elevated levees and spoil banks, and revegetating barren sites facili- 

 tates hydrologic restoration. 



Delta Marsh 



Seismic preexploration . 

 1. Activity sequence 



Because a delta marsh is composed of many water bodies partitioned 

 by sections of land - either broad expanses or narrow - completion 

 of an entire series of seismic lines frequently involves a combina- 

 tion of techniques. On land surfaces, surveyors enter by marsh 

 buggy or foot, stake the lines, and mark the areas where shot holes 

 and geophones will be placed. Vegetation may be cleared along a 

 path 3 to 4 m (10 to 15 ft) wide. A drilling rig and one or two 

 support vehicles enter next; holes are drilled, charges and record- 

 ing equipment are placed, and shots are fired. Geophones are 

 retrieved and placed for the next series of shots. After completion 

 of the terrestrial portion of the shot line, the holes are plugged 

 and the crew exits. 



In areas accessible only by vessel, nonexplosive methods are com- 

 monly used - sparkers, air guns, or vibrators that emit high- 

 frequency pulses. Navigation equipment near the water's edge guides 

 the vessel on straight-line courses back and forth across the water 

 body until sufficient data have been collected. The vessel and 

 support crew then leave the area. 



2. Primary ecological alterations 



2.1 Removal, trampling, and crushing of vegetation 



2.2 Localized change in direction of water flow 



2.3 Creation of pathway for increased rate of flow of runoff 



2.4 Creation of pathway for increased inundation (salt water or 

 fresh water) 



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