SEISMIC METHODS 731 



approximately five miles is necessary to obtain sufficient data on penetra- 

 tion of 4,000 feet. 



Using the simple analytical velocity-depth function found by fitting the 

 travel-time curve to Equation 86, it is easy to compute data for depths 

 well below the deepest point at which refraction velocity information is 

 available. This requires the assumption that the linear increase of velocity 

 with depth gives the best available extrapolation of data to these depths. 

 In this case the computations of very deep reflections are to be regarded 

 as predictions subject to correction if a later measurement of velocities at 

 greater depths shows the extrapolation to be in error. 



The low velocity layer has a similar effect on both refraction and reflec- 

 tion data and the corrections are of the same nature, except that during 

 refraction shots the rays travel a much greater distance through this layer. 



Both the reflection and refraction methods give way to a far more 

 accurate method, i.e., to the direct measurement of seismic velocity by the 

 method designated as seismic velocity well surveying. 



Well Surveying Method 



In this method, a seismometer is lowered into the well by an insulated 

 conducting cable. f First arrival-times are measured for waves from shots 

 which are fired from a shot-point located at the surface and near the well. 

 This method utilizes direct waves and measures depths directly, thereby 

 securing accurate velocity data. 



The well seismometer is of special construction, being designed to 

 withstand the high pressures encountered at the bottom of deep wells. 

 Cylindrical and of small diameter, the seismometer is built to minimize the 

 risk that it will be mired in the settled mud at the bottom of the hole and 

 to permit entrance in narrow casing. The cable supporting the seismometer 

 must be strong, well insulated electrically, and accurately measurable in 

 length during operation. Fortunately, such cables are available in the 

 equipment of electrical logging firms. The length of cable under tension 

 is accurately known and a continuous reading of depth of seismometer 

 is supplied by the cable trucks. 



Data on the low velocity zone are obtained at the well by up-hole 

 shooting. A surface seismometer at the shot-point serves as a check on 

 depth of shot and a distant surface seismometer is used to check time-break 

 during well velocity determination. 



Figure 446 shows the results obtained on shooting two wells in the 

 San Joaquin Valley, California,* The interval velocity is decidedly erratic. 



t See also H. M. Rutherford, "Reflection Methods in Seismic Prospecting," A.I.M.E. Geo- 

 physical Prospecting, 1934, pp. 395-396. 



* These results cannot be taken as typical of the Valley. It will_ be noted also that 

 the velocity V = 6,000 + O.Sh which was attributed to many regions of California 

 differs from the values shown here. The variation of velocities between these two 

 wells only one mile apart indicates the need for frequent velocity adjustments, if the 

 seismic mapping is to represent a particular prospect accurately. 



