Chap. 11] GEOPHYSICAL WELL TESTING 867 



rapidly damped out than the longitudinal component which follows the 

 pipe, the ratio of the two components at different frequencies (tuned 

 amplifier) as a function of distance gives a clue to the location of the leak. 



D. Fluid-Level Measurements by Sound Reflection 



The depth to the fluid level in a deep well may be measured by recording 

 the reflection travel time of an acoustic wave. The wave may be initiated 

 by the release of compressed gas from a tank or by the firing of a cartridge. 

 In the latter case the higher frequencies are filtered out mechanically by a 

 tube which may be combined with a flame arrester.*^ The reflections 

 from the fluid level and other obstructions in the well (such as tubing 

 collars, tubing catcher, and the like) are picked up by a microphone, 

 stepped up by a selective amplifier provided with automatic volume con- 

 trol, and are recorded photographically, or by a pen and ink recorder. 

 The sound velocity is not constant under all conditions and depends on 

 the composition of the gas mixture in the hole. An incremental velocity 

 arises from the expansion of the gas used for initiating the impulse. The 

 record may be readily calibrated, however, (1) by an auxiliary (coiled) 

 tube of known length, (2) by the reflections from tubing collars of known 

 depth, or (3) by the reflection from the tubing catcher or other reflector 

 purposely placed at a known depth. 



E. High-Frequency Measurements in Open Holes 



In the earlier days of geophysical exploration, much thought was devoted 

 to the possibilities of high-frequency methods for determining the charac- 

 teristics of formations in an open hole. These ideas probably received 

 their impetus from the demand for some method of ascertaining wall thick- 

 ness in connection with the freezing method of shaft sinking. Small leaks 

 in the freezing pipes, spreading of the freezing pipes at the bottom, and 

 other causes leading to a break in the ice wall were known to produce 

 disastrous results. It was thought that the measurement of the damping 

 of an antenna lowered into the shaft would give good leak indications, 

 owing to the considerable difference in the conductivity of brine, and ice 

 or frozen ground. The method was then extended to measure antenna 

 capacity in open holes, with the object of determining the dielectric con- 

 stant of the surrounding formations. These procedures were covered by a 

 number of patents, since expired. Beyond a few brief references in the 

 literature^^ nothing further has been published about them. 



"J. J. Jakosky, Petrol. Tech., 2(2) (May, 1939). 



'« G. Leimbach, Phys. Zeit., 14, 447-457 (1913). H. Lowy, Phys. Zeit., 12, 1001- 

 1004 (1911); 20, 416-420 (1919). 



