Geophysics at the Gorgan Wall, Iran

Hunting the Red Snake – part II

Back in Iran, this time with my brother, Martin, to continue the hunt for the remainder of the Gorgan Wall.  Last year I was here with marine geophysical equipment to try and find the remains of both the Gorgan Wall and the Tammisheh Wall as they enter the Caspian Sea as part of a project with the University of Edinburgh and the National Museum, Iran.  From a previous blog (GorganWall I) I showed the results for the Tammisheh Wall extending well over 2km from the southern shores of the Caspian into a large lagoon behind the Miyankale Peninsula.  The sidescan sonar survey and sub-bottom profiling showed the remains of the brick wall with its accompanying ditch/canal.  Unfortunately we so no manifestation of the Gorgan Wall extension – so back to try again.

The Gorgan wall projection to the west towards the Caspian Sea

The Gorgan Wall (the Red Snake) extends from the Alborz Mountains in the east across the Gorgan Plains and was built from the 4^th C AD as a defensive structure to keep out the marauding Turks from the north.  The wall was constructed of bricks fired from local clay in kilns spaced 40-80m apart along the length of the wall. It was at least 2m wide and tall expanding to a more substantial feature at numerous towers and where defensive forts occurred.  In front, to the north, of the wall was a 10-30m wide, up to 3m deep, canal.  The remains of the wall and canal are relatively well documented to the east. H,owever to the west, evidence of it remains elusive with historical writings describing the wall splitting into different parts and extending down to long-lost towns.

Despite being of similar age to the Tammisheh wall and being relatively close to each other the geomorphological setting for both is somewhat different.  The Tammisheh wall extends into the Caspian at a place where the sea is relatively sheltered with fairly benign sedimentation gently silting up the almost closed lagoon.  The projection of the Gorgan Wall however is across an area of coast where there have been highly dynamic changes due to the interplay between the Gorgan Ricer discharge and longshore drift bringing sediment onshore from the Caspian.  The present day shoreline demonstrates these active processes where the waves can reach impressive magnitudes due to a fetch stretching the length of the Caspian.

CMD Mini-explorer and Explorer ground conductivity meters

This year we have returned to attack the challenge of finding the remaining western part of the Gorgan wall as it progresses towards the Caspian Sea. We come armed with electromagnetic instruments, the GFInstruments CMD Explorer and Min-explorer.  Both are frequency domain ground conductivity meters, each with three coil spacings, in order to look to a range of depths.  We have a Leica dGPS for positioning control and for ground truth work a more traditional shovel and trowel.

The western limit of the wall is manifest by a series of “robber” pits that were used by locals to mine the decaying bricks for use in other projects.  The wall and remains of the canal that ran along the north side of the wall run in a straight course here for at least 20km.  Our first geophysical grids were laid over the robber pits and known wall/ditch locations in order to characterise their geophysical signature. The figure below shows this mapped on the satellite images.  Note that the background satellite images clearly define the agricultural system of 200x200m fields and also parts of the old natural landscape with its river channels and back barrier bars with sloughs.

EM Conductivity over robbed-out portion of wall

We continued to map the wall to the west and after approximately 1km a large modern drainage ditch was encountered and gave us the opportunity to ground truth the geophysics.  A 2D interpretation of the geophysics together with the surface mapping suggested that wall and ditch might exist here.  Excavation proved this to be true and samples have now been taken for analysis.

Using this characteristic signature mapping was continued a further 5km to the west with a complex pattern developing of linear features mixed with sinuous features.  Our current thinking has an interpretation based on a landscape evolution of shorelines behind which the old Gorgan River fights its way to the sea and through which the wall and canal/ditch is cut.  It is likely that the dynamic natural landscape was always an issue for maintaining the wall and canal here and a much larger geophysical and ground truth investigation is going to be necessary before a clearer picture of the history is revealed.

New geophysical results showing extension to the west from last known position

So, more geophysics to come hopefully in 2017. 

During the trip we also tried making maps with and AUV operated by Georgian colleagues.  While the purpose was to make maps along the wall projection we also took time to investigate some of the more interesting geological features.

Dormant mud volcano,Gorgan, Iran

This was way too short a trip as usual but a highly enjoyable one.  Our biggest issue this time was getting the equipment through customs – lessons for the next trip.  Our biggest hazard encountered (apart from the driving)  ........... watch out for those camels!

  

AUV hits Orkney

In the week of large scale remembrances of the Battle of Jutland that will include amassing parts of the British Naval Fleet, the German Fleet and possibly parts of the Danish fleet we made our own bit of history by testing for the first time one of the new generation of survey vehicles the Gavia AUV in the waters of Orkney.  The Gavia (Teledyn, Iceland) Autonomous Underwater Vehicle is a type of propelled platform designed to deploy geophysical instruments for survey beneath sea.  The vehicle looks like a small torpedo and in our configuration has onboard a complex navigation system, sidescan-bathymetric sonar and a high resolution camera.  The sonar is used to map the seafloor and to obtain backscatter images of the features on the seafloor.  The camera can likewise be used to identify seafloor features and to mosaic complete maps of what is there. There are two big advantages of using AUVs over conventional surface-based survey methods.  The first is that by flying an AUV beneath the surface it will not be affected by the noise and motion of waves and thus be able to “fly” in a steadier manner.  The second is that it can be pre-programmed for the survey and launched from either a boat or the shore.  With a forward looking sonar onbaord for collision avoidance the AUV does all the work while you sit back and have a cup of coffee – at least that is what should happen in theory!

