Tag Archives: geophysical survey

Day 54

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

Yesterday was the last day of the 2017 season at Gorhambury. Apologies for the slight delay in posting… we went down the pub for supper!  We have completed 54 days of survey at Gorhambury over the three seasons.  As well as the 35.7ha of magnetometry we completed last season, we have now completed 14ha of GPR survey.  Just pushing the machine along the strings is about 280km.  We didn’t manage any usable Earth Resistance survey in 2015, but we have done quite a bit in 2016 and 2017.  Last year we had terrible problems with the very hard and dry soil.  As a result, many grid squares have been done twice.  We have, however, completed a 5.6ha survey at 0.5m intervals.  That is 224,000 resistance readings, or sticking the machine into the soil 112,000 times.  That doesn’t include three days with the beast which is a further 9,600 survey points resulting in 67,200 readings.  Here is the story in pictures.

Fig. 1: crude mosaic of GPR time slices showing the extent of the survey so far.

Fig. 2: the 2016 and 2017 Earth Resistance surveys at Gorhambury.

Figure 1 shows a (very crude) mosaic of time slices for Gorhambury just to show the entire extent of the survey.  There is going be a great deal of work reprocessing these to get the best out of them and to get the various blocks to match.  I also need a faster computer.  I tried out the kriging option last night and it took several hours to process the data, but the images were no doubt sharper.

Figure 2 shows the 2016 and 2017 Earth Resistance surveys.  At the moment the two seasons are just images put together in Google Earth.  I need to see if I can join the two into one big survey and get the edges to match properly.  I also need to see if I can get rid of the line caused by the deluge this season.

Firstly, let us look at the Earth Resistance survey results.  On the last day we redid four squares from last year, and then completed five awkward partials around the corner of the enclosure for the theatre.  Why did we redo those four?  Figure 3 shows last year’s survey with the block marked.  The hot dry conditions gave very noisy and unsatisfactory results.  I thought it was worth a morning’s effort to get those re-done.

Fig. 3: the 2016 resistance survey showing the duff grids.

Now the improved grids.  Note that the slight difference between the two surveys is due to minor differences in how I processed the data.  I will produce a more standardised plot.

Fig. 4: detail of the area surveyed at the end of the 2017 season.

Fig. 5: the Earth Resistance survey with the blocks from 2017 high-pass filtered.

There are some interesting things happening at the north-eastern corner of the plots, and into the area to the north we haven’t surveyed.  My guess is that the stratigraphy is probably deep and complex in this area.  Let us compare this area to the magnetic data (Fig. 6) and the GPR data (Fig. 7).

 

Fig 6: the magnetic survey with the area of the Earth Resistance survey from days 17 and 18 indicated by the cyan outline.

Fig. 7: the GPR survey with the 2017 Earth Resistance survey area indicated in red.

Fig. 8: the GPR survey with the res data overlain on it.

From all three data sets we can see that there is a lot going on in that bit of the field near the drive and the theatre, but it also appears there has been a good deal of robbing to add confusion to the picture.

How about the GPR team?  A month of nice weather with a bit of rain has made the grass green and lush.  Lovely for sheep, but a pain to push the GPR through.  They completed two 40x40m grids on a hot humid day, excellent progress in the conditions.

Firstly, here are a set of time slices (Fig. 9).

Fig. 9: GPR time slices from the area surveyed on Day 18.

The vast majority of the area surveyed appears to be empty.  There are hints of earlier agricultural practice but not much else apart from the top edge where part of a building can be seen.  This connects to the area surveyed earlier in the season.  I reprocessed the earlier version of that block today using kriging to give a sharper image.  Here are the slices together (Fig. 10).

Fig. 10: the GPR survey from day 18, along with a re-processed block from day 12.

We clearly have a nice L-shaped building.  I suspect this is Insula XXX, Building 4, Niblett and Thompson Monument No. 461 which is known from aerial photographs from 1976.  They only have part of the plan, however, and ours looks quite different (hence my doubts).  The mag data shown in Fig. 11 shows some slighter, more ephemeral buildings to the SW along the line of the SW-NE street 25 which can be seen very easily.

Fig. 11: the mag data in the area of the GPR survey.

