2. REFRACTION AND REFLECTION TECHNIQUES AND PROCEDURE 



G. G. Shor, Jr. 



1. Introduction 



In basic essentials, seismic refraction work at sea is quite simple and uniform 

 ill practice among the various institutions participating. One to four hydro- 

 I)hones (pressure detectors) are placed in the water below the depth of dis- 

 turbance by wave action, and charges of high ex])losive are fired by a moving 

 shi]) at varying ranges along a line radial to the hydrophone group. The shots 

 are normally fired by means of a time fuse at a sufficient depth such that a 

 gas bubble is created which oscillates at low frequency and does not blow out 

 to the water surface. The sound of the detonation is picked up on a towed or 

 hull-mounted detector on the firing ship and is used, with appropriate correc- 

 tions for distance of the shot from the detector, as the zero time for recording. 

 Small shots at close spacing are fired near the receiving hydrophones ; as 

 distance away increases, the charge size and distance between shots is rapidly 

 increased. 



Receiving hydrophones, generally high-sensitivity crystal or ceramic units, 

 are supported by floats in such a manner as to be either neutrally buoyant or 

 very slightly heavy, depending on the mode of handling. Signals from the 

 hydrophones are pre-amplified, transmitted to the recording equipment by 

 wire in the case of a two-ship operation or by radio on a one-ship operation, 

 and then separated into multiple-frequency bands, amplified and recorded on 

 an oscillograph as a "wiggly line" record. Additional information recorded on 

 the oscillograph includes the firing mark from the shooting ship, fork-controlled 

 timing lines and time-ticks from a break-circuit chronometer. 



When the details of field procedure are examined more closely than in the 

 brief summary above, variations in technique between institutions become 

 evident. Many of the variations have originated from the efforts of different 

 groups to solve the same problems independently, and when two equally 

 successful methods have been obtained they have continued to exist. In many 

 cases superior methods of operation originated at one institution have been 

 generally adopted by all. Some of the differences are explicable because of 

 differences in ship type or availability. 



Published information on instrumentation and techniques has been scattered 

 through articles on data analysis or ])ublished as technical reports. The principal 

 reference on methods and instrumentation for seismic refraction used at the 

 Scripps Institution of Oceanography (SIO) is by Raitt (1952) ; instrumentation 

 at Woods Hole Oceanographic Institution (WHOI) is described by Dow (1952) ; 

 instrumentation at Lamont Geological Observatory (LGO) by Sutton (1952) 

 and by Luskin and Koelsch (1958). A summary of field techniques presently 

 used at Lamont and Woods Hole appears in Officer et al. (1959); earlier in- 

 formation can be found in Ewing et al. (1937, 1939, 1940, 1946, 1950), Officer 

 et al. (1952) and Hersey et al. ( 1 952). ^Vork at Cambridge LTniversity is described 



[MS received Jidij, 1960] 2U 



