808 EXPLORATION GEOPHYSICS 



Disadvantages of Mechanical Seismometers 



The mechanical type seismometer requires a solid foundation, such 

 as hard ground or a heavy wooden platform. This greatly handicaps 

 use of the instrument, particularly in the numerous areas where marsh 

 and open waters prevail. In addition, the single trace record necessitates 

 many instruments placed at different locations for each shot or else many 

 shots for a single instrument. 



RECORDING EQUIPMENT FOR VISUAL ANALYSIS 



The output of the seismometer, obtained by converting the energy of 

 the seismic wave into electrical energy, is amplified, filtered and then re- 

 corded by photographic means. In addition, it is usually necessary to 

 employ manual or automatic gain control to prevent overloading by the 

 stronger waves and to bring out the weaker waves. 



The minute currents generated by the dynamic action of the seismometer 

 are usually transmitted through an insulated two-wire cable to the recording 

 truck which houses the filtering, amplifying, and recording instruments. 



Filtering. — The seismic wave to be recorded is a complex wave con- 

 taining many different frequency components, each with varying ampli- 

 tudes, rates of attenuation, and phase relationships. The predominant fre- 

 quency range of the bands most useful in visual analysis is from 5 to 40 

 cycles per second for refracted waves, and from 20 to 100 cycles per second 

 for reflected waves. The quality of the record for visual interpretation 

 often may be improved by judicious selection of frequencies that are to 

 be recorded. As an illustration, the filters and overall response range for 

 reflection equipment are often chosen to cover the range from 40 to 75 cycles 

 per second. The lower cut-off is preferably quite sharp, while the upper 

 cut-off should be rather broad so as not to destroy the sharpness of the 

 reflections. Surface disturbances caused by light winds can often be largely 

 eliminated by attenuation of waves of frequencies above about 80 cycles per 

 second.f 



Filters are employed to obtain maximum current or voltage at certain 

 desirable frequencies, and to reject or suppress the effects of the undesirable 

 frequencies. Electrical literature contains many articles dealing with filter 

 design and tuned circuits, and the reader is referred to these articles. $ 



Amplification Control. — Following the strong burst of energy at the 

 first part of a field record, the level of reflected energy will diminish down 

 the record. The ratio of legible, reflected energy at the first and last part 



t E. M. Palmer, "Seismograph Recording Apparatus," U. S. Patent 2,440,970, May 4, 1948. 



W. W. Young, "Vibration Recorder." U. S. Patent 2.420,025, May 6, 1947. 

 t Reuben Lee, "Tuned Circuits and Filters," Industrial Electronics Reference Book, John 

 Wiley and Sons, New York. 1948. 



A typical band pass filter, which is capable of making a frequency selection in the amplifier, 

 is described by H. R. Prescott and F. L. Searcy, "Method of Geological Exploration," U. S. Patent 

 2,049,727, issued Aug. 4, 1936, and by H. R. Prescott, "Method and Apparatus for Making Geo- 

 physical Explorations," U. S. Patent 2,053,841, issued Sept. 8, 1936. 