The SSW–NNE running street 25 which runs along the NW side of the building we have been discussing, shows very clearly indeed but street 10, which is supposed to have run WNW–ESE just to the south of our new building and the “House on the Hill”, does not show at all in either the mag or the GPR data.  It was observed in excavations by Frere near the modern road, but not this far west.  In terms of the town plan, we seem to have two lines of buildings running SW-NE, one along Street 25, and another along street 23, but a large open area with nothing very much it in apart from a few quite large pits.  One can almost see some alignments in those pits.  Are we seeing backyard areas divided into blocks?

Although our season at Gorhambury has come to an end, we will be undertaking surveys elsewhere, and probably in Verulamium Park once more.  I started this posting with some numbers, so I thought I would end with some as well.  This is the 131st posting on the blog.  Those postings take-up 583mgb of our 3gb free allowance and include 693 images.  There have been 68 comments, but we have been protected from 9,402 pieces of spam!  The blog has been viewed 32,150 times by 11,016 separate visitors (in practice, this means that number of IP addresses).  Our best month was the first season at Gorhambury in 2015.  This August has been down on the previous two (2015: 1.8k, 2016: 1.7k and 2017: 1.4k) but the average number of visitors per day has gone-up over the year and we are likely to reach 9,000+ views by year’s end.

I would like to thank everyone who helped this season once more, both with pushing machines, moving strings, laying-tapes and moving equipment.  You are all stars in my eyes, and I think we have created a stunning survey.  We all got a bit tired towards the end, especially in the rather hot and humid conditions over the couple of days, including CAGGs loyal follower, Fergus (Fig. 12).

Fig. 12: Fergus sleeps off a busy day on site.

If anyone is interested in joining in with some of CAGG’s activities, drop us an email.  We are a friendly bunch, and on-the-job training is given.

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Just two more days

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

I did consider using antepenultimate again, but I thought you might think me pretentious…

Everyone worked extremely hard today.  Mike and Jim on the GPR finished yesterday’s block and managed another 40x40m grid square.  No easy task over the long grass and thistles.  Ruth, Dave and Julia completed five earth resistance squares, including two that had to be done in two parts and joined together in the software later due to an inconvenient hedge!  Good job everybody, and many thanks.

Here is the Earth Resistance survey, both normal, high-pass filtered, and the magnetometry data from the same area.

Fig. 1: the Earth Resistance data at the end of day 16.

Fig. 2: the Earth Resistance data at the end of day 16, high pass filtered.

Fig. 3: the magnetometry data for the area shown in Figs. 1 and 2.

I hate to say it, but our five squares, including the two annoying partials, appear to lie between the buildings along the road to the north of the hedge line, and to the north of the buildings we found yesterday. We do, however, have a nice tidy area surveyed now.  We couldn’t have left quite such a silly hole in our survey data.  Tomorrow we head north to survey along the northern edge of the block we did last year.

The first block of GPR data from today was a continuation of yesterday’s

Fig. 4: the GPR time slices from the block completed on day 16.

Nothing very exciting jumps out from the plots, although there are some things to check out.  Slice 6 (second from top on left) has a strange upside-down M shaped feature (in black) and slice 8 (bottom-left) has something semi- or sub-circular near the northern edge.

How about the second block?

Fig. 5: the GPR time slices from the second block completed on Day 16.

Not a great deal there either.  Sorry guys!

Tomorrow sees the GPR crew filling-in an odd gap between last year’s survey and this years.  The plus side is that the mag shows lots of buildings, so tomorrow’s results ought to be much more interesting!

Many thanks to everyone who worked so hard in the sun today.  Just two more days.

Just in time

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

Not long after we arrived home after a busy week on site, the heavens opened. We’ve had 7.2mm of rain in the last 24 hours. Hopefully, not enough to create problems with the Earth Resistance survey, unlike the deluge we had the other week.

Mike Smith sent me this entertaining picture.

Fig. 1: Red Flag © Mike Smith.

The Earth Resistance meter was manned by Ellen, Pauline, Graham and Fergus (CAGG’s mascot).  They completed an excellent seven 20x20m grid squares including two partials with a big oak tree in the middle.  Figs. 2 and 3 show the data “normally” and high-pass filtered.

Fig. 2: the Earth Resistance survey.

Fig. 3: the Earth Resistance survey, high-pass filtered.

