Tag Archives: earth resistance survey

The final results from Alba Iulia (Apulum)

We managed the final ten squares on the last day completing the area I hoped we would cover. We managed 2.6ha in seven days of survey.  The final four grid squares had an old excavation trench in the middle of them and a sea of mud from the spoilheap being bulldozed back into the hole.

Fig. 1: Stefan and Wyatt surveying in the mud.

I have left the grid on the next image (Fig. 2) which gives you some sense of scale.  The grid squares are 20x20m, and site north is to the right.

Fig. 2: the surveyed area with grid for scale.

In the next image (Fig. 3) I have labelled a few of the features.  One can easily see many of the buildings.  Because of the large differences between areas of low readings and areas of high readings I have applied a log-transform to the data to make some of the features more visible.

Fig. 3: the 0.5m survey with log-transformed data.

An alternative (and very common) method of evening-out the differences is to apply a high-pass filter (Fig. 4).  The downside to this is that areas of very high readings will often end-up with a area of very low readings next to it which are purely an artefact of the filter.  It is essential, therefore, to have several images and compare them so that you do not create features where there are none.

Fig. 4: the data after the application of a high-pass filter.

As well as the main survey with a 0.5m probe spacing (thus ‘looking’ 0.5 to 0.7m below the surface, approximately), we were simultaneously collecting 1.0m probe spacing data (which ‘looks’ about 1.0m into the ground), but at only two readings per square meter rather than four.  This extra data did not add that much more to the survey (Fig. 5), but does make the north-south road show more clearly.

Fig. 5: the 1m probe spacing data, high pass filtered.

There is quite a bit to do in terms of interpretation and drawing-up maps of the results, but I am very happy with the new results which have greatly improved an already excellent set of results from 2002/3.

The journey home seemed to go on for ever, but at least I had some entertainingly-named beer on the way!

Fig. 6: Munich Hell.

Many thanks to the whole team, and especially to Stefan, Wyatt, Doru and Ian.

Advertisements

Somewhat tired

On Friday the snow kept on coming and we had to abandon the survey for the day. It gave us the chance to explore the citadel and visit the museum.

Fig. 1: Carol’s Gate, Alba Iulia.

The snow had stopped yesterday morning so we headed out to site once more. During the day the snow cover slowly melted and we managed an excellent eleven grids, although two of those were small partials.  Today, the weather improved…

Fig. 2: Stefan surveying.

I even ended-up with sun-burnt ears!

Having lost time through airline incompetence and snow, we were determined to try and catch-up and we managed twelve 20x20m grids at 0.5m intervals with just the two of us.  To say our feet ached would be an English understatement.  We did, however, get some lovely results.

Fig. 3: the half-meter probe spacing survey at the end of Sunday.

We believe that the thin line on the right hand side of the plot running diagonally is the line of the town wall.  We seem to have picked-up a set of three small buttresses on the inside of the wall.  We have some more buildings at the top of the plot.  Obviously, there is a great deal going on in this data and a detailed interpretation will take a little longer.

Tomorrow our target is the ten grids along the top edge of the survey to complete the block and join the survey up with the earlier excavations.  It is tempting, however, to try and survey a little more at the top to see what those two pairs of parallel lines are doing!

I’d like to thank the whole team but especially Stefan who has worked really hard, and Wyatt who has also helped a great deal, and also Ian Haynes and Doru Bogdan who made the whole trip possible.  The whole team have been great to work with and made what could have been a trying experience great fun.

Of snow and sheep

Yesterday started off well. The weather was a little cold, but other than that it wasn’t too bad.  Then it started to snow.  The snow was OK, not too much.  Then it snowed some more.  And a bit more…

Fig. 1: surveying in the snow.

Just to add to the delights, we had a visitation from some sheep.  They charged past me where I was laying in the grid for the next square, and surrounded the team using the res.

Fig. 2: The sheep visit the survey.

Fig. 3: The sheep visit the survey (heavily cropped photo).

By lunchtime, it was getting silly, and we packed-up.

Fig. 4: Getting silly.

We did, however, get some very nice results from both the 0.5m probe-spacing data and the 1m -probe spacing data (the latter “looks” a bit deeper into the soil).

Fig. 5: the 0.5m probe-spacing data.

Fig. 6: the 1m probe-spacing data.

The dark line running diagonally across the plot is a Roman road with a subsidiary road running off it at an angle.  The excavations were at the far left-hand side of the image.  The buildings along the road are pretty clear.  Both these images have been high-pass filtered to bring out the structures.

Although the landscape is covered in snow, it isn’t actively snowing at the moment, so I suspect we will be going out again today.

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.

Midwinter madness

Who on Earth goes out on midwinter’s day to do fieldwork? Well, we do! Mike, Ruth, Peter and myself met up with Rex and Nick from FRAG to extend the magnetometry and Earth Resistance surveys at Durobrivae.  It was a foggy morning but at least the temperature was well above freezing and, in fact, it was quite pleasant working conditions… for midwinter!

Fig. 1: “… and this bit goes here”. Building the mag at the start of the day, Photo by Mike Smith.

