Preexploration 



Broad reconnaissance. Broad reconnaissance methods include magnetometer 

 surveys, inspection of aerial and satellite photos, reconnaissance for oil 

 seeps, and the use of logging data from existing wells over long distances. 

 Since these methods do not affect the environment in any direct manner, they 

 will not be pursued further in this discussion. 



Site-specific geophysical survey - land areas. Three major site-specific 

 geophysical methods are used on land: gravity measurements, explosive charges, 

 and thumpers. Gravity measurements made on site utilize a pendulum, the period 

 of which is inversely related to the gravitational force in the area (Conner 

 et al., 1976). Changes in gravitational force can indicate formations that 

 bear petroleum. The instrument can be carried to the measuring point by hand. 

 Thus for measurement, vehicle movement into and out of the area with minor 

 clearing is usually all that is required. Ease and speed of measurement is 

 such that numerous measurements at different points can be made in an hour. 

 However, the measurements do not yield as much structural information as do 

 seismic techniques. 



Explosive seismic methods require entry of several vehicles into a site. 

 Generally, survey lines are established on maps before vehicles enter the area. 

 On the gulf coast, seismic activities may take place year-round, though summer 

 dry periods are sometimes favored, as drier marsh conditions decrease problems 

 of vehicle movement. Surveyors may enter the site first to accurately stake 

 the survey lines and mark the areas where shot holes and geophones, or detectors, 

 will be placed. Since the placement of the charges and sensors must be accurate 

 to within a few meters, there is often a moderate amount of clearing of brush 

 and debris in upland areas to facilitate vehicle movement and remove obstruc- 

 tions for the laying of connecting cables. 



The survey and clearing usually require several traverses of the survey 

 line by vehicles. The cleared or beaten-down survey line is 3 to 4 m (9 to 

 13 ft) wide. When the line is ready, a drilling truck, supply truck, and 

 measuring van enter the shot line. Entryway and exit for these vehicles are 

 usually by the most direct route unless the local land manager specifies an 

 access pathway. The drilling van drills a shot hole, and a casing is placed in 

 the hole to prevent caving-in due to unstable sediments (Robnett, no date). A 

 charge (often ammonium nitrate with a detonator) is placed in the hole. The 

 casing may or may not be completely pulled from the hole before the charge is 

 detonated. 



The holes are about 10 cm (4 inches) wide and are generally shallow (15 to 

 120 m, or 50 to 396 ft, deep). They are often 75 to 350 m (248 to 1,155 ft) 

 apart, depending on the substrate. The detectors (often 24) are strung along 

 the shot line at prescribed intervals between the holes. When laid out, they 

 are connected to the recorders in the measuring van and two to three holes are 



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