It's not often I leave my comfort zone of doing geophysics surveys on Roman sites in the south-east of England, but my latest survey was definitely outside, being on a WWII site in France. At Hébécrevon was part of the German main line of resistance against the allied advance into France after D-Day. It was designed to slow down the advance rather than completely stop it, and in that regard was successful. The section being looked at near Hébécrevon came under attack by US forces, and is being investigated by the Battlefield Archaeology Group.
The first site is a small hill overlooking a stream valley. The side of the hill had German positions among the trees, which kept the US forces at bay for a while. The field at the top of the hill was originally 3 fields, divided by hedgelines running north-south, which have now been removed. It is known that WWII positions were dug into the side of the hedgerows, to help hide them from aerial reconnaissance. Therefore, it was important to survey the field walking east-west, to highlight the ditches either side of the hedgerows into which the foxholes were dug. The northern part of the field was already under archaeological investigation, so the geophysics was limited to the southern part of the field. So what does the magnetometry show?
Field A: +/-5nT
The first thing to notice is the huge number of metal dipoles across the field, due to shrapnel from bombing and the battle at the site. This makes it difficult to spot weaker features. The concentration of metal drives up the spread of readings, so if you use standard deviations to set your display bounds, you would end up around +/-10nT. You will only end up seeing the metal, and any softer features will be invisible. Conversely, if you set the display bounds too low, you would see some softer features, but you would also be swamped by thousands of tiny pieces of shrapnel, as shown below.
Field A: +/-2nT
The trick is to look at images using multiple display bounds in order to get a better picture of what is going on. Take the below image for example. The display bounds are much higher, and you are left with the larger pieces of iron. This is important information, because all of the shrapnell will be small, but larger dipoles could represent pieces of iron equipment from WWII. Couple this with the position of the hedgerows, where you would expect WWII features, and you have a set of targets to excavate. A list of likely targets, A-K are shown on the image below.
Staking out the targets can provide further information. The rough position is found and marked, then the precise position is found by setting the range on the magnetometer higher and scanning for the highest reading. We then used a metal detector to get information about depth, size and composition of the target, to decide whether or not to dig it. Below is what we learned about each target. Many thanks to Brian for the pictures.
Feature A is the first of five features arranged in a crescent shape on an old hedgeline. It was a horizontal dipole and the metal detector suggested it was a metre deep. Excavation found the remains of an air to ground rocket fired from a typhoon, buried in the natural.
Feature B was a vertical dipole, with the metal detector suggesting the feature was half a metre down. Excavation revealed a foxhole dug into the hedgeline which contained American 105mm artillary shell cases and other artifacts.
Feature C was a vertical dipole, with the metal detector suggesting it was half a metre long and a metre down. Excavation revealed the long artifact the metal detector was picking up was a case for a German M42 machine gun barrel within another foxhole.
Feature D was a vertical dipole, and the metal detector suggested the feature was sizable and half a metre down. Excavation revealed 3 gas masks and infantry equipment for a german soldier.
Feature E was a horizontal dipole. The metal detector suggested the feature was half a metre down. Excavation found a foxhole containing more American 105mm artillary shell cases.
Feature F was by far the strongest feature on the magnetometry results. The metal detector suggested it was 1m down and roughly 2m square. Unfortunately, you can't win them all, as it turned out to be a section of modern metal cable.
Feature G is a feature of two parts. There is a horizontal metal dipole on the west side of what appears to be a sizable cut. Cut features are difficult to see in these results with the amount of metal everywhere. I suggested that the cut might have been where a tank had been dug in. The metal detector suggested that the dipole was only 20cm down. At that depth, it was likely to be modern, and it did indeed turn out modern, as it was a metal pole used to hold up an electric fence. A slot was dug across the cut, but the results were inconclusive.
Feature H was a horizontal dipole in the eastern of the two hedgelines that once divided this field. The metal detector suggested it was only 20cm down, so it was decided to quickly dig it out. The metal detector turned out to be picking up something closer to the surface, and the feature generating the large dipole was actually a metre down. It was an open ammunition case lying on top of a large amount of belted ammunition.