The main feature of interest in the new area is the nice building in the top left-hand corner of the plot.  The apsidal end of a room facing NE shows especially clearly.  This building was known previously, partly from an aerial photograph taken in 1977 and partly from the English Heritage survey undertaken in 2000.  We have much more of it, however.  The buildings also show nicely in the mag data (Fig. 4).

Fig. 4: the magnetometry data for the area of the Earth Resistance survey.

We will fill in the odd missing squares on Wednesday.

The GPR was in the deep south of the Gorhambury side of the town, up on the hill near the King Harry Lane roundabout with marvellous views down across the theatre and the River Ver with the fields of ripe wheat on the opposite hill slope.  That is after you have caught your breath…

The GPR team completed the block I was hoping to get done, and started on the next one slated for Wednesday. Good job!  Here is the composite of six time slices (Fig. 5).

Fig. 5: The GPR time slices 3 to 8. See labels for times and approximate depths.

Slice 4 (Fig. 5, top right) appears to show a wall running NNW–SSE with a rounded corner.  Here it is on Google Earth.

Fig. 6: time slice 4.

Slice 6 (Fig. 5, middle right, and Fig. 7) starts to show a clear building in the NE corner. This is supposed to be along a road which runs approximately parallel to the modern road, but which I cannot see in the geophysical data. The wall seen in the previous image is still visible on the northern side but does not show on the west.

Fig. 7: time slice 6.

Slice 8 (Fig. 5, bottom right and Fig. 8) shows the wall again on the western side as well as the northern.  Curiously, the corner appears different. Comparison to the mag data (Fig. 9) is informative.

Fig. 8: time slice 8.

Fig. 9: the mag data in the area of the GPR survey.

The line of the wall is clearly following the magnetic anomaly in this area, with two large circular anomalies on the corner.  I had thought the anomaly was a ditch, but now I am less sure.  It is clearer to see the relationship if one traces over the wall line from slice 8 (Fig. 10).

Fig. 10: the mag data showing the wall line from GPR time slice 10 marked in yellow.

The differences in the corner can be seen if one then looks at slice 4 with the wall line from slice 8 marked (Fig. 11).

Fig. 11: the wall line from slice 8 marked on slice 4.

As can be seen from Fig. 11 the wall lines match perfectly apart from the corner.  Was it rebuilt at some point, or do the reflections in slice 4 represent some wall collapse?  I’ll need to examine the radargrams carefully to decide what is happening.

The ditches are marked on the “Urban Archaeological Database”.  Isobel Thompson informs me that they have been interpreted as field boundaries for “Little Wynyards” in the 17th century.  Perhaps a vinyard? As Isobel points out, there are the Vintry Gardens near the abbey which are in a walled enclosure.  Just one more avenue of research to pursue to be able to interpret our complex data set.

Many thanks to Pauline, Ellen, Graham, Mike, Jim and Nigel for all their help. We’ll be back on Wednesday for the final week.  Doesn’t time fly when you are enjoying yourself!

Sawtooth Sunday

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

Today saw the GPR team doing the sawtooth edges of the field. A bit fiddly to do, but very fiddly to survey-in and process.  I have only processed one block of the three, I’ll work on the others tomorrow and also start joining the various blocks together into one big survey.  The  Resistance team also managed a very respectable six grid squares, including some that covered the east wing of the “House on the Hill”, otherwise known as Insula XXVI Building 2 or even Niblett and Thompson Monument 445…  This building was first published by Corder in 1941 and has only been seen through aerial photographs until we came on the scene.

First the underlying magnetometry data from 2015.

Fig. 1: the magnetometry data underlying the 2017 Earth Resistance survey.

As can be seen from Fig. 1 we can see the building quite clearly in the mag data as a series of white (low magnetism) lines.  The east wing does not show very clearly.

Fig. 2: the Earth Resistance survey.

The results from the Earth Resistance survey show the building beautifully.  The image has been high-pass filtered.  The large room on the east wing shows very clearly, although the “apse” on the west wall of it is still a little unclear.  It looks like a large dining room.

Let’s look at the 2015 and the accidental 2017 GPR surveys.

Fig. 3: the 2015 survey of the “House on the Hill”.

The slice from the 2015 survey (Fig. 3) shows some of the detail more clearly than the Earth Resistance survey, and some less so.  The other details may show more clearly in other slices and I’ll reprocess the data in due course. It will be interesting to see what the Resistance survey makes of the building to the far east of the GPR data.

