 Welcome to this lecture 29 of ground water hydrology. The topics to be covered in this lecture are subsurface investigations of ground water. So, this is continued and within this we are geophysical methods and under this geophysical methods, the resistivity method then followed by spontaneous potential method followed by radiation method. So, these are all the methods of logging and under the radiation method, this natural gamma logging then gamma logging then neutron logging. So, these are the topics which we will be covering in today's lecture. So, we will go to, we will continue with this geophysical method which we are discussing in the previous lecture. So, here there is the geophysical log in a consolidated rock or formation. So, in the previous lecture we discussed the geophysical logging in an unconsolidated rock or formation and in this lecture we will consider the geophysical logging in a consolidated rock or formation and here we have a consolidated rock or formation with various, so on and various formations at different depths. Let us say number these formations as 1, 2, 3, 4, then may be say 5, 6, then 7, 4, 1, may be 2 and so here this 1 represents clay or clay stone number 2. Formation number 2 represents say marley limestone formation number 3. Let us say it represents limestone formation number 4 rock formation consolidated rock formation number 4 represent marl stone consolidated rock formation rock or formation number 5 represents sandstone the consolidated rock or formation number 6 represents sandstone with salt water say let us say this is a higher density or say dense sandstone and 7 let us say this is sandstone with salt water. So, let us say these are the rock the 7 formations in the consolidated rock and let us say the spontaneous potential is indicated in this green color. So, this is the spontaneous potential and here this is a positive on the left side and negative on the positive on the right side and negative on the left side and in this consolidated this one. So, it could this is a clay will have a very small positive this one or almost 0. Then it is followed by, so this marley limestone will be slightly higher then followed by limestone it will be even higher then followed by again marl stone. So, that will also have almost negligible spontaneous potential then the fifth one is sandstone. So, the sandstone will have some spontaneous potential then the sixth one is dense sandstone it will even have a higher spontaneous potential and the seventh one is this one with salt water. So, it will have a negative spontaneous potential then followed by 4 that is marl stone which is negligible spontaneous potential then followed by it is clay stone which also has negligible potential then lastly it is sandstone, but with salt water. So, again so that also has slightly negative, but negligible this is the second one is the marl limestone it will have a slightly this one. So, this is the variation this is the geophysical logging in terms of the spontaneous potential and similarly the geophysical logging in terms of the resistivity. So, this is here this case this is the increasing direction of. So, this is increasing direction and the resistivity for the this one that is clay or limestone is quite less then for marl limestone it is higher then it is the limestone it is even higher then the marl stone it is less then the fifth one is sandstone. So, the sandstone is higher resistivity and the sixth one is dense sandstone it will have slightly lower resistivity and seventh one is sandstone with salt water and that will have much less resistivity then the fourth one is marl stone that will have slightly more resistivity than sandstone with salt water then again this is clay stone with low resistivity then again this is a marl stone. So, like this so this is the resistivity log and then the third one let us say the same thing if we represent the gamma ray. So, in this case and again this gamma ray so it is the this is the increasing direction. So, in the first that is clay stone it has some gamma ray potential then for the marl limestone the gamma ray potential is less then this limestone it is even less then this marl stone it is slightly more than this the fifth one is this is a sandstone. So, it is less then the sixth one is also less then the seventh one. So, this is a seventh one is a it is a even less slightly less then four it will be slightly is one and again so this one and then two. So, this is the gamma ray logging so this is a geophysical logging in a consolidated rock or foundation. So, now we will specifically go to the resistivity method of logging and in this so basically so here this potential current and potential electrodes are lowered in an uncased well and to measure electrical resistivity of surrounding media thereby to obtain their variation with respect to depth and here. So, the multi electrode method is most the most commonly used method is the multi electrode method and in this so there are basically short normal long normal and so within this the short normal that is electrode arrangement then the long normal electrode arrangement and then the lateral. So, these are the electrode arrangement. So, let us briefly discuss the short normal electrode arrangement. So, in which so this is the short normal electrode arrangement. So, in this so this is a an uncased well and here one side is so this is the voltmeter and here so this is the point M and so this is the reference point and so this is the A and the distance between A and M the vertical distance between A and M is very small. So, and of course so this is a this is taken from the source that is keys and McCrary in 1971 and they have for the short normal. So, they have they adopted the distance of AM which is symmetrical with respect to this reference point as a 0.4 meter. So, that is M is 0.2 meter above this reference point this midpoint and A is 0.2 meter below this one and from A the other electrode is the current electrode is going and here so it is the current that is the emitter and there is an AC generator. So, this is AC generator and from the AC generator. So, there is another current electrode which is going point B and the distance between the vertical distance this between B and M is around 15 meters and so this is the short normal electrode arrangement and now we will go on to the another the second type of arrangement for the resistivity method of logging that is the long normal electrode arrangement. So, in this the so this is the uncased well this is the ground level and here the reference point. So, this is the reference point and this the electrode A which is a current electrode and in this case the earthing or the earthing is with the current electrode in the earlier case the earthing is with the potential electrode and in this case the earthing is with the current electrode and so here there is an AC generator. So, this is the AC generator and from the AC generator there is an arrangement to measure current and there is this grounding and this is the point B and here this the potential electrode. So, that is this is N and so here there is an arrangement to measure the voltage and this one so then the other electrode that is on the top side of this reference line. So, this is the or the this is the reference point and so the this is M and the distance this is M or M dash. So, this distance adopted is about this AM dash is 1.6 meters which is symmetrical with respect to this reference point and again this M dash N this is again 15 meters. So, this is also taken from the same source that is keys and McCrary 1971. So, the only difference between this and the short normal electrode arrangement is the distance is between the grounded current electrode which