Saturday, 12 September 2015

Transient Electromagnetics Survey - the final push! Days 7th, 8th & 9th September

So our final survey is also geophysics. Transient Electromagnetics or TEM allows for deeper penetration and therefore deeper imaging of the sub-surface, approximately 150m. 

TEM can take quite a while to set up, but the survey is generally quite quick to run. The aim was to use the GPR to guide the laction slection for the TEM. This was possible on some occasions, but on others I had to use other information, hypotheses and inferences to guide location selection. 

The set up involves laying out a copper transmitter coil, with the aim of the larger the coil the greater the penetration depth. So as often as possible, the 100m x 100m coil was used. This had to be laid out in a square, which straight away on such terrain was challenging. Disappearing over a hill so the start point is out of sight often led to diamond shaped areas that had to be corrected as best as possible. Laying this out took four os us. Yes there are three of us, so we had a huge amount of help from our driver. 

Me sat at the receiver computer with the receiver coil in the foreground. It reminds me a little of the globe drinks cabinets popular in the 80's.

Once we were happy with the coil layout, we then had to layout and connect the transmitter, receiver coil and the receiver computer along with a preamp. Everything needs protecting form the sun and moisture, hence umbrellas and plastic bags! Not very scientific.

TEM is used to record the electrical resistivity of the sub-surface. As lithologies change with depth, so does the resistivity. If there is a fault, the offset should be picked up based on the offset of the resistivity. 

The principle's of TEM are based on Faraday's law of induction and Lenz's Law. The copper transmitter coil is energized by a direct current which after a time appropriate for the coil size, is quickly cut off. A nearly identical current is induced in the sub-surface to preserve the magnetic field produced as a result of the original current, or eddy currents. Ohmic losses means that the induced surface currents dissipate causing changes in the magnetic field. These changes cause subsequent eddy currents. The net result is a downward and outward diffusion of currents in the sub-surface which resemble an expanding smoke ring. The currents produce a a magnetic field, the changes (flux) at the surface of this field is measured and the way the currents diffuse in the subsurface depends on the conductivity/resistivity of the sub-surface lithologies. 

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I hope that makes sense! 

I haven't seen any of the data produced by this survey, as it all needs processing with specialist inversion software when I return to the UK. 

Our final day with the geophysics equipment involved taking everything with us. Trying to squeeze it all in the car made for an interesting half hour. 

Our lunch joins the GPR antennas on the roof. We just about fit everything in. Talk about a logistical jigsaw puzzle!

The final day didn't go to plan as the transmitter didn't record a current when we first turned it on. We tried everything to eliminate the problem in the hope that we would be able to tweak the survey. Batteries changed, cables changed, coil sections checked, connections checked and even a different coil, but no luck. So after almost 4 hours of trying, we packed up with 2 TEM surveys not completed, one of which is potentially very important! Very frustrating, but what can you do?

Well that's it, all 5 surveys done in 5 weeks, with a few days off and a few big hiccups thrown in for good measure. 54 GPR seismic lines, 5 TEM surveys, 34 open extensional fractures measured, 7 fumaroles observed with 2 surveyed in detail, almost 320 CO2 efflux data sets and just shy of 700 soil gas samples collected, we are done. It is going to be a busy few months going through everything to interpret the meaning of it all. 

End of fieldwork treat, we are off on a Game DRive at Nakuru National Park. Wish us luck to catch the lions who are famous for being in the trees.

Lala Salama from Kenya!

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!

Wednesday, 2 September 2015

Direct Use projects and sundogs - 1st September

After a morning of GPR surveys (see a separate blog) and seeing the best sundog, it was time to visit the direct use projects taking place in the caldera.

Great example of a sundog

One of the many things that the people associated with the Menengai Geothermal Project are keen to achieve is more than electricity. The team are trying to find and develop as many direct use applications as possible. Last year when I visited ( Day 4) There was a direct use trial involving fish and plants.

This year whilst here, not only have they further developed some of their ideas but there was an official two day launch of some of the direct use projects, with further projects given the green light.

It is very exciting to visit these projects and see the passion in the team. The applications to you and me might seem comparatively small, but what these applications could mean to the local communities around the caldera and the potential positive impact to the lives of many, is huge.

There are four direct use schemes that have just started to run in the caldera, linked together in a cascade system. The schemes have been set up on the well pad of MW-03, which is currently venting, with power generation from this well at 2-3 MW.

A schematic of the cascade system

Tito, who is one of the leads on the projects first explained how the heat exchanger worked. Cold water from nearby tanks arrive at the well pad under gravity and is fed in to a large coil. This coil is surrounded by super-heated brine water that is rising from the well. The interface between the super-heated water and the coil is the point of heat transfer to the cold water. The cold water is heated to around 75°C before being piped to the direct use projects. Once the brine water from the well has been used, it is transferred to a pond, where there are plans being developed for further uses. Nothing is wasted.

The heat exchanger

The first project is The Dairy. Here the hot water is used to pasteurise the milk. The milk is inside a drum constantly being stirred. This milk is surrounded by a jacket that is filled with the water that has been heated by the heat exchange process. Once the milk reaches a temperature of 68°C, the hot water is removes from the jacket and replaced by cold water, the milk is constantly stirred throughout the entire process.

Tito talking to Mairi about The Dairy

Some of the hot water from The Dairy is then moved to the The Laundry and used to wash items in a washing machine and the heat is also used for drying. Vincent our driver volunteered his clothes to be the first washed in the 'Geothermal Laundry'.

The remaining water from The Dairy is transferred to an area where aquaculture is taking place. The water is still at a temperature of around 40°C and is mixed with cold water to lower the temperature to the optimum fish temperature of 29°C. This temperature along with feeding is carefully monitored, producing fish that grow at twice the rate of fish in 'normal' environments. I did ask if such rapid fish growth had any affects on the fish, but apparently not. It was compared to humans flourishing when they find themselves in an environment they love (our holidays on sunny beaches!).

Discussing the aquaculture geothermal project with Tito

As with last year, the water from the fish tanks is filtered away carefully to keep the tanks clean. The ammonia from the fish excrement is filtered out of the water so that once again it is clean. This fish water is then used to water over 876 tomato plants inside a geothermally heated greenhouse. The greenhouse is only heated at night, as the day time sun heats it during the day. It has been estimated that each of these tomato plants will have a yield of 30kg per season, with two seasons' per year. The ammonia is diluted and used to feed the plants as the plants will extract the nitrate from the ammonia.

In the geothermal greenhouses

These small projects are now up and running and it is hoped that local farmers will use The Dairy and that the community will benefit from the aquaculture and agriculture methods in the very near future.
The people who live in and around the caldera do not have much, but these projects will soon help them provide further for their families, improving the lives of many.