Tag Archives: earth resistance survey

Back to the fire

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

Week three started well with all three machines collecting data.  The Earth Resistance survey was the poor cousin as regards person-power but Ellen and I, helped by Rhian, completed three grid squares after lunch.

Figure 1: Ellen and an earth resistance meter.

The grids are over the fascinating burnt building seen in the mag.  Figure 2 shows the mag data in this area.

Figure 2: the mag data in the area of the res survey.

The black line snaking across Figure 2 from top-left to bottom-right is the aqueduct.  The very bright black-and-white area in the NW corner of that figure is probably a burnt building which was never replaced in stone.  Figure 3 shows the Earth Resistance data.

Figure 2: the Earth Resistance data.

Figure 3 is a crude composite of the data collected in 2016, 2017 and 2018.  The three squares at the north edge are this year’s grids.  We have clearly picked-up a long wall running NW-SE, and some square areas of higher resistance (?floors, maybe).  This survey makes an interesting comparison to the GPR in this area (Figure 4).

Figure 4: the GPR survey in the same area as Figures 1 and 2.

There is a lot of work to do tracing off walls and features from the three surveys.

The GPR crew completed another 80x40m block, although the slope was quite a challenge.  Figure 5 shows the time slices.

Figure 5: day 11 GPR time slices.

Slices 4 and 5 (top-right and middle left) seem the most interesting.  No stunningly clear buildings but lots going on.  Figures 6 and 7 compare the fifth slice with the mag data.

Figure 6: Fifth time slice from day 11 (indicated by the purple line).

Figure 7: the mag data in the area of the day 11 GPR data (shown by the purple box).

Notice how the square of higher mag response shown in darker tones towards the bottom of the purple rectangle are an area of light “low reflections” in the GPR data.  It is possibly something like an earth floor?  Off the west corner of that square in the mag data is a lighter coloured line running to the SW which is matched by a black line of high reflections in the GPR data.  That is clearly a wall, probably made of flint. The very narrow section of the aqueduct which runs east-west across the plot shows very clearly in the GPR data whereas the broader sections do not.  Something odd happens with the aqueduct at the eastern edge of the GPR plot. A  great deal more to tease out.

One problem we have had this year is the sheep.  In general they keep away from us.  The main issue is that some of them think the flags are tasty… (Figure 8).

Figure 8: Tasty! A nibbled flag in Mobbs Hole with the mag crew in the background.

The mag crew consisting of Jim West, Peter Alley and Dave Minty had three annoying partials to do before marching eastwards across the field.  I’m afraid I have not finished processing those annoying squares but I have added in the complete ones to Figure 9 so you can see progress.

Figure 9: mag data in Mobbs Hole after day 11.

Unfortunately, today was a bust as it rained 8.5mm.  The forecast for tomorrow is looking good though.

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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.

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…

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.

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.