SEISMIC METHODS 903 



In some instances the electronic surveying gear is located on a surveying 

 boat which precedes the recording boat and marks locations in advance 

 with anchored balloons or other markers. In other cases the gear is 

 mounted on the recording boat, which navigates to predetermined sets of 

 radial or hyperbolic coordinate readings. This latter method eliminates the 

 need for the survey boat and balloon markers. A gyro-compass is valuable 

 for getting the proper bearing as well as location. The shot location with 

 respect to the geophone spread is often determined from the high-frequency 

 water-transmitted "first breaks." 



Agocsf has recommended a method for determining the lateral distance 

 from shot to recorders, the depth to bottom, and the time break on seismic 

 records obtained at sea, using direct "first arrivals" through the water, and 

 first and second reflections. The method was later extended to apply to 

 sloping sea bottoms.:]: 



Interpretation for Water Shooting. — Interpretation procedure and 

 problems in water shooting are quite similar to those in land shooting. 

 Reflection times are reduced to either the water surface elevation or a bot- 

 tom datum. Weathering corrections are often neglected, since it is usually 

 difficult and expensive in water operations to obtain accurate weathering 

 data. The use of "first break" times is complicated by the fact that energy 

 travels both through the water and through the bottom sediments. In cases 

 where these two arrival times can be separately determined, the water- 

 transmitted breaks can be used to determine relative distances of geophones, 

 and the bottom-transmitted breaks furnish information concerning relative 

 velocities in the bottom rocks. The latter in general carry much less high- 

 frequency energy than the former. By use of dual channel recording with 

 suitable frequency characteristics, the separation of the two events is 

 facilitated. High fidelity recording, with later frequency analysis, will 

 clearly show the two events. 



The air-water interface at the surface of the water acts as a nearly 

 perfect reflector of seismic waves. The water-bottom interface also in 

 many cases acts as a fair reflector. Particularly in water more than 200 

 or 300 feet deep, multiple reflections between the surface and bottom may 

 be prominent enough on the seismograms to obscure reflections from the 

 deeper reflecting horizons. The multiple reflections may usually be identi- 

 fied, as described on pages 891 to 895. In shallow water multiple reflections 

 are usually not bothersome. 



In areas of steep dips with rough or irregular bottom topography and 

 rock outcrops, a number of extraneous events with high time moveouts 

 may be observed on the seismograms. It is usually difficult to determine 

 the exact origin of these events, but they appear to represent waves travel- 

 ing horizontally along or near the water-bottom boundary, with a velocity 



t W. B. Agocs, "A method of determining the true break on deep sea seismic records from the 

 water sound arrivals," Gcot^hysics, Vol. IX, No. 2, April, 1944, pp. 163-174. 



t W. B. Agocs, "Sea bottom slope determination from water sound arrivals," Geophysics, Vol. 

 XIV, No. 2, April, 1949, pp. 123-132. 



