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!!