Tag Archives: Geoscan

End of week two, part 2

Anyone new to this blog or geophysics in archaeology is recommended to read the material on the “Geophysical survey in archaeology” page.

Just a quick update as week 3 will be starting in about eleven hours and I’d like some sleep!

The GPR crew on day 10 completed three areas of “sawtooth”.  Well done all for putting up with such an annoying, fiddly job, but it does look good along the edge of the survey.  It took a bit of setting-up, processing-wise, but all was well.  Sadly, not much showing (Figure 1).

Figure 1: the GPR survey in the northern area after day 10.

Starting from tomorrow, the crew will be working their way slowly southwards, back up the hill.  The downside is the hill, the upside is that they will be covering areas which clearly have buildings in them!

The earth resistance meter, operated by myself and Ellen, managed a modest two grids once we had set-up the other two machines.  The results were good, however, and clearly show many of the details of this building in the top-corner of the Theatre field.  The next three images show the mag, GPR and earth resistance results for this area.

Figure 2: mag data in the top corner. the building shows as white lines of low magnetism.

Figure 3: the GPR data showing this building very clearly as black lines of strong radar reflections.

Figure 4: the earth resistance data for the same building.

Although the GPR data appears very clear, the Earth Resistance and mag data appear to show more walls between the main range and the road.  There is a suggestion, also, that the “corridor” to the SW of the main range is in fact another phase.  It would be odd for a corridor to have subdivisions.  Plenty of room for debate over the details of this building.

Many thanks to all for your excellent work in the first two weeks.

Advertisements

One hundred and fifty

Anyone new to this blog or geophysics in archaeology is recommended to read the material on the “Geophysical survey in archaeology” page.

We managed a full day today, and I’m just about keeping up! The mag team completed two grids yesterday, half of one in the aforementioned deluge. Today they completed 11 grids: three partials and eight complete ones. Way to go! Well done everyone. Figure 1 shows the survey so far.

Figure 1: the mag survey after day 9.

One really does wonder if that break in the mag data is an entrance.  It doesn’t seem like it on the ground.  I have downloaded the LiDAR data but haven’t had a chance to process it yet.

The GPR crew finished their 80x40m block, and then did some of the next “sawtooth” section, another 14m worth.  Figure 2 shows the time slices.

Figure 2: day 9, time slices 3 to 6.

Nothing jumps out at one, although there are some curious “light” lines in the fourth slice (top-right) which are parallel to the aqueduct.  Figure 3 shows that slice in context.

Figure 3: GPR survey after day 9, slice 4.

After all the rain I thought it would be worth trying the Earth Resistance survey (Fig. 4).  I spent the morning laying in grids for the mag, but managed some survey in the afternoon.

Figure 4: Earth Resistance survey in action.

Although the rain has softened the surface, it won’t have penetrated 50cm yet, and I was concerned that there would be no contrast at that depth.  I decided to survey a grid where we knew there was a building.  Fig. 5 shows the comparison between the GPR survey and the two squares of res I managed to complete (thanks Anne!).

Figure 5: Earth resistance survey compared to GPR results.

Given the drought, the results are pretty good.  It would be interesting to compare these to results from a normal English summer!

Tomorrow isn’t looking great.  We might get some work done in the morning.  Fingers crossed.

Many thanks to everyone who helped out today.  Especially big thanks to Mike, Ellen, Jim and Ruth who take on the responsibility of shipping the equipment back and forth.

By the way, this is the 150th blog post…

And now for somewhere completely different

Although this isn’t CAGG related, or Hertfordshire, I thought members of the group might be interested in my latest geophysical adventures.

Some 15 years ago I undertook a survey in Alba Iulia, Romania, for a colleague.   The site was part of one of the Roman cities at Apulum which grew-up alongside the legionary fortress.  The results were pretty good, but I was only taking one reading per square meter.  Since getting the RM85 I have been wanting to return and re-do the survey at higher resolution.  Well, be careful what you wish for!  Last Saturday, I found myself on the way…

Fig. 1: On my way…

Yes, you did read the time correctly.  I flew to Cluj-Napoca via Munich.  Sadly, when I got to Cluj, my luggage was still in Munich.  Thankfully, they delivered it all safe-and-sound the next day but it did mean I lost a half day of survey.

Alba Iulia has changed quite a bit in the fifteen years.  The citadel, especially, has been restored beautifully and now has a series of bronze statues decorating the area.

Fig. 2: scrumping.

