The reflections shown in the record indicate changes in structure, 

 lithology, density, porosity, and other properties which affect the 

 acoustic properties of the sediment or rock. However, a certain amount 

 of geologic reasoning and experience is involved in arriving at a valid 

 analysis of the seismic record. 



Some general statements regarding the geologic interpretation of a 

 record will be made, but for a comprehensive treatment of selected areas 

 off the California coast see, for example, Schlank (1968), Moore and 

 Curray (1962), Moore and Palmer (1967), or Moore (1960) . 



In inferring the rock type of a formation from the seismic record, 

 the internal structure of the subject formation is studied. Such cri- 

 teria as the roughness of the top surface, the relative reflectivity of 

 the top surface and bedding, and the spacial continuity and degree of 

 disturbance of the bedding are noted (Stride et al., 1969). As an 

 example, off the coast of California (Moore and Palmer, 1967), the reflec- 

 tions from Miocene volcanics are typically strong but show little coherent 

 layering. Dipping late-Tertiary sediments, by contrast, show strong 

 reflections from a series of very continuous bedding planes. Nonlithi- 

 fied Quaternary and recent sediments generally show poor internal 

 stratification. „ 



Occasionally, a sediment-bedrock contact, as between unconsolidated 

 sediments and shale, may show poor acoustic impedance contrast and, 

 therefore, generate a poor reflection. Also, within shale, the acoustic 

 impedance may be nearly constant or gradually changing such that the 

 internal structure is not easily identified. 



Although general statements as the above can be made, it is more 

 profitable to study typical annotated seismic reflection records as 

 presented by Schlank (1968) or Hoskins (1964). 



Besides information about the rock and sediment type present, the 

 seismic reflection record provides information about the structural 

 setting of an area. 



Marine depositional contacts appear smooth and nearly horizontal, 

 while erosional surfaces show irregular and ragged relief and may have 

 angular discontinuities with the underlying rock units. Folded and dis- 

 torted sediments and rock units are clearly evident, which may indicate 

 the presence of currently active stresses in the region. Slumping is 

 often seen near the base of steep marine slopes. 



Of particular engineering importance off the California coast is the 

 presence of small- and large-scale faults. Using the reflection record, 

 the location of the faults can be mapped and the regional or local stress 

 field interpreted (Ridlon, 1969). Additionally, the presence of active 

 faults would enter into the hydrodynamic and safety analysis of an 

 underwater structure (Wilson, 1969). 



