Chap. 2] METHODS OF GEOPHYSICAL EXPLORATION 23 



parent velocity in the second part of the travel time curve, and the inter- 

 cept moves toward the shot point. If the profile is down dip, the apparent 

 velocity is less than the true velocity, and the intercept moves away from 

 the shot point. Dip and depth may be obtained from 2 profiles (perpen- 

 dicular to strike), one up and the other down the dip. To determine 

 strike and dip, it is necessary to shoot two up- and down-dip profiles at 

 right angles to each other. Interpretation of refraction data may be 

 simplified where considerable velocity contrasts exist, so that the rays 

 may be assumed to be perpendicular to the interfaces. This leads to 

 simplified field technique, widely used in mining exploration and weathered- 

 layer reflection correction, known as "method of differences." It in- 

 volves forward and reverse shooting of a refraction profile with one re- 

 ceptor set out at the end of the forward profile away from the rest of the 

 units. This location serves as the shot point for the reverse profile with 

 the other receptors in the same location. In most refraction problems 

 interpretation is based on the assumption of straight wave paths, that is, 

 uniform velocity within each medium. In areas with great thicknesses of 

 unconsolidated formations, a continuous depth-increase of velocity occurs 

 and manifests itself in a curved travel time diagram. In that case special 

 interpretation methods are used. 



Reflection methods. Reflection impulses, or "echoes," always appear 

 in a seismic record after the first arrivals. Since there is no way of dif- 

 ferentiating between a later refraction impulse and a reflection in a single 

 record a multiplicity of receivers is used in a number of shot distances. 

 In a six- or twelve-receiver record, reflection impulses stand out by their 

 almost simultaneous arrival (see Fig. 2-11). Important factors controlling 

 the appearance of reflections in a seismogram are the placement (depth) 

 of the charge and the distance between the shot point and receiver 

 locations. 



Depths are calculated from reflection records by timing the reflections 

 for a mean receptor distance, and multiplying the time by one-half of the 

 average velocity. This is true for nearly vertical incidence. For 

 greater distances a "spread correction" is applied. Although reflection 

 rays are curved, it is usually satisfactory to calculate depths on the basis 

 of straight ray propagation. If beds are dipping, at least two profiles 

 must be shot up and down dip. For the determination of dip and strike, 

 two profiles at an arbitrary angle with each other, shot up and down dip, 

 are required. 



Relative depth determination may be made by plotting travel time 

 only. For absolute depth determinations the average velocity must be 

 known. It may be determined by recording reflections from known 

 depths, by shooting in wells, or by surveying a long reflection profile at 



