Friday, 6 November 2015

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. 

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