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Tuesday, 15 April 2014

Tasik Chini Fieldwork - Geophysics

Assoc. Prof. Dr. Shaharin, our Environmental Physics last semester, the nicest lecturer that we have ever met that he always treated us like his children, talking to us softly and kept reminding us to have breakfast before class, haha. Despite of his age, he still could going up and down the hill just like any of us, erm, maybe I should say, better than anyone of us. He is going to retire officially in 2015. We are going to miss him. *tears*

I don't really fond of Physics as I grow older (actually it's because all the Physics formulae accumulate until the point that I am lazy to memorise all of them, hahaha) but all the three Sciences are important in the environmental studies. The measurement of earth resistivity was the first session of fieldwork that my group did on the day we arrived at Chini Lake. I had not thought that Physics can be so cool when it is combined with Geology! Let's see what we did there.

Every material has their own resistivity towards electrical current depending on their characteristics. For example, metal has low resistivity, so it is very conductive towards electricity. On the other hand, sand has very high resistivity, so it is a very good insulator, as we know. So we filled up the sample cells with earth materials such as red soil, coarse sand, mineral water, lake water and stones respectively to measure their resistances using the Geohm meter which is a type of ohm meter. The difference between Geohm meter and normal ohm meter is, we have to adjust the meter on the Geohm meter until the pointer reaches zero. So the reading on the meter is the resistance of the substance contained in the sample cell. Its resistivity can be calculated using the formula.

The Geohm meter with the sample cell filled with red soil.
That was just a simple part of earth resistivity. The most interesting part was to profile the change in resistivity of the earth, which means we can know about what's going on underground without digging into it! The Terrameter will show the resistivity and induced polarisation (chargability) of a cross sectional area of the ground so that we can detech the availability of groundwater, the presence of certain metals and also groundwater contamination by analysing the data processed using computer. We covered a length of 80m straight line of the ground (the photos above showed only a small part of the set up, it actually involved a hill, woo~) using two bundle of multi-core cable with about 40 electrodes inserted beside the cable. Usually scientists will cover a length of at least 1km because the longer the cable, the deeper the ground that we can investigate (due to some theories that we yet to understand, hehe) It was tiring that we had to hit the electrode into the ground as the ground was quite hard. After connecting the electrode with the cable, we added a little bit of water at the ground where electrodes were inserted to increase the conductivity of electricity. When choosing the location to set the instrument up, we have to avoid places with rocks too as it would affect the resistivity measured.

The orange colour box was the Terrameter while the yellow colour axle was for keeping the 40m long multi-core cable.  
Jumper cable connecting the electrode inserted into the ground with the multi-core cable.
When the water content of the ground is high, the resistivity will be low as water increases the conductivity of the soil particles. Same goes to the salinity of the soil. The higher the salinity, the lower the resistivity of the ground. Thus, referring to this two characteristics, we can know whether there is ground water, or is the ground water being contaminated. It is true that if the ground has low resistivity, there is water. But if the resistivity level is lower than a standard, then probably the ground water there is being contaminated by salts. We can see the importance of Geophysics here for the environment. It is one of the ways we use to look for resources or pollution that we cannot see with our eyes. They are hidden, deep underground. Measuring the earth resistivity to do the profiling often takes whole day to be done.

The amazing tough book with touch screen. It has been used for at least 10 years for the Geophysics research.

P/S: Due to the battery problem, our group was unable to carry out the profiling of the change in earth resistivity. So I just explained how it works. There are much more to learn. This was simply the introduction for Geophysics - Earth Resistivity.

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