Field A: +/-30nT
Staking out the targets can provide further information. The rough position is found and marked, then the precise position is found by setting the range on the magnetometer higher and scanning for the highest reading. We then used a metal detector to get information about depth, size and composition of the target, to decide whether or not to dig it. Below is what we learned about each target. Many thanks to Brian for the pictures.
Feature A is the first of five features arranged in a crescent shape on an old hedgeline. It was a horizontal dipole and the metal detector suggested it was a metre deep. Excavation found the remains of an air to ground rocket fired from a typhoon, buried in the natural.
Feature A
Feature B was a vertical dipole, with the metal detector suggesting the feature was half a metre down. Excavation revealed a foxhole dug into the hedgeline which contained American 105mm artillary shell cases and other artifacts.
Feature B
Feature C was a vertical dipole, with the metal detector suggesting it was half a metre long and a metre down. Excavation revealed the long artifact the metal detector was picking up was a case for a German M42 machine gun barrel within another foxhole.
Feature C
Feature D was a vertical dipole, and the metal detector suggested the feature was sizable and half a metre down. Excavation revealed 3 gas masks and infantry equipment for a german soldier.
Feature D
Feature E was a horizontal dipole. The metal detector suggested the feature was half a metre down. Excavation found a foxhole containing more American 105mm artillary shell cases.
Feature E
Feature F was by far the strongest feature on the magnetometry results. The metal detector suggested it was 1m down and roughly 2m square. Unfortunately, you can't win them all, as it turned out to be a section of modern metal cable.
Feature F
Feature G is a feature of two parts. There is a horizontal metal dipole on the west side of what appears to be a sizable cut. Cut features are difficult to see in these results with the amount of metal everywhere. I suggested that the cut might have been where a tank had been dug in. The metal detector suggested that the dipole was only 20cm down. At that depth, it was likely to be modern, and it did indeed turn out modern, as it was a metal pole used to hold up an electric fence. A slot was dug across the cut, but the results were inconclusive.
Feature H was a horizontal dipole in the eastern of the two hedgelines that once divided this field. The metal detector suggested it was only 20cm down, so it was decided to quickly dig it out. The metal detector turned out to be picking up something closer to the surface, and the feature generating the large dipole was actually a metre down. It was an open ammunition case lying on top of a large amount of belted ammunition.
Feature H
Feature I initially looked like a cut feature in the middle of the field. We later found the spot was once occupied by a large tree, so is probably a silted up tree throw. We decided not to dig it.
Feature J was a horizontal metal dipole which the metal detector found to be 0.5m down and quite dispursed. It was a good target, being on the hedgeline, but it was decided that enough trenches had been opened to cover the week.
Feature K was a horizintal metal dipole which the metal detector suggested was small and 1m down. It wasn't on the hedgeline, so wasn't seen as a priority to dig.
The geophysics was a great success, identifying several features by finding their contents. Finding the ditches of the old field boundary was important to identify which features should be a priority, so walking across rather than along them was important. The combination of the magnetometer and metal detector was very useful in assessing what features should be dug and what should be ignored. It's also important on a site like this to not just accept the default display bounds for the magnetometry results, but to try different settings in order to highlight different types of feature.
Further to the west of the excavation side is a field known to have been used as part of a German road block, with tanks and machine gun positions stopping the advance of the American troops. The field was much larger than the first, and judging by the amount of metal lying about on the surface, it was going to be just as noisy as the first field.
Field B: +/-2nT
Field B: +/-5nT
Field B: +/-30nT
Again, looking at images with different display bounds is useful, with cut features showing best at lower values and the larger chunks of metal showing best at higher values. Of particular interest is a set of features highlighted with the red arrows on the 5nT image, on which it shows best. There are two sets of broken linears here, which fit within one of the historical smaller fields that make up this now much larger field. This could be a German 'square wave' trench. As I was walking north-south, and east-west sections of trench show best, with the short connecting pieces of trench between the two lines falling between my survey lines, and are thus invisible in some cases. That's an excavation for another year.
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