Tag Archives: geophysics

Batford Mill

I have a number of small surveys which remain unreported that I need to catch-up on, and so here is the first of them.

Earlier in the year, Mike Smith and I assisted Alex Thomas (University of Bristol) in undertaking a Ground Penetrating Radar survey of land lying to the north of the B653 at Batford, Harpenden, Hertfordshire (TL 148150, Fig. 1). The survey was undertaken over the weekend of 2nd/3rd April 2016. Earth resistance and magnetometry surveys had been undertaken in the area previously.

Fig. 1: Location of the site at Batford.

Fig. 1: Location of the site at Batford.

The underlying geology of the site is Lewes Nodular Chalk formation overlain in places by the Kesgrave Catchment Subgroup sand and gravel.

A Mala GPR with a 450mhz antenna was used, identical to the one CAGG borrow from SEAHA. The survey transects were at a 0.5m spacing collected in a zig-zag fashion. The survey started in the NE corner and proceeded east-west. Radar pulses were set for 0.05m intervals with a time-window of 73ns. The newer Mala systems do not allow manual selection of sample numbers which are determined by the machine, in this case 376 samples per trace.

For the amplitude slices presented here, as usual, the software system developed by Jeff Lucius and Larry Conyers was used (http://www.gpr-archaeology.com/software/). This necessitates the conversion of the Mala rd3 files into GSSI dzt files using the companion conversion program.

For this posting, the slices were 3ns in thickness starting at 3.5ns  From these, it appears that the second slice, 6.5–9.5ns represents the immediate ground surface. This agrees with the estimate of the first reflection from the individual radargrams at about 8ns as examined using RadExplorer. Beyond slice 7 (>24.5ns) the signal has completely attenuated. This means that all the usable returns lie in the band between c.6.5 and 24.5ns. This is not unusual for Hertfordshire where the clay soils do not allow for the GPR surveys to penetrate particularly deeply.

As with most of the GPR surveys reported in this blog, the numerical output from that software was turned into images using Surfer v.8. Kriging was used to interpolate the values into a 0.1 x 0.1m grid. The resulting images where then imported into Google Earth.

Six amplitude maps or `time slice’ maps were produced and are shown in Figures 2–7. The topmost map (Fig. 2) shows two strong reflections to the north-east and the south. The second map (Fig. 3), which represents the 9.5–12.5ns range, has the clearest set of features. A number of long linear features are visible, two of which I have labelled A and B. There is a odd-looking curved linear feature with two parallel lines, labelled C, into which a pair of parallel lines cuts, labelled D. Further fainter linear features can be seen, such as those at E.

slice-2

Fig. 2: Time slice 2 (6.5–9.5ns).

Time slice 3 (9.5–12.5ns).

Fig. 3: Time slice 3 (9.5–12.5ns).

The third map (Fig. 4) has fewer clear features, most of which are probably `echoes’ of the features seen in the previous map. The next three maps (Figs. 5–7) have successively fewer features in them, none of which are especially clear. By the last map, the GPR signal has largely attenuated and little can be seen. At best, we are getting a depth penetration of about a meter, probably somewhat less.

Fig. 4: Time slice 4 (12.5–15.5ns).

Fig. 4: Time slice 4 (12.5–15.5ns).

Fig. 5: Time slice 5 (15.5–18.5ns).

Fig. 5: Time slice 5 (15.5–18.5ns).

Fig. 6: Time slice 6 (18.5–21.5ns).

Fig. 6: Time slice 6 (18.5–21.5ns).

Fig. 7: Time slice 7 (21.5–24.5ns).

Fig. 7: Time slice 7 (21.5–24.5ns).

The question arises, therefore, as to what the long linear features may be. If Fig. 3A is a wall, it would be at least 35m long, and Fig. 3B would be at least 55m long. One possibility is that they represent old field boundaries. Looking at the 1898 OS map (Fig. 8), there is nothing to suggest an origin for those features. The 1799, map now in the Westminster Abbey Muniments Room does, however, show a field boundary behind some buildings to the north of the road. A crude overlay of an extract of this map (Fig. 9) on the Google Earth image with the GPR data, shows a remarkably good correlation between the field boundary and the one of the linear features (Fig. 3A).

Fig 8: The survey overlain on the 1898 OS map.

Fig 8: The survey overlain on the 1898 OS map.

Fig. 9: The 1799 Westminster Abbey map overlain on the Google Earth image.  Westminster Abbey map used with permission.