Fig. 4: The 2017 GPR survey overlain on the other data.

Just for completeness sake, here is one of the time slices from the 2017 survey (Fig. 4).

Further south (and further up the hill!), the team completed a block to the south of the nice building we saw yesterday.  The new software can create an image with all the slices on.

Fig. 5: slices 3 to 10 of the day 4, block 3, GPR data.

This is useful to be able to see how the details change as one goes deeper.  (Take the depths given with a pinch of salt as 0.09m/ns is just a guess at the moment.)

The next series of images shows slices 4-8 from the above plotted next to yesterday’s building (just yesterday’s slice 5). The joins between blocks will improve when I process it all as one big survey.

Fig. 6: slice 4 (see Fig. 5 for depths).

Fig. 7: slice 5 (see Fig. 5 for depths).

Fig. 8: slice 6 (see Fig. 5 for depths).

Fig. 9: slice 7 (see Fig. 5 for depths).

Fig. 10: slice 8 (see Fig. 5 for depths).

As can be seen there are various buildings in this area.  It looks like there might be another small town house just the SW of the one we saw yesterday.  There is also something quite large parallel to the SW-NE road which shows in the deeper slices.  The large ditch which runs parallel to that building, just to the west which can be seen in the mag data, shows as a whiter line of “no reflections” in the GPR data.  There is quite a lot going on in this little block of data.

Tomorrow I will work on the other blocks and try to integrate the first four days of survey.  We are not out on site again until Wednesday.

Many thanks to Ellen, Jim, Mike, Graham, Nigel, David and Pauline for all their hard work, and especially to Mike, Jim and Ellen for transporting all the equipment as well as myself!

 

CAGG on tour: Chisbury, Wiltshire

At the request of Andrew Reynolds and Stuart Brookes, medieval archaeologists at the Institute of Archaeology, UCL, members of CAGG headed out on tour to Wiltshire.  The site we were asked to survey was the Iron Age hillfort of Chisbury, near Great Bedwyn, Wiltshire.  One may ask why Chisbury is of interest to early medievalists?  An early 10th century document called The Burghal Hidage records a site called Cissanbyrig (Baker and Brookes 2013, p. 228) which may be Chisbury.  The Burghal Hidage records the defended settlements (burhs) of the Kingdom of Wessex set-up after the defeat of the Vikings in the late 9th century.  The Historic England Scheduled Monument listing notes that:

Although no formal excavations have been carried out within the hillfort, observation of 20th century disturbances has produced evidence of urns, bronze swords and of storage pits containing Late Iron Age and Romano- British pottery.

The site also has a well preserved 13th century chapel under HE Guardianship.

Fig. 1: the chapel at Chisbury.

The interior is quite plain but has some interesting details.

Fig. 2: the interior of the chapel at Chisbury.

One of those details is a surviving consecration cross painted on the back wall.  Apparently these were painted on the wall when the church was dedicated.

Fig. 3: The consecration cross.

Peter Alley’s high-level photograph from the UAV shows how the chapel lies right across the defenses of the hillfort.  Maybe this position was significant in somehow slighting the earthworks?

Fig. 4: high level photo showing the location of the chapel.

The defenses are very well preserved around most of the circuit but they are covered in trees so hard to see and photograph.  From the satellite image the site is an extended oval of trees.

Fig. 5: Chisbury from Google Earth.

Jim West took a good image showing part of the defences to the west.

Fig. 6: Chisbury defences from the west (photo: Jim West).

The weather was wonderful and we all got a bit sun burnt.  The bluebells and primroses were out in force.

Fig. 7: bluebells on the defences.

The theme of the week was, however, definitely “horses”!

Fig. 8: Nosey horsey.

Unfortunately, horses are quite magnetic due to their ferrous footwear.

Fig. 9: Horseshoe alert! (Photo: Mike Smith).

Those horses are also a bit careless with their shoes…

Fig. 10: Missing footwear.

