Thursday, 10 September 2015

Geophysics begins - the tough stuff! Days 30th and 31st August, 1st, 2nd, 3rd, 4th and 9th September

So it's time for the tough stuff, GNSS, GRP and TEM. What the devil are they I hear you ask? Well I shall try my best over the next few blogs, to take you through them.

First things first, the GNSS. The Global Navigation Satellite System is a constellation of satellites providing signals from space transmitting positioning and timing data, it provides global coverage and the in our case accuracy of 2mm.

First attempt at setting up the GNSS

As you can probably tell, the first attempt to set up took some time and completely baffled the memory afetr training took place about 5 weeks prior. This set up took about an hour and a half. The set up on the last day, took about 20 minutes!

It is very important when doing large surveys, to have as accurate a location as possible. The above image contains the base station, there is also a rover, which came everywhere with us and was even connected to the GPR (see below).

Beth (right) has the rover on her back in this image, though I appreciate it is not the best image (she is communicating with aliens apparently!) Mairi with the GPR computer

The GPR is Ground Penetrating Radar. It allows us to see shallow structures that in the case of Menengai, have been buried by the young lavas. GPR has never been done in this area, so we really didn't know what results we would get, if any at all. It's penetration depth is estimated to be around 10 meters for this kind of environment. 

We had with us 4 different antenna strengths, 25, 50, 100 and 250 MHz, the lower the frequency, the deeper the electrical signals will penetrate, or the greater the attenuation depth. But with increased depth, you sacrifice clarity of imaging. We used the 50's and 25's which surprisingly sometimes gave us penetration down to 16m and 32m respectively.

Survey team ready to go!

Construction of the 25MHz antenna

GPR survey with the 25MHz antenna. Beth leads controlling the pace size, I'm in the middle monitoring the image that is slowly growing on the computer screen and controlling the 'firing' of the , Mairi bringing up the rear checking how parallel the antennae are. 

For those of you reading this who are familiar with some geophysics field techniques will undoubtedly notice the fence next to us. Thankfully it was only for the first 80 meters of the section. But the importance of completing a comprehensive geophysical survey anywhere has been highlighted during the past 5 weeks. Menengai is well on the way to being a fully developed centre for power production. The pylon bases are cropping up everywhere and the huge steam pipes are being installed as well as the start of construction of the sub-station. 

Trying to find survey areas where the signals won't be affected by ringing caused by the presence of metal was a huge challenge. My very personal opinion of this locality is that not enough surface surveys were completed, surveys that include geophysics. 

Comprehensive and detailed surveys ranging fully from geophysics, gas sampling, fluid sampling, temperature surveys, geology mapping and cross sections and trial wells, among many many others should be completed. Some of these surveys were completed, but in the area of geophysics, the surveys were minimal. Yes, they are costly, but will save in the long term because money is not spent putting in deep, expensive wells that then don't discharge. 

So what is GPR?

GPR is a geophysical method that uses radar pulses to image the subsurface. It is a nondestructive method that uses electromagnetic radiation in UHF/VHF frequencies of the radio spectrum. The high frequency radio signals are transmitted in to the ground and the reflected signals are returned to the receiver and stored. The computer calculates the time taken for a pulse to travel to and from a target in the subsurface.

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So why are we doing this kind of survey?

The equipment is on loan from NERC Geophysical Equipment Facility in Edinburgh. 

The location of all the faults and fractures within the caldera are inferred at the very best, and even less is known about which are actively used for fluid and gas movement. The reason the locations are unknown is because young lavas from eruptions as recent as 200 years ago, have buried the surface traces of these structures. Knowing where these features are and which are active, is of vital importance when planning to utilize and develop power from geothermal sources. 

Menengai is now well developed as I say, but it has provided the opportunity to develop hypotheses after last years visit and develop methods by which to test these hypotheses, as well as test the equipment capability in such environments which will contribute to guiding future research techniques in the field. 

Mairi grabbing a much needed 40 winks after a 6am start to avoid the 2pm storms

So for now, Lala Salama from Kenya!

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