Having lost half a day, we got started in the afternoon.  Three whole grids and partial that day, seven whole grids a four partials (including one very silly small one) the next day, and eleven yesterday.

Fig 3: Wyatt helping with the Earth Resistance survey.

Yesterday, going along the first line seemed fine with the wind at my back.  Then I turned into the howling gale and snow…  The effect was like star-trails in a science fiction movie as the snow blew past me horizontally.  Thankfully, the weather got better during the day.

There were some software issues to begin with, but thanks to David Wilborn’s excellent customer support, those were quickly resolved.  The results look pretty good.  In the next image I have applied a high-pass filter to even out the big changes in range that occur in this data set.

Fig. 4: the Earth Resistance survey results at the end of Day 3.

I’ll update you all as I go along.  I have six more days to try and complete the whole survey.  I suspect I’ll be a little tired by the end.

Your foreign correspondent.

Back to Durobrivae

Anyone new to this blog or geophysics in archaeology is recommended to read the material on the “Geophysical survey in archaeology” page.

Firstly, many apologies for the time it has taken to write this report.  The data were quite complex, and the day job comes first.  This is a monster blog post, so make a cup of tea and settle back somewhere comfy!

Last year CAGG teamed-up with local groups to undertake some survey at the Roman ‘small town’ of Durobrivae, near Peterborough. If you have ever driven on the A1(M) past Peterborough you would have passed by the site. Our aim was simply to determine which geophysical survey techniques would give good results at this site.  The answer was: all of them!  A short note on the results has just been published in the International Society for Archaeological Prospection‘s newsletter.

We decided to add to our original survey by undertaking another three days work from 4th to 6th November.  Unfortunately, it decided to rain in the morning of the first day and so we lost some time.  We had enough helpers to run the mag, two Earth Resistance meters (the Welwyn Archaeological Society‘s and UCL‘s) and the Malå GPR we had on loan from SEAHA.  A small group of us returned on 26th November to expand the magnetometry survey and undertake a topographic survey using the dGPS.   The first three figures just show how much we have done so far.

Fig. 1: the complete mag survey as of the end of November 2017.

Fig. 2: the complete GPR survey as of the end of November 2017.

Fig. 3: the complete Earth Resistance survey as of the end of November 2017.

In the following post I am going to firstly discuss the western block of data, and then the eastern block over “the tumulus”.

Last year we completed a 80m x 360m transect of mag data across the town.  We also completed two blocks of GPR data, one 80×80, and one 80x40m.  This year we wanted to fill in the gap between those two blocks so surveyed another 80x40m block giving us one contiguous 80x160m survey.  Unfortunately, matching GPR grids is quite difficult, especially when there is a year between when they were collected, and therefore quite different ground conditions.  I did, however, manage to produce some “OK” time slices by applying a zero-mean traverse to each line of GPR data.  Hopefully, I will be able to create better slices in future, but these will do for now.  I also noticed that Larry Conyers had produced a much clearer plot of the temple by using a much thicker time slice.  I usually aim for 3ns thick slices.  Larry, however, used 8ns slices.  Here I have compromised by using 5 1/4 ns slices with a 50% overlap.  Fig. 4 is a composite of 12 slices starting at the surface.

Fig. 4: twelve time slices of the western area. Each slice is 5.27ns in thickness.

Apart from Ermine Street cutting across the top right hand corner, the first three slices are not really showing anything much of interest. Let us now look at the individual slices in more detail.

Fig. 5: GPR time slices 4 and 5.

In Figure 5, left, we can see the temple (A) starting to show as an area of lower reflections.  Larry Conyers was able to demonstrate that the interior of the temple building was clear of rubble, and thus there is little to reflect the radar waves.  In slice 5 on the right, we can see a linear feature (B) to the west of the temple.  This lines up perfectly with a strongly magnetic feature and is therefore a narrow cut feature like a ditch or possibly a wall foundation.  There are faint hints of buildings with robbed-out walls at C and D, showing as light areas of low reflections.  Similarly, at E, we can see some of the buildings alongside Ermine Street.

Fig. 6: GPR time slices 6 and 7.

In Figure 6, left, at A, we can see one of the minor side roads off Ermine street starting to show.  The building at B is still visible (just), and we are can start to see a wall parallel to the linear feature we saw previously (Fig. 5, B).  This suggests to me that we are dealing with a cut feature rather than a robbed wall.  Slightly deeper, in Fig. 6, right, at D we can see more buildings alongside Ermine Street.  The square building at E is now showing more clearly.  The large building at G is beginning to show quite clearly at this depth.  I am puzzled, however, that the road coming from the east seems to end in a sharp angle at F, as though something has cut through it.