The AUV (named “Freya”) was brought to the site by a team from the Scottish Association for Marine Science out of Dunstaffnage (SAMS) and was part of a small grant awarded to us by the Marine Alliance for Science and Technology Scotland (MASTS).  For John Howe, Karen Wilson and Colin Abernethy it was a first for not only visiting Orkney but also for using the AUV on an archaeological project.   Time was split between surveying the loch of Harray, near to the Ring of Brodgar and Ness of Brodgar archaeological sites and surveying in the Bay of Firth close to areas where we have previously mapped and reported unusual features on the sea floor.

Context

The Rising Tides project has been investigating the area around the Ring of Brodgar for a number of years.  Most recently we have completed a full bathymetry and sub-bottom sonar survey of the loch of Stenness to the west.  The purpose of this work was to reconstruct the palaeo-landscapes and environments when the Ring of Brodgar and Ness of Brodgar were being built and used. What our work demonstrates is that the loch was smaller at that time and further that there was a complex history of sealevel rise associated with the infilling of sediments with a step change in sediment input coincident with changes in vegetation in the catchment at about 6ka BP. For more details on this see our publication “The environmentalcontext of the Neolithic monuments on the Brodgar Isthmus, Mainland, Orkney”

Caroline and I have also been experimenting with digital reconstructions for the area that can be seen on our test site "Ancient Lands" 

AUV in the Loch of Harray

In order to match the work in the loch of Stenness we brought the AUV to the loch of Harray.  This loch is a shallow, freshwater loch to the east of the Brodgar isthmus.  Previous surveying with the sub-bottom profiler showed it to not only very shallow at the southern end (less than 4m water depth), but also that the sediments were gas saturated for the most part.  Because of the very shallow water the AUV was not able to “fly” beneath the water and had to be operated as a surface vehicle.  This, combined with the fresh water challenged both the vehicle and the operations team to come up with novel ways of ensuring the AUV did not either crash into the bottom or into any of the small fishing vessels on site! Unfortunately, the weather also proved problematic with strong winds.  The vehicle was able to survey a few lines of data but the automated tracking features did not work correctly and so the vehicle did not manage to stay on course in order to survey a complete grid of data.

AUV in Bay of Firth

Following the tests in Harray (also useful because it was a confined loch and so there was no chance of the AUV being lost out to sea!) we took Freya to the Bay of Firth.  Here she was much more at home back in saltwater and also with a greater depth range to work in (up to 9m water depth).  A survey grid was programmed in the centre of the bay over areas where we have previously recorded and dived on unusual stone features.  Freya was deployed from a small rib and perfectly performed her planned mission of bathymetry mapping, sidescan sonar imaging and photography.  The preliminary results, processed within minutes of return to shore (another huge advantage over old methods of survey) showed a resolution that we had not seen before.  Fantastic new images of the seafloor which we will now ponder over before our return to dive on them in August.  Look out for results in a later post.

The AUV opens up a new era for us with archaeological survey.  There are so many sites that could benefit from this type of survey and many new and exciting discoveries to be made.

The AUV team with Richard and Caroline in Orkney - check out the renewable power sources!

 PS – on a historic note, the Gavia is approximately half the size (half the length and diameter) of the original WW I torpedoes and weighs about the same as the explosive head that these carried

Recon on Lewis

Recon Survey to the Outer Hebrides

to see this in 3D go to GoogleSpheres here

Over the last 10yrs or so I have made a number of visits to the Outer Hebrides for both work and holidays. Most of the research trips have focused on marine biological habitat mapping and assessment of the impacts of storm activity on the coast lines. However, as with most of the Scottish landscape, the archaeology of the islands tells a compelling story, especially in comparison to the work we are currently doing up in Orkney.  So when Donald Herd, a colleague and native of Lewis, mentioned the possibility of a trip to visit his ancestral stomping grounds with the possibility of building collaborations for some future work on the islands I readily agreed to go even if it is sometimes dodgy weather in March!

Donald hails from the village of Suainebost in the far north of Lewis where there is a fantastic community historical society – Comunn Eachdraidh Nis.  This group is working to preserve island history, language and culture for both local community now and in the future as well as providing a resource for visitors to the area. This north part of the island has a diverse range of heritage on offer from some of our most ancient rocks in the Lewisian Gneiss to the heritage of the people. A great example of this is the recently completed investigations published in two books by Chris and Rachel Barrowman on the archaeology and heritage of Ness and an investigation on the unusual 16C-17C remains on the sea stack of Dun Eistean.  Hopefully we will be able to find funding to work on some of this in the future.  

The island of Lewis is justifiably famous for its archaeology, the centre piece of which are the iconic standing stones of Calanais (Callanish).  The best estimate of when the stones at Calanais were raised is about 3000BC, nearly at the time that farming began during the Neolithic here. However it is incredibly difficult to date the actual erection of stone circles, especially when almost no other structures, either houses or burial chambers have been discovered in the surrounding environment. We visited this site in order to check out the wider landscape as a potential place to conduct palaeo-landscape studies.  Since it was a quiet time of year with no visitors, Donald also took the opportunity to fly the site for aerial photography.  The results were quite stunning and fit nicely into the quick 360 image that we took at the centre.

We experimented with photographing a number of the stones as the structure displayed in the Lewisian Gneiss that the Neolithic people used is quite stunning!

As the day was such a good one we visited a much later broch on our way back to the north of the island.  Dun Carloway is a remarkably well preserved Iron Age broch (a type of fortification found throughout Scotland) likely constructed between 100BC to 100AD. It is of typical double wall construction which can be seen in the east side. The remains were in use, laterally as a kiln until at least 1000AD.  Folklaw has it that the clan Morrisons of Ness (Donald’s ancestors) used to hide in it while they went on cattle raids of their rival clan, the MacAulays! 

Now looking forward to a trip back, if for no other reason than to try some of the island's famous product - Black Pudding!  trouble is there are so many to try .......