Sunday, 2 February 2014

Chew Bahir, ERT 2013

I am just about to leave for Tanzania to do some offshore archaeological surveying with Ted Pollard from BIEA (I will be blogging on this on my return) but I thought before I went I would post a short video that was compiled during a field campaign late last year to the Rift Valley in southern Ethiopia. 
I made this trip together with Dei Huws (Bangor Uni) and Tigistu Haile (Addis Ababa Uni) as part of a much larger, international project that is drilling a number of locations throughout East Africa that are linked to key Hominid sites.  The overall aim of this is to be able to provide dated information about environment and climate at key stages of hominin development and migration.

The Chew Bahir site is located in SW Ethiopia close to the boarder with Kenya.  The large valley generally drains to the south into Lake Stephanie but the northern part where we hope to drill our core is only flooded on an infrequent basis.  As with many of arid areas, when it rains it does so often causing drastic and violent floods from the surrounding uplands that wash coarse river gravels far out into the generally flat valley floor.  Since our drilling objective is to obtain as simple a geologic sequence as possible the first task is to locate an appropriate site that has not been affected by the alluvial fans on the valley side or any of the ancestral rivers that migrate across the valley bottom. 
So, in November we made a preliminary field visit to the site armed with magnetic and electrical resistivity tomography/imaging (ERT) equipment in order to find appropriate sites.  Our choice of locations for the geophysics was based on some regional background work on geophysical signatures and also on some local knowledge kindly shared by Tullow Oil Plc.

The site is quite special and like nowhere I have worked before as you can see from the video.  We managed a couple of sites during our survey, one in the centre of the basin, the preferred core location, and one over an alluvial fan sequence at the edge of the basin.  The ERT gave some great results despite the arid conditions mainly I think because of the relatively high conductivity of the lake sediments – likely a result of the periodic wetting and drying. The alluvial fan was readily recognised in the ERT sections at the basin edge but no signatures of this were recorded at the centre of the basin where a fairly uniform sequence was modelled.

The next stage from us it to return to do a seismic reflection survey in order to image the top 400m (the core depth) in much greater detail. 

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