Question

2. 5 marks] List five strategies for dealing with non-uniqueness in geophysical inversions and give a specific example of each, for the case of a non-unique DC resistivity inversion in a groundwater survey.

Answer 1

3. In the IP or induced polarisation method, we try to induce an electric current in the ground by passing electricity through the ground using electrodes. This causes a potential difference in the ground which we measure and interpret. This potential difference is because of polarisation of the rocks. 2 main kinds of polarisation usually take place i.e. membrane polarisation and electrode polarisation. The basic phenomenon is that under normal conditions the pore water-rock interface is negatively charged and causes positive charge attraction. This positive charge attraction is fulfilled by electron transfer between the interface and the material in the pore space (either pore water or metals). In the case of metal ores, this electron supply is provided by them. And metals being very good electron donors the electron supply exceeds the demand and hence a charge build-up takes place in the ground which causes polarisation which we call electrode polarisation. Now in case of disseminated ores as the surface area is more, therefore, the polarisation is also more, hence we get a stronger signal. Therefore disseminated metal ores are easier to prospect and detect using the IP method as compared to the resistivity method which would yield a very weak signal as it measures bulk rock resistivity. But in case of massive ores, the signals in both the IP and resistivity method would be strong enough and will help in prospecting.

4. Different kinds of apparatus and settings used in resistivity, IP and SP methods are shown below: -
Ca) wenner Mehod → Equal electrode (paung b/w CU'rrenC4- voltage ele ctro e ou au V → Voltage mcaured → cwren't supplied

2L' a.

neme Dipo/e D.pol LO Schlumberger C p

CW SP meaquuement mehoob Ong enve ther

5. Carbonate-hosted Pb-Zn-Ag deposits usually form in the basin margin region in a reef type setting. These kinds of deposits are sometimes called Mississipi Valley Type deposits as they are exclusively found in carbonate rocks. Now, these deposits occur in clusters as they form simultaneously as a basin evolves and expands. The hot metal-rich brines which are fed by the underlying mineral rich magma being constantly exposed at the bottom of the basin as it is expanding are the main source of these deposits. So due to these factors, these deposits commonly occur in clusters and their distribution is fault controlled. So the IP method helps to detect and prospect these deposits easily as they are spread out and distributed in a large area in the form of clusters i.e. disseminated in nature. This nature of distribution gives rise to the electrode polarisation in the ground while doing an electric survey and hence their prospecting becomes easy.

6. If the pore waters in a disseminated lead sulfide ore were replaced by saline water, the frequency effect, as well as the metal factor, would decrease. The reason for this is as the salinity of the pore water increases the polarisation effect decreases as the charge transfer and electron exchange becomes easier. Due to this what happens is the difference of apparent resistivities in the frequency domain calculation i.e. in the numerator decreases, thereby decreasing Frequency effect and metal factor both.