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4 <br />1 will indicate what type <br />;slum ratio can also be <br />uranium-[o-potassium <br />~tial ofshale can thus be <br />.,S <br />ncident GR is greater <br />.spectrum, and this is <br />ated to the number of <br />peci(~c gravity of the <br />4 <br />4.3.1. Density log <br />NUCLEAR WELL LOGGING <br />157 <br />As shown in Fig. 4.4b the wall o(the borehole is irradiated with a GR source. A counter <br />mounted on the same pad and protected from the direct radiation of the source picks up <br />the gamma rays diffused by the formation toward the borehole. The signal observed is <br />related [o the density of the formation since it represents the number of gamma rays <br />having undergone Corrtp[on diffusion as de(tned above. <br />Pad <br />iper arm <br />Fig. 4A. -Density sonde. <br />(Scldunrberger documenQ <br />b. Principle. <br />So as to correct for the effects of the mud cake, because this log elves goad results only in <br />an open hole, a second nearer counter is added on. [n this way a density log is obtained <br />directly and is sometimes given in terms of porosity on the log by applying the following <br />equation: <br /> <br />where <br />Sys. This absorption pq is the bulk density read on the loe, <br />:urn is linked to the pm, is the density of the grains of the matrix rock, <br />for another log, the p f is the density of the interstitial fluid, generally mud (dtrate, <br />d5 is the porosity. <br />(4.3) <br />.em mm <br />N(%) <br />a. Device. <br />