Fig. 9: The 1799 Westminster Abbey map overlain on the Google Earth image.

The origins of the curved and parallel linear features can be seen if one takes into account the location of the machine-dug test trench marked in Figure 10.  These parallel lines, only some 1.8m apart, represent areas of soil compression from the machine used in the excavation of the test trench.  Examination of one of the radargrams (Fig. 11) would seem to confirm this.  The origin of the reflections, marked with blue arrows, occurs at the very surface and is highly suggestive of compression rather than construction.

Fig. 10. Slice 2 showing the location of the machine dug test hole.

Fig. 10. Slice 2 showing the location of the machine dug test hole.

Fig. 11: Radargram showing areas of surface compression.

Fig. 11: Radargram showing areas of surface compression.

The survey results appear to be largely connected to (a) earlier agricultural use of the land in the form of hedgerows and so forth or (b) the recent impact of the excavation of the test trench.  It appears highly unlikely the GPR results indicate any sort of structure although the golden rule of ‘absence of evidence is not evidence of absence’ must be applied.  The lack of pottery or ceramic building materials on the surface makes it unlikely that a building is indicated.

Perhaps I should have saved a more exciting post for #100!

Bank Holiday Monday

The August Bank Holiday lived up to its reputation.  It rained, pretty much all day.  According to our rain gauge we had another centimeter and the roads were all flooded once more.  So, folks, that it is for Gorhambury for the next 11 months.  Plenty to do, however, with data processing, analysis and interpretation.

Today Ellen and I went back to site to collect the last remaining pegs.  Ellen organised us into walking back and forth across the field systematically to see if we could find my missing survey book.  On our second pass…

A very damp notebook!

A very damp notebook!

It was very soggy, but thanks to being a “rite-in-the-rain” product it looks OK and is now drying in the airing cupboard.  Yay!  Inevitably, as we were walking around picking up pegs and finding the book, it started to rain and we were soaked by the time we left.

On Sunday, I had some fun making timelapse videos.  Here is a condensed version of surveying three squares.

I did a poor job of setting it up for the GPR, but here it is anyway:

I have quickly processed the GPR data from using the large 160mhz antenna. We surveyed a block 20x40m which went over the “sinuous ditch” in the hopes we might be able to see it more clearly. We have picked up the walls that we saw with the 450mhz antenna.  In the radargrams (the vertical slices which is how the data is collected) I think I can see the ditch… but I need to check with someone more experienced at reading them.  Here are the time slices.  Note that we used a 1m spacing between lines which leads to a cruder picture.

Time slices from a 20x40m block at 1m spacing using the 160mhz antenna.

Time slices from a 20x40m block at 1m spacing using the 160mhz antenna.

I thought it would be useful to show one radargram from the day 15/16 block.  This is line 1893 which was surveyed from north to south, i.e., from the top of a block (as I usually present them) to the bottom.  Here is a screen grab from “radexplorer” showing unprocessed data.

An unprocessed radargram of line 1893.

An unprocessed radargram of line 1893.

It is very difficult to see much.  This is because most of the signal is the loud “noise” near the surface.  The first thing to do is tell the software where ground level is, i.e., the start of the first ‘loud’ reflection shown as black band at the top of the radargram.  Then we need to apply ‘gain’.  This is simply amplifying the lower reflections which are much weaker than the ones near the surface.

Line 1893 after the ground level has been input and gaining (using the autogain function in radexplorer).

Line 1893 after the ground level has been input and gaining (using the autogain function in radexplorer).

We can see much more now, but there is still a great deal of banding.  The bands are, essentially, echos and can be removed with the “background removal” function.

The processed radargram of line 1893.

The processed radargram of line 1893.

There is a great deal going on in this line of data.  Note, however, how the vertical column of echos under where I have put the label “wall” starts down into the radargram where as the column of echos where I have put “ring” starts at the top.  The “ring” is a small incipient mushroom ring which I noticed as we pushed the GPR over it.

A small incipient mushroom ring.

A small incipient mushroom ring.

If we look at the upper time slices we can see the ring:

A time slice from near the surface showing line 1893. The dark blob near the centre of the line is the mushroom ring.

A time slice from near the surface showing line 1893. The dark blob near the centre of the line is the mushroom ring.

For the wall, however, we need to look at a deeper slice.

Slice 3, 14-18ns.

Slice 3, 14-18ns.

In this slice we can see a small rectangular building showing faintly in the data. This building explains some of the things we can see in the radargram.  Remember that north is to the left of this radargram.