Unfortunately, the mag developed some problems and although we managed two grid squares before it died completely, I am unable to download them at the moment.  Fingers crossed I can rescue the data in due course.

The Earth Resistance survey was more successful.  Given the short day I was only aiming for four grids to extend the previous survey to see if the outer circular feature did actually join-up on the eastern side.  Figure 2 shows the results.

Fig. 2: The Earth Resistance results over the “tumulus” as of December 2017.

As can be seen, the “outer circle” does indeed form a continuous curved feature on the eastern side.  There also seems to be two features running off this curved edge eastwards.  I have marked these in Figure 3.

Fig. 3: the Earth Resistance results. Red lines mark the features mentioned in the text.

We have clipped the corner of a building in the bottom right hand corner of the survey.

How do these match up with the mag data?  Figure 4 shows it for the same area.

Fig. 4: the magnetic data for the same area as Figs 1–2.

The northern linear feature picked-out in Figure 3 shows clearly in Figure 4 as being strongly magnetic (it ranges from -10 to +15nT).  It is probably a ditch full of rich organics and/or burnt material.  The southern linear feature is harder to see in the mag data so in Figure 5 the lines from Figure 3 have been superimposed on the image.

Fig. 5: the mag data with the linear features from the Earth Resistance data superimposed.

The northern line in Figure 5 fits nicely on the magnetic feature as we expected.  The southern line joins-up three magnetic anomalies ending up with a big one which is about 10m by 5m.  It looks like parts of the ditch were filled in with non-magnetic material, and parts with highly magnetic material suggesting several phases of infilling.

It clearly worth continuing to expand the Earth Resistance survey yet further.  It would be good to see what is happening on the western side, but also to see if we can pick-up the plan of the building to the south where we have just got a single corner.

Nerdy bit…

For those of you interested in this sort of thing…  Since getting the new RM85 I have been using a “pole-pole” configuration for the probes.  That means having the remote probes 20m or more away from the edge of the survey area, and separated by at least 10m.  This contrasts with the more usual set-up (called “twin probe”) where the remote probes are often only half a meter apart.  The reason for doing this is that is removes, usually, the need for grid-matching, i.e., getting an image where one cannot see the joins between grid squares.  If the survey is done over only a few days, this works well.  If, however, the two blocks of survey are separated by several weeks of winter weather, the differences can still be seen (Fig. 6). This requires a bit of extra processing, but still not as bad as having to get the grids to match every time one moves the remote probes.

Fig. 6: plot showing the girds of Earth Resistance data before grid matching.

The CAGG team are a sociable group, and we get to sit and eat lunch in entertaining places (Fig. 7).  If you would like to come and join us during our surveys in 2018, please get in touch.  We provide training, and commitment is minimal.  We simply add everyone to a mailing list, and when there is a survey coming-up, we ask who can make it.  Often, we do not want a team bigger than seven people apart from the big survey season at Verulamium in August, and maybe one or two other sites where we would like to run more than one machine.

Fig. 7: the crew having lunch accompanied by the dull roar of the A1(M). Photograph by Mike Smith.

I hope you all have a very Merry Christmas, and wish us all a successful year of surveys in 2018.

 

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.

Just too claggy

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

A group of us headed out to Little Hadham today with the aim of extending both the Earth Resistance and magnetometry surveys.   By the time Nigel had pushed the mag across the field, the wheels had diameters several inches larger than they should.  Jim tried a line or two of data collection, but the odometer was over-running by two meters and the wheel needed trowelling clean every transect.  We decided, therefore, to abandon the mag survey, and concentrate on enlarging the area of the Earth Resistance survey.  We managed another six 20x20m squares giving us a total of 100m by 80m, some 32,000 readings in total.

Figure 1, below, shows the initial results.  As before, the data is dominated by striping caused by the cultivation pattern.  A 2D fast fourier transform (as implemented in TerraSurveyor), quickly removed these stripes.

Fig. 1: the Earth Resistance data overlain on the mag data.

In Figure 2 I have applied the filter to remove the striping.  To the right I have put the mag data for comparison.

Fig. 2: The resistance data after processing with the 2DFFT. The mag data of the same area is shown to the right.

Unusually, most of the features show in the res and mag data.  The res data has nicely picked-up many of the linear features more normally only clearly seen in mag data.  In Figure 3 I have labelled a few points.

Fig. 3: the res and mag data with labels.

Ditch features A and B show nicely in both the res and mag data.  What is clear from the res data, however, is that the ditch continues between the two and they are one distinct linear feature.  If one draws a straight line along A and B, it lines up perfectly with the linear feature C we found across the road in 2014.   Linear feature D shows equally well in both data sets.  At E, something complex is happening.  In the mag data it looks almost as if A is turning and runs alongside E, whereas in the res data is looks more like AB cuts across the linear to the west of E.  The parallel lines to the west of E show quite well in both, and are probably some form of trackway.

Many thanks to Jim (CVAHS; both for surveying and transporting the equipment and myself), Nigel (NHAS), Caroline, Peter, Amanda and Mark (BAG).  Hopefully we can get to do some more when the field is less claggy.