and comments from at least eight people in the audience.