The arc we can see at the bottom of the top slice starting at about 27m west and going to 34m west is the northern edge of a very big mushroom ring.

A large mushroom ring.

A large mushroom ring.

The interaction between the fungus and the grass is quite complex.  The rings are showing in the radargrams probably because they are retaining water.  This can make the grass grow lush, or in extreme circumstances kill the grass.

A mushroom ring killing the grass.

A mushroom ring killing the grass.

Some of the rings merge and become quite complex.

A complex mushroom ring.

A complex mushroom ring.

And some of them cause mad photographers to get to their knees.

yummy circle

Yet another mushroom ring.

Yes, I became a bit obsessed.

Although we have finished for this season, there are lots of other things on the horizon, as well as working through all this great data.

I know I have said it before, but it is worth saying again.  Many thanks to everyone who contributed.  We collected great data, and we have found some really intriguing things.

In the frozen north?

A short while ago we were in the bottom pointy bit of the theatre field. We are now in the top pointy bit! We have surveyed with the mag a substantial proportion of the field. A quick check with QGIS shows that we have surveyed 17.3 hectares of a 27.3 hectare field.  Good job everyone.

To start, let us see the overall progress.

The area surveyed after day 18.

The area surveyed after day 18.

The northern area has some very interesting features.

Detail of the northern area.

Detail of the northern area.

The sinuous ditch continues westwards up the dry valley towards the wall.  A basic aquaduct seems a good possibility.  Less Pont du Gard and more muddy ditch, as aquaducts in Britannia tended to be.  There are also quite a few small and quite faint ditches which look like small field boundaries.  Perhaps evidence of agriculture outside the town when the ‘1955 ditch’ was the boundary and before the wall was built?

There is a nice find in the topmost area.

A building parallel to the road just inside the Chester gate.

A building parallel to the road just inside the Chester gate.

A rectangular building can be seen a little back from the line of Watling Street just inside the Chester Gate.  Snack bar or tax office?  This will be a nice building to target with the GPR… next year!

On the subject of GPR, the crew managed a 20×40 block with the large 160mhz antenna, and two 40x40m  blocks with the usual 450mhz antenna.  Here are the time slices from the two blocks with the 450.

The more southerly block of GPR data. The slices are 8-12, 12-16, 16-20 and 20-24ns.

The more southerly block of GPR data. The slices are 8-12, 12-16, 16-20 and 20-24ns.

The more northerly block of GPR data. The slices are 8-12, 12-16, 16-20 and 20-24ns.

The more northerly block of GPR data. The slices are 8-12, 12-16, 16-20 and 20-24ns.

An excellent result showing a whole series of buildings.  Joining up the 16-20ns slices in Google Earth we can see how they relate to the magnetic data.

The day 18 GPR blocks overlaid on the magnetic data.

The day 18 GPR blocks overlaid on the magnetic data.

The same area as the previous image for comparison.

The same area as the previous image for comparison.

And finally for this evening, the context of those GPR blocks.

The GPR results around the day 18 data.

The GPR results around the day 18 data.

Quite an impressive series of buildings in the heart of the town.  Clearly, expanding the GPR survey next year will be well worth the effort.

The weather for tomorrow, our final day, is somewhat uncertain.  We will see…

 

The antepenultimate day

“Antepenultimate” is my second favourite word. At least I get more use of it than I do of my favourite word, “defenestrate”. I used to defenestrate my Christmas tree until I moved and now I can only deport it. Anyway, enough of this nonsense.

Today the mag re-did a few problematic blocks of data before moving on the survey a batch of new grids.  Here is the overall plan.

The total area surveyed after day 17.

The total area surveyed after day 17.

I think we should all be very pleased with the progress we have made.  Looking at the new area in more detail.

Detail of the area surveyed on day 17.

Detail of the area surveyed on day 17.

The large ditch which runs from the SW to the NE of the westernmost part of the survey is the return leg of our old friend, the 1955 ditch.  It looks like the northern part of this feature has been substantially backfilled with non-magnetic material as it is only visible faintly.  The sinuous ditch clearly crosses the 1955 ditch  and heads west up the shallow valley.  At least we can be sure now that the sinuous ditch post-dates the 1955 ditch.  Part of the western part of the sinuous ditch was found in the 2000 English Heritage survey.  The line of pits which appears to join to the thinner NW-SE ditch could be all one long linear feature.  This area may not have the big stone buildings found in the central part of the town, but it still asks a great many questions.