Fig. 7: GPR time slices 8 and 9.

In Fig. 7, left, at A we can now see the building to the north of the temple very well.  It would appear the walls have been robbed but some of the floors left intact.  At B, the large building to the east of the temple is showing very clearly now. The wall along the west side of the temenos of the temple (C) is showing clearly at this depth.  Starting to show, but more clearly in the next depth slice at D, is a long wall running across the site.  There seems to be an almost entrance-like feature in it at the western end.

Fig. 8: GPR time slices 11 and 12.

Skipping a slice and moving to No. 11 (Fig. 8, left), we can see the wall to the west of the emple at A very clearly.  The possible floor of the building to the north at B still shows.  In the deepest slice I have generated, we have a curious series of curved features at C.  I have no idea what these are.  Answers on a postcard, please, to…

Fig. 9: the Earth Resistance survey (lower half) overlain on the GPR data (faded out a little).

Figure 9 shows the Earth Resistance survey.  We added a single line of grids on the eastern edge of the block we did last year.  There is a strange speckly effect in the new strip.  This block of grids we did with WAS’s TRCIA meter.  The resistance values were very high, and the meter had to keep swapping range which, apart from slowing us down in the field, may be the cause of the rather odd looking results.  The main result in the new strip is the high-resistance line running WSW to ENE which is probably a road.

A comparison with Stephen Upex’s transcription from aerial photographs is quite informative (Fig. 10).

Fig. 10: Transcription of the aerial photographs for the temple complex by Stephen Upex.  The image has been rotated to match the geophysical surveys. © Stephen Upex, reproduced with permission.

Some of the details between the aerials and the geophysics agree quite well.  The temple itself, and the temenos are pretty good.  What about the circular shrine?  I reprocessed the GPR data from just that section using 6.5ns thick slices this time.  I have produced an image of slice 6 (16.04 to 22.49ns) in the variety of palettes offered by GPR Slice (Fig. 11).

Fig. 11: GPR time slice of the area to the immediate north of the temple in a variety of palettes.

The building to the north of the temenos shows quite well.  There are hints of a circular structure just to the north of the main temple building lying underneath a robbed rectangular building.  The circular feature shows quite well in the last palette, and the antepenultimate one.  The rectangular building is clearer in the second slice where black is showing areas of low reflections.

There is clearly a great deal which can be teased out of this data, but let us move on!

The second area we surveyed was over the so-called tumulus towards the western side of the town.  The mound showed quite nicely last year when the evening mist rolled in on the last day (Fig. 12).

Fig. 12: The mist shows the location of the “tumulus” beautifully.

Our aim was to survey the mound using all three techniques.  Unfortunately, the half day we lost to rain resulted in not covering quite as much ground as we hoped.  The mag results were especially interesting, hence our return to expand the survey area a few weeks later.

Fig. 13: the magnetometry survey of the eastern area over the “tumulus”.

Figure 13 shows the magnetometry results.  Ermine Street and the minor road running off it show well.  The town wall also shows clearly.  The zig-zag look to the wall is not “stagger” in the usual sense of the odometer being incorrect, but a result of the cart going up and down a steepish slope resulting in the sensors not being vertical.  There are indications of more long, thin buildings coming off Ermine Street at right angles, and plenty of other pits, ditches and other features.  The really curious aspect though, is the empty space in the middle, under the “tumulus”.  This seems to have a polygonal linear feature around it, showing as a magnetic positive and therefore either a cut feature, or a brick-built wall. To the east / south-east of the tumulus is a largely open area, somewhat fan shaped in appearance.  How very curious.

I wanted to check the relationship between the results and the topography so I undertook a topographic survey with the dGPS taking readings every six paces (just under 5m).  This differs from using the UAV.  The GPS survey will give us a digital terrain model (DTM) which is the actual surface, but at a cruder resolution.  The UAV will give us a digital surface model (DSM) which gives the surface and thus maps the tops of stinging nettle patches and so on, but at a much higher resolution.  We saw this at Darrowfield. Neither method is better than the other, it depends on ones aims, but using the UAV is certainly very much quicker in the field!

Fig. 14: dGPS topographic survey of the area around the tumulus.

As can be seen from Figure 14, the tumulus shows as an elongated feature running SW-NE.  My guess is that this shape is a result of plough damage.  How does this relate to the mag results?

Fig. 15: the topography with the mag data overlain on it and made partially transparent.

