4. Data Collection Planning . 



Before the field data collection effort, tentative offshore seismic survey 

 tracklines were established and plotted on navigation charts of the survey area. 

 Position, spacing, and length of the tracklines were determined by several 

 factors. The primary factor was spacing the lines to achieve maximum coverage 

 of the study area. In the Central Basin, the inshore boundary averages about 

 1 kilometer from shore, whereas the offshore boundary averaged about 7 kilo- 

 . meters. Average water depths in the nearshore were about -7.5 meters (about the 

 minimum depth for obtaining good quality seismic profiles), and in the offshore 

 were about -14 meters. In the Western Basin, the inshore boundary averaged 

 about 2 kilometers from shore, whereas the offshore boundary averaged about 5 

 kilometers. Average water depths in the nearshore were about -5 meters, and in 

 the offshore were -8 meters. 



A second factor was to lay out the seismic lines so that geologic features 

 with a high potential for containing sand would be crossed and show on the 

 seismic profiles. Preliminary core sites were selected on the basis of bathy- 

 metric information; however, final core sites were chosen after all the seismic 

 data were collected and subjected to preliminary interpretation. After the 

 survey tracklines were selected, the locations of the shore stations for the 

 navigation system were determined. Of high priority were stations at elevated 

 positions (for adequate line-of-sight) which also offered good triangular po- 

 sition in relation to the survey ship and adjacent shore stations. (Acceptable 

 results are achieved when the a'ngle of range Intercept of the vessel is greater 

 than 30 and less than 150 ; optimum range angle intercept is 90 . ) A total of 

 39 shore navigation stations were used along 284 kilometers of coast between 

 Conneaut and Toledo. Occasionally, positions and spacing of the predetermined 

 tracklines were altered to gather additional information on geologic features 

 such as buried stream channels, sediment contacts, and lake bottom outcrops of 

 possible sandy material. 



After seismic profile records were collected, preliminary^ analyses and in- 

 terpretations were made to select coring sites with the greatest potential, 

 based on^past experience, for finding sand and providing subsurface information. 

 Use of seismic data to decipher geologic conditions before selecting final core 

 sites enables a selection based on the best information available. Thus, this 

 procedure maximizes the usefulness of both sources of data and provides the most 

 efficient use of funds. 



During phase II the vessel GS-1, with the positioning system aboard, was 

 used to relocate fix positions selected as coring sites by duplicating the range 

 values from the shore stations. The vessel first maneuvered until one of the 

 ranges was duplicated and then an arc was run on that range until the other 

 range was intersected, at which time an anchored float was used to mark the core 

 location. Core sites were located and marked in this manner because of the 

 limited maneuverability of the barge. Without using the GS-1 to first buoy the 

 core site, much additional time would have been required to maneuver the scow to 

 the precisely determined core locations. The GS-1 crew located a core position- 

 in minutes and dropped a float marker; the tug and scow then moved in on the 

 marker, anchored, and the core rig was lifted from the deck of the scow and set 

 on the lake bottom next to the float. Meanwhile, the vessel proceeded to the 

 next selected core station. Once on the bottom the coring device was energized, 

 the core barrel was driven into the lake bottom, and within about 15 minutes the 



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