SEISMIC METHODS 853 



to place all the seismometers on one side of the shot-point for the first 

 spread and then move to the other side for the second spread — a single 

 equivalent long split spread will effectively be obtained. The reverse 

 of this procedure is sometimes used when a single symmetrical disposition 

 of seismometers at one shot-point serves also for shots from adjacent 

 shot-points, the process being repeated for each disposition of seis- 

 mometers. Here more than one set of dip data is obtained per seismometer 

 disposition, and both "close" and "open" spreads are supplied. (See 

 following section.) 



Relation Between Surface Waves and Spread. — The ground sur- 

 face waves usually have a velocity of about 1000 ft./sec* The rela- 

 tive magnitudes of the surface wave velocity and the wave velocities 

 of the deeper beds may be important in that they may govern the 

 choice between "close" and "open" spreads in reflection shooting. In the 

 "close" spread, seismometers are deployed near the shot-point. This 

 spread is used where depth of investigation is sufficient to allow the 

 reflections in question to arrive after cessation of the surface wave. In 

 the "open" spread, seismometers are deployed far from the shot-point 

 so that reflections are received prior to arrival of the surface wave. The 

 consideration of "close" and "open" spreads is required only when the 

 disturbance from the surface wave is prominent enough to require 

 attention. 



Field Methods for Improving Detection of Reflections 



Multiple Detection 



Where difficulty has been encountered in obtaining good reflections, it 

 has become almost a general custom to use several seismometers in series 

 or parallel instead of a single seismometer. This practice is called "multiple 

 detection. "f A group of seismometers which are connected in series feeds 

 a single channel of the recording equipment and is represented by one 

 recording trace. Usually there are two distinct advantages in such a 



* van den Bouwhuijsen has investigated the velocity of surface waves by placing 

 seismometers in very short spreads between 5 m. and 100 m. from the shot-point. He 

 found that the time-distance curve of these waves is not linear. The slope of the curve 

 at the origin corresponds closely to a velocity of 330 m./sec. The slope then decreases 

 and finally attains a limiting value denoting a constant velocity. The final value of the 

 apparent velocity in many regions is about 500 m./sec. It appears that the velocity 

 at the surface is equal to the velocity of sound in air and that in the near-surface 

 the air in the pore space plays a major part in the determination of the velocity of 

 the longitudinal waves, while with increasing depth and decreasing porosity the bulk 

 of the material which forms the upper layers becomes of greater importance in deter- 

 mining the velocity. In the experiments under discussion, where the top deposits 

 were alluvial quartz sands, a definite break in the time-distance curve was observed, 

 corresponding to a depth of about 10 m. The velocity of the second bed was of the 

 order of 1200 m./sec. 



t H. G. Taylor, "Method of Recording Seismic Waves," U. S. Patent 1,799,398, issued 

 April 7, 1931. 



