Seismic Reflection - Chew Bahir

Return to Chew Bahir – the Seismic Reflection Story

Two years ago we (St Andrews, Dei Huws at Bangor and Tigistu Haile at Addis Ababa University) visited the Chew Bahir basin in order to acquire geophysical data in advance of the HSPDP (Hominin Sites and Paleolakes Drilling Project) drilling a hole to investigate the palaeo-climate of the last 500 thousand years.  The overall aim of this programme is to investigate palaeo-climate at key African Hominin sites. During the first geophysical survey we used electrical resistivity tomography near to where previous shallow (20m) boreholes had been drilled by the HSPDP group on the lake bed at Chew Bahir.  In addition to our shallow geophysical surveys Tullow Oil had also completed a programme of deeper hydrocarbon seismic reflection investigation.  The results of both these were used together with the shallow drilling to pick the location of the deeper borehole.  In 2014 this hole was drilled with much success and is now undergoing extensive investigation at a number of research institutes.

So what were we doing back in the basin? An important part of trying to understand the core sequence and determine the environmental change that it represents requires that we have confidence in how the core fits with the larger geological story.  This means a better knowledge of how to extrapolate the data away from the core.  This requires geophysical data and the best type for this is high resolution seismic reflection.  So this was what we went to do.

Our data was acquired with a 96 channel Geometrics GeodeSystem with a 40kg Propelled Energy Generator impact seismic source.  After a walkaway survey we decided to shoot the data with a 32m source offset and 4m geophone interval.  We shot the data with 72 fold coverage along two crossing reflection lines over the borehole site and an additional 3 refraction lines to evaluate near surface velocities.  The seismic crew consisted of the previous team joined by Dr Erica Galetti from Edinburgh University and Yemane Kelmework from Addis Ababa.  Tim Raub (St Andrews) also came along to add his geological savvy to the project and to look at a number of other geologically-relevant sites (see later blog on this for some stunning new sites!). The initial data looks very encouraging with numerous reflectors identified.  Processing will tell how good the final data will be and that will happen over the next few weeks.

Despite the relatively dry year with almost drought conditions we managed to grab the data just in advance of the little rains starting.  Downpours on the last morning flooding the local roads and turning the dried lake bed into a gloopy mass that was impossible to drive on. 

During the field work we camped at the village of Arbore and employed a fantastic cook from a nearby town.  The field work would not have gone so smoothly without her keeping us fantastically fed on traditional Ethiopian foods (injera, shiro, and of course some local goat) and also without the help of our two drivers, Yared and Solomon. 

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.