Blog Post 3 – Gradiometer Fire Rock Experiment – Waltowa Wetland Geophysical Study

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 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.

Figure 2: The limestone fire rock that was tested.

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