SEISMIC METHODS 851 



of the ground, their cases are usually designed with a flat extended bottom 

 and streamlined upper portion. The surface of the ground is usually scraped 

 to remove loose material, and the seismometer is then firmly seated or 

 pressed in place. 



If possible, the seismometers should be placed on firm ground and at a 

 reasonable distance from trees, telegraph poles, irrigating ditches, pipe lines, 

 concrete roads, and other structures that may transmit vibration to the 

 nearby earth. Field men in the vicinity of the seismometers should remain 

 motionless w^hile the record is being made. In w^indy weather, trees, wheat, 

 high grass, etc. often prove troublesome as they are a source of noise or 

 vibration. 



Extraneous vibrations are especially troublesome in the case of reflection 

 prospecting. The higher the noise level in any particular location, the 

 higher must be the energy level of the reflections in order to give a usable 

 reflection record. This necessitates larger charges of explosive than would 

 otherwise be necessary or even advisable. Conditions are comparable to 

 radio reception : the incoming signal must be of a higher energy level than 

 the static and local interference. Under conditions of high noise level, 

 increased amplification usually does more harm than good. In areas of 

 heavy traffic, or other disturbing factors, the operator must make his 

 records during periods of minimum disturbance. In some cases airplane 

 noise has proven to be a particularly disturbing factor. The operator should 

 never give the signal for firing if any of the galvanometer traces is unduly 

 disturbed or if interference is expected, as from approaching cars, trains, 

 airplanes, etc. Oftentimes, the difference between usable and useless 

 records depends upon the vigilance and ingenuity of the operator. 



Seismometer Spreads 



The character of the records and the nature of the information which 

 may be obtained is dependent to a large extent on the relative positions 

 of the seismometers and shot-point. The type, length, and orientation of the 

 seismometer spread are determined by such miscellaneous and often unre- 

 lated factors as magnitude and direction of strata dip, presence of particular 

 geological features, character of low velocity layer, accessibility, disturb- 

 ance energy, availability of permits, economic considerations, and degree of 

 required accuracy, i.e., whether reconnaissance or detail shooting. Further- 

 more, in reflection shooting, the type, length and orientation of the spread 

 are determined to a large extent by the quality of reflections, as shown by 

 sample records. 



General classes of individual seismometer spreads used in reflection 

 shooting include the split spread, the symmetrical offset spread, the uni- 

 directional spread, the two-hole spread, the ofifset uni-directional spread 

 and the right-angle spread. (Figure 533.) 



The split and uni-directional spreads indicated in this figure are used 

 particularly when, due to limitation of accessibility or simply to greater 



