SEISMIC METHODS 899 



assembly of geophones and cables held taut by two boats ready for the 

 shot. Some of the cable assemblies are designed for dragging on bottom, 

 and the geophones are mounted in watertight housings at fixed intervals 

 along the cable.f The geophone is usually mounted in a system of gimbals, 

 with brushes and slip rings for electrical connection, allowing the unit to 

 orient itself freely in a vertical position within the waterproof housing. 

 The cable is usually built in sections, with waterproof fittings for connect- 

 ing into the geophone housings. This construction allows relatively easy 

 replacement in case of damage to or failure of a section of cable. The cable 

 ordinarily consists of a steel center cable for strength, several pairs of 

 insulated electrical conductors, and a protective rubber or plastic insulating 

 covering. The entire assembly is quite heavy and bulky and is best handled 

 on a large power-driven reel. 



In a variation of this system, the geophone is mounted in or on a wood 

 or cork buoy,| and floats upward a short distance from the cable lying on 

 bottom. Electrical connection is made through a pigtail cable which enters 

 a waterproof fitting on the main cable. 



The bottom-dragging cable works best in relatively shallow water, and 

 over bottoms which are not too rocky. If the bottom is very rough with 

 numerous shell or rock outcrops, or if the water is deep, a floating geo- 

 phone cable system may be used to advantage. The floating-type cable 

 consists of a center steel cable, for strength, from which geophones are 

 suspended at intervals, and to which the insulated electrical conductors are 

 fastened. The entire system is suspended at intervals from floating rubber 

 bladders or metal can buoys. It is towed and kept in position by a boat at 

 each end. There is no limit to the depth of water in which the system may 

 be used, since it is completely floating. 



The geophones are mounted on paravanes or planing boards which 

 (1) keep the geophone in a vertical orientation, (2) serve as a baffle or 

 mechanical filter element in reducing noise, (3) keep the geophones sub- 

 merged and in a stable condition while being towed, and (4) provide 

 optimum coupling with the motion of the water by reducing the mean 

 density of the assembly to a value near that of water. The geophone 

 assemblies are suspended by rubber shock cords about ten feet below the 

 surface of the water. 



The recording equipment is usually installed in the boat which is used 

 to tow or drag the cable assembly. This equipment is generally the con- 

 ventional assembly of land seismograph instruments. Some minor precau- 

 tions may be necessary to avoid the efifects of dampness and salt-water 

 leakage. It is convenient to have the amplifiers provided with relays which 

 temporarily bypass low pass filters, in order that the very high frequency 

 water-transmitted "first breaks" can be sharply recorded. These data are 



t J. W. Flude, "Apparatus for Making Geophysical Surveys Under Water," U. S. Patent 

 2,423,591, July 8, 1947. 



t R. A. Peterson, "Submersible Seismometer System," U. S. Patent 2,449,085, Sept. 14, 1948. 



