578 SEISMIC METHODS [Chap. 9 



so on. (6) Sui'veyor's notes, giving locations and elevations of shot holes 

 and receivers, direction of shooting, and the like, (c) Observer's notes, 

 consisting of a schedule of records taken, charges and shot depth, distances 

 of receivers, and data pertaining to instrument control and filter settings. 



Calculation forms used in the office for the evaluation of seismic records 

 vary greatly in arrangement, depending on company procedure and prefer- 

 ence. The form given in Table 58 will be found to be flexible enough to 

 cover most weathering and reflection procedures which are likely to be 

 appUed in pmctice. It contains, in the upper portion, general data on 

 location of shot point and receivers. In the lower part follows a form for 

 calculation of weathering correction by the ABC method. Distances and 

 elevations of receivers are entered first. Of the three main columns, the 

 left pertains to the shot point and a location 200 feet away from it, which 

 is intended to give the depth of weathering Ws under the shot point. In 

 it are entered shot elevation Es , shot depth ds , vertical time U and time 

 to the 200-foot detector im corrected for elevation. Depth to weathering 

 at the shot point is obtained by adding the 200-foot time to the vertical 

 time, subtracting the horizontal time h , dividing by 2, and multipljdng 

 by the overburden velocity Vi calculated from shot depth and vertical 

 time (da/tv = Vi). 



The result will indicate whether the shot is located above or below the 

 weathered layer, and the correction ts = (Wg — ds)/vi is applied accord- 

 ingly. Added to it is a correction to datum td calculated with an (average) 

 velocity v^ of the high-speed bed under the weathered layer. In the mid- 

 dle row of colimms are entered the times ts from the reverse weathering 

 shot which are then corrected for shot depth da/vi . Next follow the 

 times Ia of the first breaks from the reflection record. They are added; 

 the time k to the sixth receiver (set out as sho^vn in Fig. 9-75) is sub- 

 tracted; the result (2^) is divided by 2; and weathering Wr under each 

 receiver is calculated by multiplication with the overburden velocity. As 

 shown in the form, it is usually sufficient to average the times and calculate 

 a mean weathering depth. Subtracting the weathering times tw from the 

 forward times Ia gives accurate times corresponding to the advance of the 

 wave in the high-speed bed underlying the weathered layer. From this, 

 an accurate value for its velocity V2 may be obtained. An average value 

 Vd (usually close to V2) determined from refraction profiles or well shooting 

 is used for the calculation of td in the first, and of U = (Er — Wr — D)/yd in 

 the last coluran for reduction to datum. The total correction (tr + fu. + 

 ta + td) is entered in the reflection calculation form and added to the 

 spread correction. 



In correlajtion shooting, times corresponding to the various reflections 

 and their phases are entered and corrected, and depths are calculated for 