is below the reference point and the ungrounded potential electrode which is above the reference point at the same distance. So, this distance is much higher it is of the order of say 1.6 meters while in the previous case it was just 0.4 meters and now let us come to the lateral arrangement. So, this is the long normal electrode arrangement and now let us consider the lateral electrode arrangement again this is from the same source that is keys and McCrary and in this lateral arrangement. So, the distance AO so this is the uncased well and so here this is the reference point and that is denoted by the letter O and the electrode the potential electrode M and N are on either side of this reference point and there is voltmeter and so this is M I am sorry this is M is above this one and N is below the reference point and the distance is much less and in this case the current electrode A so this is below or rather above the potential electrode M and there is an arrangement to measure current and there is an AC generator so this is the AC generator and the other this current electrode is the topmost that is at point B. So, this distance AO so here remember none of the current electrode pairs or the potential electrode pairs are grounded so the distance AO is of the order of say 5.7 meters and again the distance between A and B is of the order of say 15 meters so this is the lateral electrode arrangement. So, basically with each of these arrangements so we can measure the resistivity and this resistivity is so in this case the resistivity log so that is so this is the depth and here this is the resistivity so this is in ohm meter and let us say this is 00 and this is 500 and then this is 1000 ohm meter and let us consider the normal so in this case let us see this is the normal the resistivity it may vary so this is the am and then B so this is the normal resistivity that is normal electrode arrangement on the other hand the lateral electrode arrangement so this is A and this is I am sorry this is O and A so this is the lateral electrode arrangement for this the resistivity may show a different slightly the staggered this one slightly the one with the this time lag so this is the lateral electrode arrangement shown in green color. So here actually so this the resistivity so this resistivity logs are used so this is a resistivity log to determine specific resistivities of strata and so this is though this specific resistivity the specific resistivity of an unconfined aquifer unconsolidated aquifer so depends upon porosity packing degree of saturation and temperature so the the fresh water will have the fresh water sand has moderate to high values and this one the shale clay salt water have low electrical resistivity fresh water sand has moderate to high values of electrical resistivity and this cemented sandstone and say non porous limestone have very high values of high values of electrical resistivity so this the electrical resistivity of ground water so depends upon ionic concentration and ionic mobility and ionic mobility of salt solutions so this mobility is related to the molecular the electrical molecular weight and electrical charge electric charge so here so this mobility of for example mobility of this sodium chloride solution is much higher mobility of this calcium carbonate solution and so as also as ground water temperature increases it has higher ionic mobility so therefore this electrical resistivities are electrical resistivity values are multiplied by correction factor to obtain the electrical resistivity at standard temperature of 25 degree Celsius so the most common application of electrical resistivity method electrical resistivity log is to obtain or to determine the proper place for fixing well screen for appropriate lengths or say optimum lengths and in this case so the there is what is called a field formation factor for appropriate lengths that is opposite the best formations so there is what is called the field formation factor so it is denoted by f of an aquifer so this f is determined as the ratio of rho o by rho w where this rho o is the electrical resistivity of saturated aquifer whereas rho w is the electrical resistivity of ground water in the aquifer and so this the another application long normal the electrical resistivity curve is to estimate permeability so in this case so depending upon the as previously mentioned the electrical resistivity very much depends upon the whether it is a fresh water or salt water so therefore we can determine the based on the electrical resistivity chart we can determine where the fresh water and is existing and where the salt water is existing so in this case say for example that is the let us say the hydrologic conditions and this resistivity curves electrical resistivity that is er curves for wells penetrating to aquifers so in this case so let us say there is an aquifer there is a well which penetrates a salt water aquifer below the fresh water aquifer so in this case this is the well so this is a fresh water aquifer above salt water aquifer so let us say this is the salt water aquifer so this is the salt water aquifer and this is the fresh water aquifer in this case so suppose this is the water level and so corresponding to this so this suppose this is the electrical resistivity and this is with respect to the water level and in this case say so the fresh water aquifer will have a higher and let us say so because of this pumping various this one the let us say the so there is so because of the pumping the salt water has come almost very close to the fresh water aquifer so in this case so we will see that the electrical resistivity in the fresh water aquifer is much higher and so the so here this is the so this electrical resistivity access 0 so this is saline water increased salinity and decreased electrical resistivity whereas in this case so this is so this is the this indicates a fresh water region so like that we can determine the based on the electrical resistivity log we can determine whether it is the ground water is fresh water or salt water so now we will go to the spontaneous method the spontaneous potential method so this is the method of logging so in the spontaneous method spontaneous potential method so the spontaneous potential so this is the natural electric potential found within earth so that is known as the the spontaneous potential so it is measured in millivolts and so millivolts from a potentiometer from a recording potentiometer connected to electrodes and here so again the same arrangement which used for the short normal and long normal arrangement can also be used to estimate the that is so the short normal and long normal short normal electrode arrangement potential electrode arrangement can be used to estimate the spontaneous potential so the short normal potential electrode arrangement say with grounding so it can be used to estimate the spontaneous potential and the spontaneous potential is there is an equation for the spontaneous potential so suppose if we denote the spontaneous potential as sp and the equation is sp is equal to minus 64.3 plus 0.239 I am sorry it is plus 0.239 T into log of rho f by rho w so this rho f is the drilling fluid resistivity electrical resistivity in ohm meter and this rho w is the ground water resistivity ground water electrical resistivity so that is also measured in ohm meter and T is the borehole temperature so this is the borehole temperature in degree Celsius so if we know these 3 parameters T rho f and rho w the spontaneous potential can be estimated so we will continue this in the next lecture we will move on to the radioactive logging as well as other methods of subsurface ground water investigation through geophysical logging thank you.