The plan was to complete as much of the inside of the fort as we were able with the magnetometer, and to do some selected areas with the GPR and the Earth Resistance meter. Right from the beginning we were beset with problems. We arrived at lunchtime on the Thursday, and managed to complete quite a few squares in the first afternoon, but the odometer started to over-run, eventually by four or five meters.  I swapped a few emails with Pat Johnson from Foerster, and the next day we managed to cure the problem.  A couple of days later, one of the pins in the “spider” — the cable that joins the sensors to the control box — snapped so we were down to three sensors.  On the last full day, the odometer started slipping again…  We did manage to survey the whole available area of the fort, but only just and without much time to try the other methods.  Fig. 11 shows the overall survey.

Fig. 11: magnetometry survey.

Although some of the major features can be seen at this scale, I have created two images with the north and south parts of the survey and some annotations.  (You might like to look at these downloaded and at full size.)

Fig. 12: the mag survey of the northern part of the site.

In Fig. 12 we can see a series of parallel linear features which have been annotated in cyan. These look like field drains to me.  Very faintly, however, there are some circular features.  These may well be the “drip gulleys” of Iron Age circular round houses.  I have marked some of the possible ones in red.  The problem with these is that the more one stares, the more one invents!  I am sure you can spot a few more possibles if you look long enough.  There is a great deal of ferrous noise, especially around the edges from fences, gates and water tanks, but also in the field from old nuts and bolts, horse shoes and the like.  Looking carefully and the little blobs and measuring the minimum and maximum values in nanoteslas (the unit of magnetism), one can start differentiating between bits of old iron and possible pits.  I have marked a small number of the possible pits with green arrows.

Fig. 13: the mag survey of the southern part of the site.

In Fig. 13 the red line is the pipe which joins the main water tank in the middle of the field.  I have marked just one piece of ferrous rubbish with a red arrow, there are lots more.  The cyan lines mark the possible field drains.  The dark blue line is a negative-magnetism feature which runs from the edge of the water tank to the pond.  This can also be seen in the GPR data (below).  I am guessing this is some sort of drainage / outflow from the water tank to the pond. There are some areas with such high ferrous noise it is impossible to see anything, for example the north end of the eastern field.  There are, however, quite a few pits once more, and I have marked just a small selection.  What is curious, however, is how much of this area seems devoid of any features at all.

Although the mag results are not exciting in the sense of being able to clearly see a building, as we often do at Verulamium, there is quite a bit of detailed information buried in the data.

We managed just a couple of days of GPR survey thanks to some local help.

Fig. 14: the GPR survey underway. (Photo: Mike Smith.)

I used the GPR Process and Surfer programs and created 3ns time slices.

Fig. 15: GPR time slice 3 from 10.5 to 13.5ns.

Fig. 16: GPR time slice 4 from 13.5 to 16.5ns.

Fig. 17: GPR time slice 5 from 16.5 to 19.5ns.

Fig. 18: GPR time slice 6 from 19.5 to 22.5ns.

Fig. 19: GPR time slice 7 from 22.5 to 25.5ns.

Fig. 20: GPR time slice 8 from 25.5 to 28.5ns.

To be frank, not a great deal shows.  The last time slice shows the suggested pipe from the tank to the pond.  The southern area has more high reflections, especially in slice 5 (Fig. 17) which one could try to make into buildings, but I find myself suspicious that these features are close to the water tank and they may be something to do with the tank’s construction.

Giving the billing this site has, the results are not all that stunning.  There are, however, features which would be worth investigating further, and hopefully we will get to “ground truth” some of these features as part of Andrew Reynolds and Stuart Brookes’ wider project.

Many thanks to those CAGG members who came all this way to do the survey: Ellen Shlasko, Ruth Halliwell, Peter Alley, Jim West, Nigel Harper-Scott and Mike Smith.  Many thanks too to the members of the local group who came to help: Shaun Wilson, James Kay and Lynn Amadio. Lastly, but certainly not least, thanks to the stud and the farm for allowing us to play on their land.

Another day, another town

You would think I would have had enough. But no… hot on the heels of the end of the Gorhambury season, we headed off to the mysterious east side of the county. The Greenwich meridian seems to exert a powerful influence in Hertfordshire with its citizens seemingly afraid to cross the invisible line.

Back at the start of the project, we planned to do some survey in and around Braughing.  We managed just one site. The area is extremely important with multiple late Iron Age and Roman sites including the Roman “small town” on Wickham Hill.  We had an opportunity to work on the small town along with members of the Braughing Archaeological Group for a couple of days, mainly to see if magnetometry would show something useful.  The field was, however, rather rough and caused the odometer on the cart to over-run by about a meter, and the nuts and bolts needed constant tightening. On the second day I adjusted the odometer settings which improved matters a bit.