In Figure 15 I have overlain the mag data on the topography, and then made it partially transparent.  As can be seen, the mound is smack in the middle of the polygonal magnetic feature.

The Earth Resistance survey adds a little to the picture (Fig. 16).

Fig. 16: the Earth Resistance survey overlain on the mag data.

The resistance data shows little in the way of positive features apart from an area of high readings towards the south.  There does seem to be a oval of low resistance readings, normally indicative of a ditch-like feature.  There are faint hints of this in the mag data, but they are obscured by other magnetic features running up to the polygonal feature.  If the ultimate origin of the “tumulus” is a prehistoric burial mound, perhaps the res survey is showing us the outer ditch surviving, in part, below the Roman levels?  Fig. 17 shows the resistance survey with contours from the topo survey.

Fig. 17: contours overlain on the Earth Resistance data.

We managed three 40x40m blocks of GPR data.  Unfortunately, the GPR had a glitch in the second grid resulting in a single line of very high values.  This glitch caused some problems in the processing.  I have tried to get rid of them, but the line still shows, especially in the lower slices.  Fig. 18 shows the composite of 16 slices.

Fig. 18: all GPR slices from the eastern survey.

Surprisingly little shows in this survey.  The two main features are the rectangular building which shows from slice 5 onwards, and the general lack of anything much under the tumulus part from some general reflections suggesting some hard material (stones, rubble?) under the mound.  Let us look at slice 6 in more detail.

Fig. 19: GPR time slice 6.

The building towards the south of the image is fairly clear.  I wonder if it might be a bath house?  The area of higher reflections under the mound have faint hints of straight lines and rectangles, but this only shows in this one slice and my guess is that these are fortuitous rather than archaeology.  How does the GPR data relate to the topography?

Fig. 20: GPR time slice 6 with the contours superimposed.

As can be seen from Fig. 20, the higher reflections do not lie below the main part of the mound but slightly to one side.  The building lies outside the polygonal feature seen in the mag data.

A slightly deeper time slice (Fig. 21) shows the strip buildings along Ermine Street starting to show.  They appear to be missing their back walls which might be one impact of ploughing.

Fig. 21: GPR time slice 8 with topographic contours.

I made a crude interpretation map in Google Earth by marking the polygonal feature from the mag data, the building from the GPR data, and, with some guess work, the outer feature from the Earth Resistance data (Fig. 22).

Fig. 22: rough interpretation of the three data sets.

It is impossible from the data to tell if the outer feature from the res survey goes under or around the building, so I may have been a bit generous there.

So what is it?  One possible interpretation could be that we have a prehistoric feature with a mound and a ditch, presumably a round barrow.  The ditch silts up before the Roman occupation.  The mound is then fenced off and kept completely clear of structures or negative features like ditches and pits.  A building is constructed to the south of this mound, and a viewing area to the east.  Stephen Upex, solely on the basis of the aerial imagery, suggested that the feature was prehistoric, and re-used as either a amphitheatre in the Roman period or a small castle in the medieval period.  With the new data, I think we can rule out the castle (unless it was much more substantial at that date).  Although the phrase “ritual” is greatly over-used in archaeology, maybe in this case we are looking at an earlier mound which continued to be venerated into the Roman period?  Baths are often associated with religious sites.  At this stage, this is purely guesswork at the end of a long blog post.  This feature is, really fascinating and quite enigmatic.  Extending the earth Resistance and GPR surveys would, obviously, be very helpful.

The landscape around Durobrivae is fascinating from an archaeological point of view.  The nearby Roman fort is only known from aerial photographs, and just across the river and the Nene Valley Railway lies the site of Castor (Fig. 23) with its huge Roman building complex.

Fig. 23: Castor as seen from Durobrivae.

Last, but not least, many thanks to all those who helped push the mag and the GPR, and who aerated the grass with the resistance frame, or flew UAVs to map the topography.  Although the site is a long way for all of CAGGs volunteers, the site is both stunning and intriguing and, I think, worth the effort.  We hope to return to collect some more data soon.

Fig. 24: collecting Earth Resistance data with WAS’s machine.

A good day

Anyone new to this blog or geophysics in archaeology is recommended to read the material on the “Geophysical survey in archaeology” page.

Today was definitely a good day. Although it was raining early in the morning, the weather had started to cheer up by the time we were on site, and was glorious in the afternoon.  Ruth, Adrian and I worked with the Earth Resistance meter, now restored to its normal operation.  Jim, Mike and Dave were up on the hill with the GPR.

