796 



EXPLORATION GEOPHYSICS 



Fig. 484. — Moving coil type geophones. (a) refraction type seismometer; 

 (b) well shooting seismometer constructed to withstand 10,000 psi for deep 

 wells; (c) reflection type seismometer. (Courtesy of Engineering Laborato- 

 ries, Inc.) 



Variable Reluctance Type Seismometers 



A variable reluctance type of seismometerf employing electromagnetic 

 damping is shown by Figure 485. The schematic diagram of the magnetic 



Fig. 486. — Schematic diagram of a variable re- 

 luctance type seismometer, (a) out-put terminals; 

 Fig. 485. — Variable-reluctance electromagnetically- (b) coils; (c) adjustable air gap; (d) working air 

 damped seismometer. See Figure 486. (Courtesy of gaps; (e) spring suspension; (I) inertia mass, 

 Geophysical Engineering Corporation.) which is also the permanent magnet. 



circuit and mechanical arrangement are given by Figure 486. | The problem 

 of obtaining good damping characteristics, calibrated period and stability 

 for frequencies in the range between 40 and 50 cycles per second was solved 

 in the case of this instrument design by introducing an adjustable internal 

 air gap (C). This adjustable air gap places a reluctance in series with the 



t T. p. Minton, "Geophysical Prospecting Apparatus," U. S. Patent 2,371,973, March 20, 1945. 

 t K. W. Raitt, "Transducing System," U. S. Patent 2,418,953, issued April 15, 1947. 



