II. EQUIPMENT AND FIELD AND LABORATORY PROCEDURES 



1. Geographic Positioning System . 



A radar-type electronic positioning system, the Motorola Mini-Ranger III, 

 was used to determine position of the research survey vessels during the 

 seismic survey (phase I) and the vibracoring (phase II) . The system detemnines 

 the position of the survey vessel with respect to two known reference points 

 on shore and is restricted to line-of-sight operation (the stated accuracy is 

 ±3 meters) . The basic system consists of a master mobile unit mounted aboard 

 the vessel and two shore-based transponders. The master unit triggers reply 

 pulses from the transponders; each transponder pulse is received separately and 

 the elapsed time between the transmitted pulse and the individual transponder 

 reply pulse is converted to a measurement of distance. Each distance (range) 

 from the two transponders at the known shore stations is displayed, in turn, on 

 the range console. This range information, together with the known locations 

 of the shore stations, is then trilaterated and plotted on hydrographic charts 

 to obtain the position (fix) of the survey vessel. Navigational fixes during 

 the seismic survey were obtained about every 2 minutes and each fix was keyed 

 to the seismic records by an event mark on the records. 



2. Seismic Reflection Profiling . 



Seismic reflection profiling is a technique widely used for delineating 

 geologic features such as bedding surfaces, faults, rock outcrops, channels, 

 and structures beneath the lake floor. Continuous reflections are obtained by 

 generating repetitive, high-energy, sound pulses near the water surface and at 

 the same time recording "echoes" from the lake floor-water interface and from 

 subbottom interfaces between acoustically dissimilar materials. This is done 

 while the survey vessel is moving. In general, the compositional and physical 

 properties (e.g., porosity, water content, relative density) which commonly 

 differentiate sediments and rocks also serve to produce acoustic contrasts which 

 show as dark lines on the seismic paper records. 



The seismic reflection data were obtained by towing sound-generating and 

 -receiving instruments behind the ODGS Research Vessel GS-1 (Fig. 9) which fol- 

 lowed predetermined survey tracklines (Figs. 2 to 8) . In phase I of this study, 

 two seismic subbottom profiling systems were used simultaneously. An Ocean 

 Research Equipment, Inc. (ORE) 3.5-kilohertz pinger system was used to gain high 

 resolution of the upper 10 meters of lake floor; an Edgerton, Gremerhausen and 

 Greer (EG&G) , Inc. UNIBOOM system operating on 300 joules of energy was used to 

 decipher geologic conditions to depths from to about 30 meters below the lake 

 bottom. Data from each system complement the other and were used to achieve 

 maximum understanding of the subbottom geologic character. A vertical scale on 

 the profiles was determined from a sound velocity of 1,550 meters per second in 

 water and 1,800 meters per second for typical sandy sediment. Actual velocities 

 will vary depending on sediment properties but 1,800 meters per second was found 

 to be a reliable average value for sand. Additional information on various 

 seismic profiling techniques is discussed in Ewing (1963) , Moore and Palmer 

 (1967), Barnes, et al. (1972), and the American Association of Petroleum Geolo- 

 gists (1977). 



16 



