IN BOREHOLES, BASED ON A FIBER-OPTICS CABLE

However, the results obtained in this way do not give a sufficiently complete representation of the properties of rocks in their natural occurrence. The reason is that when sampling and extracting a core sample, sludge, and fluid, the properties of the rock and the liquid saturating it are changing, part of the samples during drilling-out is destroyed and is not carried out to the surface. At the same time, drilling with core sampling is more complex and is more expensive than coreless drilling. One of the routes for increasing the efficiency of geological prospecting operations is the remote-controlled and high-speed determination of the properties and composition of rocks in their natural occurrence, i.e., directly in the boreholes, with depths amounting to 5-10 km or more, by nuclear geophysical methods [I]. For prospecting mineral deposits, first and foremost petroleum and gas, and for actively monitoring the exploitation of the deposits, methods are used based on measurements in the boreholes of the parameters of ionizing radiation fields, steady and nonsteady neutron fields, the y emission of natural radioactivity, inelastic scattering of fast neutrons, radiative capture of thermal neutrons, induced activity, and scattered y radiation. Gamma spectrometry, directed at the quantitative determination of the content of natural-radioact ive and principal-petrog enic elements, is of particular importance. The success in interpreting the results obtained depends on both the degree to which the effect of the borehole conditions is considered and on the degree to which the borehole measurements approximate the laboratory measurements. One of the problems of this approximation is the operative transmission of the measurement results, with minimum distortion and losses, from the detectors located in the borehole equipment to the surface recording equipment. For this it is necessary to remove the incompatibility between the volume of signals received and the transmitting capacity of the available borehole telemetry channels, in consequence of the extremely low pass bands of the communication lines -- the logging cables. For example, when recording the y emission of radiative capture in the case of the use of a neutron generator with an output of 107 neutrons/sec and a frequency generation of 400 Hz at recording time delays of 0 ~t ~200 ~sec, pulsed integral loads for a u quanta energy of more than I00 keV amount to not less than 105 pulses/sec, which requires a cable pass band of 1-2 MHz for scintillation spectrometry. The pass band for a 3-km cable with a fluorop!ast insulator amounts, in all, to about i0 kHz. The problem mentioned is very urgent for borehole geophysical instrument design, in view of the improvement of individual methods of measuring the fractions of ionizing radiations, and in view of the combination of these measurements with measurements from elastic oscillation, electromagnetism, and other fields, with a constantly increasing depth of the boreholes being investigated. It can be shown[2] that in the construction of borehole telemetry channels possessing the required transmission capacity, it is most advantageous to be directed to the use of linear telemetry channels using fiber light guides as the guiding structures for the electromagnetic signals.