79 



miles west of San Diego. In this area the bottom is flat, hence there are no con- 

 fusing echoes from hills and valleys, such as are frequently observed in rough 

 topography. Two sub-bottom echoes, Bj and B2. are observed on the low fre- 

 quency channels, but not on the higher frequency channels. Evidently the echoes 

 are caused by a gradual change of elastic properties, rather than an abrupt 

 transition. Hence the long waves received by the low frequency channel are 

 strongly reflected while higher frequencies are not. This demonstrates that 

 frequency selectivity is useful for "tuning in" the important sub-bottom echoes. 



Even when bottom echoes are of the comparatively high quality illustrated 

 in Figure 17 they do not yield directly very much information about the nature of 

 the reflecting interfaces. However, when combined with refraction measure- 

 ments in the area, velocities can be measured, depths calculated and strata 

 roughly identified. Reflections may then be used to supply detailed information 

 about variation in depth to the important interfaces. 



DISCUSSION: Frank Press 



Dr. Raitt's complete discussion of seismic refraction measurements 

 requires very little additional comment. The instrumentation and techniques 

 developed at Scripps Oceanographic Institution closely parallels that evolved by 

 the Columbia University -- Woods Hole groups. It has been the experience of 

 both groups that an outstanding problem still remains to be solved -- the reduc- 

 tion of noise associated with the motion of the hydrophone through the water. 



Accumulation of seismic refraction data for the ocean floor is now pro- 

 ceeding at a rapid pace. Most of the measurements consist of isolated reversed 

 profiles or several profiles along a line. This reconnaissance technique serves 

 to point out the major features of suboceanic crustal structure. Thought should 

 now be given to the use of continuous profiling methods over distances of the 

 dimensions of ocean basins. Only in this way can the details of ocean bottom 

 structure be mapped. Somie of the most significant contributions using seismic 

 measurements will be made from investigations of the key structures of the 

 ocean floor -- continental margins, deep sea trenches, submarine mountain 

 areas. A major problenni facing these studies is the determination of a proper 

 topographic correction to be applied to refraction data obtained over rough bot- 

 tom. Certainly detailed fathograms and a multiplicity of shots in the first few 

 miles of the profile will be required. 



Continuous reflection shooting should be made part of every cruise since 

 negligible ships time is required and data essential to proper understanding of 

 refraction results and sedimentation studies will be obtained. Deep sea reflec- 

 tion techniques at this present stage of development are crude compared to 

 methods developed by petroleum geophysicists for use in shallow water. There 

 is room for much improvement here. 



A detailed evaluation of the British technique of making refraction meas- 

 urements from a single vessel using telemetering detectors should be made. 

 Although some of the obvious disadvantages such as limited profile length and 

 increased time are immediately evident, the tremendous saving in cost and the 

 more frequent availability of a single ship is a major factor in its favor as an 

 alternative method. 



One does not ordinarily think of a seismograph as an oceanographic in- 

 strument. Significant information has been obtained, however, by seismo- 

 graphs placed on oceanic islands. These observations supplement the point by 

 point seismic refraction measurements by data averaging conditions over long 



