High energy electromagnetic radiation sources are used in well logging for various applications, most principally for measuring the bulk density and lithology of earth formations. The current state of the commercial art in formation density logging tools is to use a radioactive (chemical) source, usually .sup.137 Cs, two gamma ray detectors, typically NaI, with suitable data processing circuitry and algorithms to derive mudcake and/or standoff-compensated density measurements. A photoelectric effect P.sub.e measurement (compensated or uncompensated) may also be made from the low energy part of the gamma ray energy spectrum from the density tool detectors, from which information of the lithology of aformation may be derived.
The presence of a radioactive source in such tools, however, gives rise to radiological safety hazards during use, transportation and storage of the tools. Also, the maximum energy and radiation fluxes attainable with radioactive sources are limited by the size and type of the source, which parameters are also affected by the aforementioned safety and handling considerations. Moreover, as radioactive sources emit photons continuously and isotropically, they are not readily usable for timed or focused measurements.
Efforts have been made to overcome the foregoing limitations of radioactive sources by using linear particle accelerators in well logging tools. Linear accelerators of the standing wave type are disclosed for this purpose in, for example, U.S. Pat. No. 3,976,879 to Turcotte, U.S. Pat. No. 4,093,854 to Turcotte et al., and U.S. Pat. No. 4,713,581 to Haimson. While such linear accelerators afford advantages relative to radioactive sources with respect to radiological safety, higher flux and energy outputs, and pulsed operation, they are comparatively expensive to manufacture and maintain. Their complexity and lack of reliability are also drawbacks.
The use of a betatron for borehole logging has also been proposed, at least theoretically. In a paper entitled "Compact Betratron for Petroleum Logging", Proceedings of the 7th International Conference on High-Power Particle Beams, Vol. 2, pp. 1485-90, 1988, Fisher et al. describe a type of betatron developed at the University of California, Irvine, which the authors assert could be sized for borehole use. Monte Carlo simulations of such a borehole-sized device indicate that it would compare favorably with the conventional cesium source for logging purposes. The UCI betatron, however, differs from the classical, circular betatron in that it is elongated, or stretched, in the axial direction and the charged particles move in helical, rather than circular, orbits. This device employs a torodial magnetic field in addition to the conventional betatron field to increase the circulating electron current. However, the elongated structure means that the magnetic field needs to fill a larger volume than does a conventional betatron of comparable energy. Thus, the excitation energy per pulse is higher and the repetition rate is lower than in circular induction betatrons; a disadvantage. Furthermore, the. elongated structure makes flux containment difficult in the borehole geometry.
In classical circular betatrons, focusing is typically achieved by using two opposed magnet poles to provide a magnetic field traversing the substantially circular electron orbit between the poles. This type of focusing is quite weak, and by itself does not permit sufficient electron charge to be trapped and accelerated to the full desired energy. Auxiliary focusing, while useful in surface betatrons, is not practical for borehole applications because of space limitations in the borehole.
Consequently, conventional circular betatrons have been either too bulky and inefficient or of too low electron current for use as a borehole photon source.
There is, therefore, a continuing need for particle accelerators which meet the constraints imposed by the hostile borehole environment, e.g., high temperature, restricted space, limited power supply, etc., while at the same time affording the desired photon output requirements in a low cost, reliable package.