The survey underway on Wickham Hi

The survey underway on Wickham Hill.

We managed to complete 13 grid squares which was pretty good going, especially as the data logger crashed three lines before the end of the fourteenth square and we lost the rest of the grid.  The results, after a bit of work in TerraSurveyor, were very interesting.

The survey results.

The survey results.

The broad line running east-west towards the south of the surveyed area is the road.  It can be seen in the Google Earth image in the background.  Towards the west, the very dark band must be where the road becomes a sunken way as it goes up the slope.  What is very obvious is the difference to the planned public town at Verulamium.  This site was clearly a very different type of settlement.  What we have clearly shown is that it is worth expanding the magnetometry survey to cover as much of the settlement as possible.  Hopefully, the field surface will be a little more benign when we return!  One thing won’t change, however, and that is the slope…

On top of the hill.

On top of the hill.

Many thanks to Jim West for coming all the way from Chorley Wood to run the mag on the first day while I lay-in the grid, and also many thanks to all the members of BAG who joined in. Looks like we’ll be back!

A picture is worth a 1000 words

The Verulamium magnetometry survey.

The Verulamium magnetometry survey.

I suppose I cannot really get away with that simple a post.

First of all, CONGRATULATIONS to everyone, it is a fantastic achievement and I am so proud of all of you.  Secondly, a big thanks to everyone who turned out for an extra day on Bank Holiday Monday to complete the Macellum field.

How about some numbers?  Well, Verulamium is the third biggest Roman town in Britain, after London and Cirencester.  It is, however, the largest Roman town in Britain which doesn’t have a modern settlement built over most of it.  We have surveyed 64.5ha of the total area of 81ha.  It has taken us 83 working days starting in the summer of 2013, but we didn’t do much at Verulamium in 2014.  It took 12,900,400 readings to cover those 64.5 ha.   That, of course, doesn’t include the grids we did twice because of frozen sensors or other problems. People pushing the cart walked about 322km, not including having to go back to the start for partials, getting to the squares in the first place, or laying in the tapes and strings.

Let us look in more detail at the last bit surveyed in the Macellum Field.

The area surveyed during day 37) high contrast).

The area surveyed during day 37) high contrast).

Several things come to mind.  Firstly, there is very little there!  Towards the NE and along the western side there may be a ditch feature, although it is quite faint.  Other than that, the main (and annoying) thing are the strong magnetic anomalies along the edge of the field.  Some of you may remember the 12″ gas main which runs across the Park… well here it is again.  What I do not entirely understand why there are differences between the negative and positive readings along our grid lines.  Jim and I spent some time making sure I put the composite together correctly, and we are sure it isn’t a survey error.

This end of the field is know to contain two Romano-Celtic temples.  These are known from aerial photographs taken in the hot summer of 1976.  I wonder if this area of the town was kept clear of encroaching buildings, pits, ditches and the like deliberately?  If we turn the contrast down (i.e., clip the image at +/- 40nT instead of +/- 7.5nT, we can see one of the temples close to the hedge as a faint white line.

Low contrast version of the area surveyed on day 37.

Low contrast version of the area surveyed on day 37.

Yet another target for the GPR next year!

Some of the team (many thanks Ellen, Mike and Jim!) helped re-do a number of areas of the res survey, plus one extra bonus square.  The biblical deluge of Sunday night (Lamer Lane was flooded once more) was not ideal.  This is the final area completed in 2016:

The 2016 resistance survey.

The 2016 resistance survey.

It is a pretty good result.  There is almost no use of the “edge match” feature of the software to get the various grids to join neatly.  It could be improved.  The very high contrast of the temple rather makes the buildings faint, but either the creation of selective composites (i.e., processing bit of the survey separately), or use of a high pass filter, would improve that.  The survey is quite big for a res survey: 2.5739ha according to TS (or 2.6ha to sane people who round numbers), which equals about 103,000 resistance readings.  That, of course, doesn’t include the large numbers of squares we re-did due to the dry conditions.

There is a great deal more to do in terms of data processing and interpretation, but I think we all deserve a well-earned rest.  Well, at least until Thursday…!