Feasibility investigation and design study of optical well logging methods for high temperature geothermal wells. Semiannual technical progress report, 30 September 1976-31 March 1977

Abstract

Measurement of the absorption-edge in the compound semiconductor GaAs shows that changes in temperature over a range from 200 to 500/sup 0/C produce a cut-off transmission wavelength approximately coincident with the optimum spectral transmission region of quartz optical fibers. Thin GaAs elements are thus shown to be suitable for use as passive optical temperature transducers in the logging of high temperature geothermal wells in conjunction with fiber-optic cables. Stress effects on the optical transmission properties of quartz fibers indicate increased attenuation amounting to approximately 5%, due to tensile stress of 90 ksi, or an elongation of 1%. Temperature effects on optical transmission were found to be negligible at 500/sup 0/C. Preliminary results of this study indicate that construction of a borehole geophysical cable based on single-fiber quartz waveguides is entirely feasible. Present upper temperature limits in successfully measuring certain geophysical parameters in boreholes can be extended appreciably by the use of such a cable in conjunction with passive optical transducers which require little or no down-hole electronics.