The GPR has now covered quite a large area.

All areas surveyed using the GPR overlain on the mag data.

All areas surveyed using the GPR overlain on the mag data.

Today they covered another 80x40m block to the south of the “uber magnetic” building.

The GPR block surveyed on day 17.

The GPR block surveyed on day 17.

The same area for comparison to the previous image.

The same area for comparison to the previous image.

We picked up quite a bit more of the building which was in the SE corner of yesterday’s block, as well as this missing corner of the corridor building to the SE.  The road does show, but not very clearly in the time slices.  There is also quite a solid area in the western part of today’s block; it maybe another building.

Tomorrow may be the last day if the weather forecast is accurate.  Fingers crossed that it isn’t!

 

At 1pm precisely…

The Met Office predicted rain at 1pm, and at 1pm it began to rain.  We had, however, managed to do four mag grids before lunch, and finished off another one and a res grid in the drizzle.  Many thanks to everyone who turned out on a unpromising morning and helped us complete a fair bit in the time we had.

The bad news first.  The Chess Valley Archaeological and Historical Society were kind enough to lend me their resistance meter and cable to try and work out what is wrong with ours.  Unfortunately, it looks like it might be the wiring on the resistance frame as the results we got today were utter nonsense.  Many thanks for John Glover for lending me the equipment and Jim West for bringing it over.

The five mag grids were, however, extremely interesting.  First the overall image:

The mag survey after Day 14.

The mag survey after Day 14.

Weather permitting, we should be up by the drive by the end of tomorrow, and hopefully will have picked up more of the ‘sinuous ditch.’ Zooming into the area we did today reveals some interesting features.

Detail showing the area surveyed on day 14.

Detail showing the area surveyed on day 14.

An annotated version of the previous image.

An annotated version of the previous image.

We have picked up the cross-roads between streets 11 and 25 very nicely.  Along street 25 there seems to be a line of smaller buildings.  More modest dwellings, perhaps, or maybe workshops and shops?  On the north corner, however, is a weird looking and extremely magnetic feature, probably a building as it seems too rectangular to be something else, and is approximately 20m by 10m in size.  Perhaps the building burnt down, or maybe it is an industrial feature?

Tomorrow we should be able to survey the grid next to our first “uber magnetic” building as well as picking up, we hope, some more of the infamous sinuous ditch.

We have also made the news on the Institute of Archaeology‘s website!

Rain stops play

Yesterday we had warning that it was going to rain in the early afternoon, but we went out anyway.  We managed a 40x40m block of GPR data and all the annoying partials around the end of the hedgerow with the magnetometer.  Then, in the early afternoon, it started to drizzle.  Luckily we decided to head home as later it really started to rain.  Today has also been washed out.  We’ve had 31mm of rain in the last 24 hours.

The break has, however, enabled me to catch up on some data processing, especially with the GPR.  Firstly, we can see how much we have done so far.

The magnetometry survey after day 13.

The magnetometry survey after day 13.

As can be seen, we have already covered a good sized area of the theatre field.  Still quite a bit to do, however, and the weather forecast is looking a little grim.

Zooming into the area we are working on at the moment…

Mag survey near the hedgerow.

Mag survey near the hedgerow.

We have a part of the SW-NE road (street 25 in Niblett and Thompson 2005) and buildings alongside it.  It looks like we have buildings around the outside edges of Insula XXX and a quite open area in the middle.

The rain has given me a bit of time to work on the GPR results.  Here is the overall plot. Please remember this is a simple mash-up in Google Earth and that I should be able to get much better plots eventually.

All the GPR results up to day 13 plotted together in Google Earth.

All the GPR results up to day 13 plotted together in Google Earth.

The same area as before without the GPR data.

The same area as before without the GPR data.

If you compare the mag and GPR data you can see that some features that look like walls in the mag data show clearly in the GPR data, but some show as faint negative (light) areas. This is probably the difference between buildings which have had their footings robbed and those where they survive.  The temple does not show very clearly in either data set.  It seems likely it was badly robbed.

The area surveyed on days 12 and 13 is especially interesting.

The block of GPR data collected on days 12 and 13.

The block of GPR data collected on days 12 and 13.

The same area with just the mag image.

The same area with just the mag image.

The corridor building shows very well, but the sinuous ditch is conspicuous by its absence.

Hopefully the rain will let up long enough for us to manage a little more survey tomorrow.