718 EXPLORATION GEOPHYSICS 



To obtain a mean datum point between shot-point and seismometer, 

 the elevation of the base of the low velocity layer to be used is : 



_ Eo + Eg Uq+Us /7o\ 



Eu-—^ 2 ^^^^ 



where Eq = elevation of shot-point above mean datum 

 Eg = elevation of seismometer above mean datum 



The problem of determining the travel-time in the low velocity layer 

 is one for which the refraction method is well adapted and for which the 

 reflection method is inapplicable. A method essentially the same as that 

 already described for a two-layer section, page 666 is used. However, the 

 velocity in the low velocity layer cannot be determined accurately from the 

 slope of the direct wave travel-time curve for short distances, because 

 the velocity in the low velocity layer is not constant in the vertical direction. 

 The most accurate direct determination of the average velocity of the 

 low velocity zone is obtained by so-called up-hole shooting: several light 

 shots are discharged at varying depths in a shot-hole and the first-break 

 times are recorded with the aid of a single seismometer placed at the shot- 

 point.* The depth versus first-break time curve will indicate the thickness 



of the low velocity zone and the travel- 

 time in this zone. (Figure 438.) (The 

 thickness of the low velocity zone may 

 also be obtained by electrical resistivity 

 measurements. The electrical method is 

 especially advantageous in that the thick- 

 ness can be determined accurately at 

 several points with relative rapidity.) 

 The use of this type of curve would 

 tJ — j„p,hof^f,ot dispose of the low velocity layer prob- 



FiG. 438.-LOW velocity layer diagram ^^m WCrC it not^ f Or the f aCt that the low 



from up-hole shooting. <„ = vertical velocity layer is scldom uniform, even 

 travel-time through low velocity layer; oygj- ^^e length of a Spread. A difference 



J7„ = thickness of low velocity layer. . , .... i • .1 ,-1 



of several milliseconds in the vertical 

 travel-time in the low velocity zone is sometimes present between seis- 

 mometers separated by only a few feet. It is impractical to drill holes 

 below every seismometer in order to secure complete low velocity zone 

 data and therefore some approximation must be made. As stated 

 in the last paragraph, the vertical travel-time but not the velocity in the 

 low velocity layer can be obtained accurately by the refraction method. 

 However, because the depth JJq of the low velocity layer and the 

 average vertical velocity F„ in this layer may both vary from point to 

 point along the surface, an estimate of the variation of each must 



* Up-hole shooting or coming up the hole at a new location is frequently employed 

 to determine the best depth for shooting. 