The Earth Resistance team managed an excellent eight grid squares today, and there is now a link to the 2016 survey. I wonder if it was a reaction to the go-slow of the last three working days?  Fig. 1 shows everything we have done at Gorhambury using the Earth Resistance meter.

Fig. 1: the whole area surveyed with the Earth Resistance meter as of the end of 17/08/2017.

The next image shows the mag data from the same area (Fig. 2).

Fig. 2: the magnetometry data for the same area.

Over the next few days we will be working our way west filling in the gap between the two surveys.  That annoying hedge line will slow us down a bit. The next image shows a high-pass filtered version of the area surveyed in 2017. The filter flattens out the background variation and thus makes the buildings stand out more.

Fig. 3: the 2017 survey with a high-pass filter to bring out the buildings.

Unfortunately, my trick of spreading the remote probes wide apart to remove the edge-matching problem didn’t work so well here, so I have to play with the data some more to get a good images.  Today’s survey picked-up the line of buildings running SSW-NNE at the top of the plot.  These buildings lie along a road which run alongside the temple.  In the next day or so we should pick-up the cross roads with the road which runs parallel and slightly to the north of the hedge line.

The area covered by the GPR in the last three seasons is quite impressive (Fig. 4).

Fig. 4: the complete GPR survey.

Fig. 4 is a nonsense mish-mash of images from multiple depths, dates and even software packages, but it at least gives one an idea of the amount we have done, and serves to remind me about that annoying hole in the data around the hedge line.

The next image (Fig. 5) shows everything we have done in 2017.

Fig. 5: the 2017 GPR survey so far.

Fig. 6 shows the area surveyed today (with the yellow border).

Fig. 6: the area surveyed on day 10.

There is a substantial building about one quarter of the way along the area surveyed. and another one parallel to the 1955 ditch.  It is useful to look at the mag data (Fig. 7).

Fig. 7: mag data from the same area as shown in Fig. 6.

There is a magnetic anomaly in the same place as the substantial building, but I am pretty sure I would not have interpreted it as a building.  Although the building plan in Fig. 6 is obviously incomplete, it is certainly there which is very interesting given its position high on the hill outside the 1955 ditch. The building which shows in the topright corner of the GPR plot, doesn’t show at all in the magnetic data.  We have seen this before when we have got towards the edges of the town.

Many thanks to everyone who helped today.  Tomorrow the GPR will fill the annoying gap and the Earth Resistance meter will work slowly westwards.

Worn down

Anyone new to this blog or geophysics in archaeology is recommended to read the material on the “Geophysical survey in archaeology” page.

After three days of using the beast, the Earth Resistance team are feeling worn down. Not with the effort, but with boredom! We have completed a 60x40m block over the “House on the Hill” and that is enough. Tomorrow we return to “normal res” hoping to cover the same size area in a single day, but with just a single depth.

Some of you may remember that the Earth Resistance meter was brand new last year.  The meter came with six steel probes to allow for various configurations, of which we have used three extensively, and three very occasionally.  Clearly we have been busy if Fig. 1 is anything to go by.

Fig. 1: the probes before and after some use.

A small problem with today’s Earth Resistance survey was resolved with some programming in the statistical programming language R.  Here are the six depth slices along with the Wenner array image.

Fig. 2: the multiple depth Earth Resistance survey.

I am very pleased with the result, and it will be interesting to compare the images with the GPR time slices for approximately the same depths.  I think we can safely say we have “done” the house on the hill.

The GPR completed another 40x80m block up on the hill to the south of us. Firstly, a composite of the time slices.

Fig. 3: The GPR time slices for day 9.

Despite some strong reflections, nothing much resolves itself into anything very intelligible.  Slice 1 to 3 are basically the topsoil, and slice 10–12 are nonsense from the bottoms of the profiles.  Slice 6 does show an empty space where the 1955 ditch is.  A composite of days 8 and 9 in Google Earth, plus the mag, makes this quite clear.

Fig. 4: GPR results from days 8 and 9 showing the 1955 ditch.

Fig. 5: the magnetometry data with the area of the day 8 and 9 surveys shown as a yellow box.

As we get up here on the heights, there is less in the way of obvious buildings showing on the mag survey, but there are some intriguing enclosures.  Elsewhere at Gorhambury was have found buildings in these areas not visible on the mag surveys.  There next few days could be very interesting, or very disappointing!

Many thanks to Anne, Pauline, Jim, Adrian, Dave and Mike for all their help today.