GB1345848A - Nuclear magnetism logging - Google Patents

Abstract

1345848 Nuclear magnetic resonance borehole logging TEXACO DEVELOPMENT CORP 9 May 1972 21501/72 Heading G1N In nuclear magnetic resonance bore-hole logging, susceptible nuclei in earth material surrounding the borehole or in a sample contained in the logging sonde are polarized by a relatively strong D. C. magnetic field Ho transverse to the earth's magnetic field He, and subjected to an alternating magnetic field Hi transverse to both Ho and He, weak compared with He and of a frequency corresponding substantially to the Larmor frequency of the nuclei, Ho is terminated or substantially reduced to H'o adiabatically (i.e. in a time interval less than the thermal relaxation time of the nuclei but greater than the Larmor period) during or before the application of Hi, and Hi is terminated abruptly so that the polarized nuclei are caused to precess freely about the resultant of H'o and He, and a characteristic of the resulting free precession condition is detected. In one form, Fig.3, the down-hole sonde has an aluminium housing (20) containing an electromagnet comprising laminated cores 25a, 25b for Ho, an A. F. coil 22 for Hi a pick-up coil 23 at right angles to coil 22, and amplifiers 27, 29. The surface electronics comprise a polarizing pulse generator 33, responsive to a trigger generator 31, to control the generation of the D.C. polarizing field Ho from times 1 to 3, The collapse of Ho at time 3 is controlled by a shaping circuit including an integrator followed by a clipping circuit. Trigger generator 31 also controls a circuit 32 which triggers a C.R.O. 44 and sends a gating pulse to an amplifier 37 to admit a burst of ascillations from A. F. O. 38 to the coil 22 to generate Hi between times 2 and 5. The N. M. R. signal from susceptible nuclei in the earth material around the borehole detected by coil 23 is summed at 42 with a phase coherent reference signal and fed to C. R. O. 44 for visual display and to an analogue boxcar integrator 36 which receives variably delayed pulses at times 7 to 8 and 9 to 10. Integrator 36 sums the results of several measurement cycles to enhance the signal to noise ratio. To correct for base-line drift measurements in the intervals 9 to 10, which occur after the N.M.R. signal has decayed, are subtracted from those for intervals 7 to 8. Measurement of relaxation times T 2 *, relaxation time of nuclei in He including effect of earth field inhomogeneity, may be simply observed on the oscilloscope 44, or a series of measurements of the decaying M vector, Fig. 4c, may be made and plotted. T 2 , true relaxation time of nuclei in He, is measured by making a series of observations with different durations of Hi (interval 2 to 5 or T lock) and plotting the logarithm of the respective initial values of the detector signal (instant 6) against T lock, Fig. 4e. T 2 is the slope of the resulting straight line. Measurement of polarization times T 1 (high field) is measured by making a series of observations Figs. 5a-5h, (not shown) with different durations of polarizing field Ho (interval 1 to 3 or Tp) insufficient to fully polarize the nuclei and plotting the respective initial magnitudes of the detector signal against Tp. T 1 (low field), generally equal to T 2 , is measured by making a series of observations in which variable decay times are allowed after the collapse of Ho. Ho may be reduced to H'o Figs. 6a-6i, (not shown), then H 1 applied and then H'o reduced to zero at a variable time Tr after the collapse of Ho; or Ho may be first reduced to zero Figs. 6'a to 6'i (not shown), then increased to H'o after a variable time Tr, then Hi applied and then H' o reduced to zero. In either case, the respective initial magnitudes of the detector signal are plotted against Tr. Petroleum or mineral logging. For petroleum logging, the frequency of H 1 is the Larmor frequency of protons in the (about 2 kc/s) and Ho is reduced to zero at time 3. For fluorspar, phosphates, bauxite, borax and spodumene, the frequency of H 1 is the Larmor frequency of F<SP>19</SP>, P<SP>31</SP>, Al<SP>27</SP>, B<SP>11</SP> and Li<SP>7</SP> respectively in a field H'o, and Ho is reduced to H'o at time 3. Alternative forms of apparatus. One arrangement Fig.3a, (not shown) is similar to Fig.3 except that the mixer 42 and phase shifter 43 are omitted and a digital integrator (36a) is used having a separate channel for each half cycle of the N. M. R. signal (e.g. 400 channels for 100 msec sampling interval). In another arrangement Fig.7a, (not shown) the frequency of A.F.O. 38 is automatically controlled by a N. M. R. magnetometer in the sonde, preferably using the same species of nuclei (e.g. protons) as those under investigation, to take account of variations in He along the bore-hole. Magnetometer Fig. 9a, (not shown) is similar to Fig.3 except that a known sample (s) is inserted in the detector coil and a multichannel digital boxcar integrator (36a) is used. Beat frequency on the averaged decay envelope is used to derive measurement of He at the sonde.