This study though not 100% successful in my opinion, has changed my thoughts in regard to hearth detection methodology. The aim of the study was to detect hearths using two different types of geophysical instrument, one being GPR and the other gradiometer. I theorised that by surveying areas known to contain hearths with two types of instrument would double the chances of success. I also theorised that using the gradiometer first to collect data over the 7 grids would be the most time efficient and successful way to detect potential hearths. After viewing the magnetic data and taking note of where potential hearths appeared in the data, the 7 grids were then surveyed using GPR to confirm the anomalies found in the magnetic data set. Unfortunately, though, there were no hearths detected in the first 7 grids surveyed with the gradiometer, this may have been because there are no hearths in any of the grids (unlikely), the line spacings in the grids were too large, or because none of the survey lines were on the alignment of any subsurface hearths.
When the 7 GPR grids were analysed the three grids at blowout A did appear to contain hearth-like anomalies, some shallow (0.4m) (figure 1) and some deeper (1m) (figure 2). These anomalies were marked on site with wooden pegs after being detected with GPR and readings were taken directly over them using the gradiometer. Readings of -7 to +7 nT were experienced over the anomalies, which corresponds to hearths detected in other studies I researched for the literature review section of this directed study. Therefore, if I was to do this study again in this type of environment I would use GPR first to look for hearth-like anomalies, mark them and then conduct magnetic surveys afterwards, confirming or discounting the anomalies detected in the GPR survey. This study has proven that the use of two geophysical instruments can complement each other, increasing the chances of success. Much more testing is required in different environments in order to develop a strong methodology for the detection of submerged hearths, however. This study has inspired me to develop some test areas (potentially in conjunction with my industry partner) with subsurface artificial archaeological anomalies (i.e. hearths, footings, graves etc) for myself and other students for practice, testing, and calibration of geophysical instruments.
I would like to say a big thank you to: Kelly Wiltshire, Heather Burke, and the NRA for all the information and help they have provided me with, throughout the duration of this study.
Figure 1 (left): is a GPR profile from grid 5, blowout A, a hearth-like anomaly can be seen at approximately 0.4m deep at the 2.4m mark in the profile. Depth is expressed in metres on the left-hand side of the scan and distance is expressed in metres on the top of the scan.
Figure 2 (right): is also a GPR profile from grid 6, blowout A, a hearth-like anomaly can be seen at 1m depth and 6 metres into the scan. Radar worked well in this environment, with the signal propagating to over two metres.
Before the commencement of the gradiometer surveys testing was conducted over known fire rocks in order to establish the intensity of readings that may be experienced. Testing was conducted at blowout A, as there is an abundance of limestone fire rock lying on the surface of the dunes. These fire rocks may well have been in-situ hearths but have since been uncovered as the dune has eroded away. When I conducted the test it became apparent that not all of the fire rocks gave a strong magnetic reading when traversed with the gradiometer. By accident a theory has arisen as to why this is the case. When testing, a particular fire rock (figure 2) was traversed and gave a high magnetic reading of -24 nT (figure 1). It was picked up off the ground and analysed and set back down in the same position, though by accident it was turned over before being set down again —meaning that the opposite side was then facing upwards. When traversed again the readings were very low (1nT), so my theory is that the face of the rock providing the high readings was the side of the rock that experienced the fire event—hence containing the remnant magnetisation—whilst the other side did not. This seemed to be the case for many of the other rocks tested in this way. There needs to be much more study done in this area, but further testing was beyond the scope of this paper, though this did bring about questions regarding the survey. If the hearths were no longer in-situ and individual stones were shuffled around due to an ever moving dune system, would they be able to be detected by magnetic methods if the fired surface of each rock was not facing upwards? The small amount of testing done during this study would indicate that they would not.
Figure 1: Field Testing – A Bartinton Grad601 sensor sitting on top of a fire rock, providing a reading of -24 nT.
Figure 2: The limestone fire rock that was tested.
As part of my directed study I was required to do some research into hearth detection projects that had been conducted in similar environments and/or with similar geophysical equipments. Upon researching this area it became evident that only a small percentage of archaeological work has been conducted with geophysical instruments in the past. This is possibly because of the perceived high cost of geophysical instruments, a lack of training available at university level and as a result of a lack of competent technicians with the ability and experience to detect subtle targets. By subtle targets I mean targets/anomalies that that may not be apparent straight away in data sets and that may require processing to extrude them from the rest of the unwanted data set. Ian Moffat and Lynley Wallis did a very similar geophysical study in inland North Queeensland using magnetic methods, whereby they attempted to locate hearths and middens in areas where they were known to be. Unfortunately their study was unsuccessful, as hearths were not easily located, although the study provided me with very useful information regarding which choice of magnetic instrument to use and the line spacings between survey lines and survey sizes. There have been plenty of successful studies of this type conducted throughout the U.S. using both magnetic geophysical instruments and ground penetrating radar, but I noticed very few successful studies in Australia. So I decided to employ both types of geophysical instrument for this study to increase my chances for success (both geophysical instruments pictured below). I found plenty of case studies done by Larry Conyers to find hearths of Native American peoples using ground penetrating radar. His methods of data interpretation and processing gave me a good idea of what to look for, the GPR profiles to be collected and the data processing that may be required to extrude these features if the profiles are full of noise.
