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- SCNLUM6ERGFR LOG INTERPRET~jLON ! PRINCIPiFS •
<br />Ra}' Lo, diNerenr radinacriciry levels, the less dense forma-
<br />tions mill appear to be more radioactive.
<br />The Gamma Ra}' Log response, after mrrecrion for bore-
<br />hole, casing, etc., is proportional ro the u•eigbr rourerrh~a-
<br />tion of the radioactive material in the fortmtion. (It is
<br />assumed that the strength of the radioactis'e material, in
<br />gamma ray flux per gram, is constant. Also, it is assumed
<br />that the density variations are due ro porosit}' and ordinary
<br />litholoe}' changes, and not ro the presence of elements of
<br />high atomic numbers which would change the absorptive
<br />cha roc terisrics. )
<br />Consider a formation series containing chiefly one spe-
<br />cific radioactive mineral. The Gamma Ray Log reading at a
<br />given level will be:
<br />GR = ppVt A, (10.1)
<br />where p, is the densin of the radioactive mineral
<br />\'t is the bulk volume Gaaion of the mineral
<br />p,a,!p,, is the concentration by wefgbt of the min-
<br />eral
<br />At is a proportionality factor corresponding to
<br />the radioactivity of the mineul
<br />When the formation contains more than one radioactive
<br />mineral the Gamma Ray response will be the sum of several
<br />cerms like Eq. 10.1. For a formation containing rn~o radio-
<br />active minerals, having differen[ densities and strengths, and
<br />present indifferent amounts,
<br />GR = P~Vt A, + p•,Va Aa (10-Z)
<br />Pt, P..
<br />The Gamma Ray Log may be "normalized" by mulriply-
<br />ing it by p„ (from the Densin' Log). Thus Eq. 10-? becomes:
<br />pnxGR=B,Vt+H_Va (10-3)
<br />where B, = p,A, and B, = p:Aa
<br />In cases where the radioactive minerals derive their
<br />radioacriciq~ from their potassium content, the relative values
<br />of the B's could be computed from the densiry• of the mineral
<br />and proportion by weight of potassium in the mineral (de-
<br />termined from its chemical formula; see Ch. 1-iS t.
<br />~C'hen. as in the case where the radioactive mineral is
<br />shale, the density and radioacticiry are less well defined, the
<br />value of B may be determined empirically for the given geo~
<br />logical section and area. (Chapter 1.1G.)
<br />In sedimentar}~ formations, the average depth of pene-
<br />tration of gamma rays is abou[ one fool.
<br />EQUIPMENT
<br />The Gamma-Ray sonde contains a derecror ro measure
<br />the gamma radiation originating in the volume of the forma-
<br />tion near the sonde. Scintillation counters are now generally
<br />used ro measure radioacticiries in boreholes. They are much
<br />more efficient titan Geiger-Dfueller councers, which were
<br />previously used. Because their active length is only a few
<br />inches, scintillation counters give good formation detail.
<br />SB
<br />The Gamma Ray may also be run in combination with
<br />several other tools; e.g., Neutron, Sonic, Density, Induction
<br />Log, Laterolog, even with a casing collar locaror on mono-
<br />cable, or with a perforating gun.
<br />STATISTICAL VARIATIONS
<br />The number of gamma rays reaching the counrer flucm-
<br />ares ecen when the sonde is stationary in the hole; the phe-
<br />nomenon is snristicsl in nature. The fluctuations are more
<br />noticeable for lower count tares. However, the number of
<br />gamma rats counted per second oeer a sufficiently long
<br />period of time will be practically constant. The period of
<br />time required ro obtain a good average value is appreciable,
<br />usually a few seconds.
<br />In order w average our the statistical variations, con-
<br />denser-resistor smoothing circuits are used in the measurinG
<br />circuits. \'arlous "time constand' may be selected according
<br />ro the radioactivity level measured. The lover the counting
<br />rare, the longer the rime constant required for adequare aver-
<br />aging of the variations. In most cases, however, a rime con-
<br />stant of 2 seconds is sufficient.
<br />In the interpretation of Gamma Ray and other radio-
<br />acticiry curses, a bed boundary is picked at a point halfway
<br />between the mazintum and minimum deflections of the
<br />anomal}'. The recorded depth of this porn[ depends on log-
<br />ging speed and the rime constant; the faster the logging
<br />speed or the longer the time constant, the more the apparen[
<br />depth of the anomal}• is shifted in the direction the cool is
<br />moving. This lag is approximarely equal to the distance the
<br />counrer motes during one time constant, according to the
<br />relationship:
<br />lag (in fU =logging speed (in ft/sec) x TC (in sec )
<br />(10-4 >
<br />In order to avoid excessive disrortion of the curve, re-
<br />cording speed is chosen so that the lag is about one foot.
<br />Thus, for a rime constant of 2 seconds, logging speed is 0.5
<br />ft./sec or 1S00 fr/hr.
<br />The dynamic measure point of a Gamma Ray logging
<br />root is then taken as being locoed below the counter a dis-
<br />tance equal ro the lag. This places the midpoint of a bed-
<br />boundary anomaly at the correct depth on the log.
<br />CALIBRATION
<br />Gamma Ray logs are now usually calibrated in API
<br />units. The radioactivities obsen~ed in sedimentary formations
<br />range from a few API units in anhydrite or salt, to 200 or
<br />more in shales.
<br />Prior to the API calibration procedure, Schlum6erger
<br />Gamma Ra}' logs were scaled in micrograms of radium-
<br />equivalent per ton of formation."' Conaersions from these
<br />units to APl units are shown in Table ]0-1.
<br />The API calibration is based on the use of a permanent
<br />calibration facility'"' to establish standard units for nuclear
<br />logs.
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