fired simultaneously. The detectors are retrieved and placed for the next 

 series of shots, until the entire survey has been completed. Shot lines them- 

 selves vary in length from several hundred meters to as long as 30 km (18 mi). 



Though there is much dragging, laying of cable, and explosions, explosive 

 seismic work is accomplished rapidly. It is unusual for a line two or three km 

 (one or two mi) long to require more than a week's operation. Because of 

 explosive material use, human activity, and vehicle presence, fires can occur, 

 particularly in dry grass. Consequently, seismic crews sometimes carry fire 

 control equipment and must be alert for fire hazards. 



At the end of the survey, crews remove the shot tubes and usually plug the 

 shot holes with earth. This is done to protect shallow aquifers from surface 

 contamination and to prevent surface areas from draining through impermeable 

 sediment layers penetrated by the shot hole. There may be debris to pick up 

 after shooting is complete. Vehicle exit ends this phase of preexpl oration 

 activities. 



In wet soils, the sequence of events is the same except that the vehicles 

 used may be marsh buggies. In the gulf coast region two types of marsh buggies 

 ire generally used. One type has large, low-pressure rubber tires (Rol-a-gon); 

 che other is a track-laying vehicle with wide treads. By supporting weight 

 over a wide area, these vehicles can successfully move over^marsh surfaces thatp 

 may tolerate soil loads of no more than 1,200 to 2,400 kg/m (250 to 500 lbs/ft ) 

 (Conner et al., 1976). In '^ery wet areas, airboats may be used to move men and 

 light equipment. In an attempt to minimize alterations of wet areas, helicop- 

 ters have also been used for transportation of men and equipment. 



Continuous running of marsh buggies back and forth along the same track 

 may form ruts. In some areas, buggy operators find this advantageous since 

 vegetation is beaten down and vehicle operation is eased. In other areas, 

 however, repeated operation over the same track can turn very plastic soils 

 into muck, making vehicle operation more difficult. In these areas, buggy 

 operators are likely to drive along adjacent, parallel pathways, while still 

 remaining as close to the shot line as possible. 



Buggies transport men, materials, equipment, and sensitive instruments. 

 Since seismic operations often involve placing men and equipment in areas that 

 are either remote or pose access problems, time is an important consideration 

 to seismic crews. The number of trips a buggy makes up and down the shot line, 

 the route of travel, and the length of the working day often depend on time 

 considerations; 



The end result of a seismic survey is a chart of the pattern of reflected 

 and refracted waves from the explosive energy source. This chart is interpreted 

 by geologists, who are looking for characteristic oil-bearing formations and 



85 