Figure 3 Figure 4
Figure 3 (above left) – GSSI SIR 3000 radar system, a 400 MHz antenna is mounted under the centre of the survey cart. Distance travelled is measured with an encoder wheel attached to the rear wheel. Figure 4 (above right) – A Bartington Grad601 single sensor fluxgate gradiometer.
The background research phase of this study provided me with some helpful information about survey methodology that may be suited specifically to this type of survey. I decided that, rather than have large survey areas, I would have small 10 x10 m plots which would result in smaller more manageable data sets. Data was collected over each 10m x 10m grid with both ground penetrating radar and gradiometer. Line spacings were set at 1m, which admittedly could be a mistake, and which proved to be the case in Wallis and Moffat’s study. I’m hoping that a gradiometer, being a more sensitive instrument to local changes in the earth’s magnetic field, will have the ability to detect hearths at 1m spacing as opposed to something like a proton precession magnetometer (used in Moffat’s and Wallis study) which was not capable of doing this. Something I have learnt regarding my magnetic survey methodology is that, rather than tightening grid spacings, it can be just as effective—if not more so—to survey a grid in one orientation, for example east–west and then in an opposite orientation, for example north-south. This methodology has the potential to detect magnetic fields on one axis of the gridded area that may not be strong or apparent on the other. I anticipate that GPR will not be so greatly affected by line spacing due to the 400 MHz antenna’s footprint (the antenna focusing range) and the ability of the signal to effectively propagate through the soil type at this particular site. I predict that GPR will be more effective at this site than many would expect, as test scans easily detected targets from 0-2m deep. Hopefully the discussion about my results in the next blog backs this statement up!!
Blog Post 1 – Study Aims /Site Location and Details/ Industry Partner
The main focus of my directed study is the detection of submerged (intact) heat retainer hearths, which are a type of earth oven used in the past by Aboriginal people to cook food. Within Ngarrindjeri country a fire was typically made in a scooped out depression and stones thrown in to the point where they retain a lot of heat; the stones were then moved to form a base layer for cooking on. I aim to find these hearths (ideally intact) with two types of geophysical instrument: one being ground penetrating radar (GPR) and the other a gradiometer. As GPR and gradiometry are very different methods I’m hoping to establish which method is most suitable for detecting hearths, or whether both will be successful and complement each other. It may even be the case that both methods are unsuitable for detecting hearths in an undulating sand dune environment.
My study area is located in the Waltowa Wetland, which is approximately 15 km from the town of Meningee in South Australia’s south east. In particular I am working within Tatiara Station. Tatiara Station is approximately 8500 acres of wetland and contains a series of blowouts and dunes, although most dunes have been stabilised by a cover of pastoral grasses. The land is primarily used as an area to farm cattle by the McClure family who own it. Waltowa Wetland contains a lot of archaeological evidence of past Aboriginal occupation, which has been recorded as part of ongoing research by Kelly Wiltshire, a PhD candidate at Flinders University.
Figure 1 – Top map indicating Tatiara Stations location on the Fleurieu Peninsula
Figure2 – Bottom map indicating the research areas within Tatiara Station .
My industry partner for this project is the Ngarrindjeri Regional Authority Inc., also known as the NRA. The NRA represents communities and organisations that currently make up the Ngarrindjeri Nation and the current individual Native Title claimants of the Ngarrindjeri and Others Native Title Claim. This project is being undertaken as part of the Ngarrindjeri Regional Authority’s Ngarrindjeri Yarluwar-Ruwe Program, which co-ordinates and manages all active heritage programs and research actively undertaken within Ngarrindjeri country.
As an initial part of the Directed Study I ran a Ground Penetrating Radar (GPR) training workshop with the NRA to provide training and contribute to NRA’s capacity building program. The NRA have just recently purchased a new GPR and we both thought it would be a great opportunity to share some of my knowledge and experiences with the people who are likely to work with the GPR in the future. I’m by no means an expert, but I’ve been working with GPR for about 6 years so I was pretty confident I could show the NRA a ‘few tricks of the trade.’ The workshop was run over one day and consisted of three topics: GPR theory, practical and data processing and interpretation. The feedback from the workshop was great!! To be honest it was easy to run, as everyone seemed to grasp the concepts very quickly and easily……and it was a Saturday!!