Sunday, 16 February 2014

Kilwa Marine Geophysics Survey, 2014

I have just returned from Tanzania working with Dr Ted Pollard from the British Institute in Eastern Africa and Elgidius Ichumbaki from the University of Dar es Salaam. Ted (also an honorary researcher at St Andrews) has been researching coastal archaeological sites along the southern Tanzanian islands near World Heritage site of Kilwa for over 10years.  His studies have documented settlements and trade back to Middle Stone Age with extensive developments from the 14C and 15C based on gold trade  that originated in modern Zimbabwe. To date, most of his work has been focused around recording intertidal sites in relation to the changing geomorphology that will have dictated the land use.  He has shown how river channels and harbours have silted up, settlements abandoned both as a result of nature and from human pressure.  There is a fascinating story in this coast line and one that despite the obvious immediate differences (its hot and the water is warm) has many similarities to the work that many of us are conducting around the North Sea.  

Kilwa Fort, Kilwa World Heritage Site, Tanzania

So this year Ted wanted to investigate the sub-tidal part of the study area.  In order to do this I have brought out our SwathPlus 468kHz bathymetric sidescan.  This is the sonar that we have been using up in Orkney, Scotland (see earlier posts) to successfully map the submerged topography (the drowned landscape) but also using to map different types of biological habitat on the seafloor.  It was deployed with a TSS DMS205 motion reference unit, sound velocity probe, Vector Hemisphere GPS and Topcon RTK dGPS.  Unfortunately here I do not have the advantage of bringing a custom survey vessel so we are relying on the collaboration with the Tanzanian Antiquities Department.  Check the video clip to see how the survey went and get a feel for the place. The boat required a fair amount of impromptu rigging and a couple of visits to a local woodworking shop - here my demands on yokes to go around the keel of a boat were a welcome change from making bed frames I think.

Charles Okeny from BIEA surveying using the SwathPlus bathymetric sonar
Over the course of the two weeks we managed to get a fair amount of the inner harbours covered including some great detail around the old fort and palace. Water depths ranged down to 65m and from the mapping it appears that there are a number of submerged coral terraces plus some great sediment features. 

As with any geophysics, it is only as good as the ground truth and so there was nothing for it here but to jump into the water to get some.  Whereas I am usually a fan of putting down an ROV (remotely operated camera) there was just not the facilities to do this and so we hired kit from a local dive centre and had to do the honourable thing – diving around coral reefs can be a hardship, honest!

The final map of bathymetry showing a deep (>60m) channel with steep coral shelf breaks

Dr Ted Pollard 
The result of the day of targeted dives proved out many of the sonar targets and gave some valuable information on seafloor type.  It also gave us the first confirmation of archaeological significance with a large pottery scatter at one site.  Ted will now be following this up with careful analysis/identification of the pottery.

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