While there is some attention paid to drainage patterns in designing the 

 route of access, there is always a trade-off between minimizing distrubance and 

 increasing construction costs. The balance between these often opposing forces 

 is usually a result of a combination of legal ownership or lease status; the 

 authority of the land manager; corporate policy; public relations; and govern- 

 mental regulation. The lack of highly detailed topographic data is a serious 

 shortcoming to this process. Large-scale topographic maps such as the U.S. 

 Geological Survey topographic maps (1:24,000) usually have a minimum contour 

 interval of 1.5 m (5 ft). Yet most of the area of gulf coast marshes lies 

 below the five-ft contour on commonly available topographic maps. Thus, most 

 of the topographic features necessary to design a sound access pathway may not 

 be discernible within the resolution of the current data base. Since a detailed 

 topographic survey of an area is an expensive proposition, design of access 

 routes is dependent on use of existing topographic data, aerial photos, and 

 impressions from site visits. 



Dredged sites. Cost considerations often cause petroleum exploration 

 companies to seek dredged access canals and wellsites. This is particularly 

 true if the marsh soil is very mucky or if the site is located a long way from 

 road access. 



The construction of access canals begins with surveyor entry by truck, 

 marsh buggy, or on foot. Little, if any, clearing is required since marsh 

 vegetation generally does not stand high enough to impede vehicles. In general, 

 surveyors place stakes to indicate the boundaries of the levee around a site 

 and the channel pathway. 



There are two general types of dredges used in petroleum canal excavation - 

 hydraulic and bucket dredges (Davis, 1973). Bucket dredges can be of several 

 sizes, but they all consist of a dragline, either track-mounted with movable 

 mats or mounted on a barge or marsh buggy. Davis (1973) notes that hydraulic 

 dredges are rarely used in oil field work, though in the process of this study 

 one has been observed doing maintenance dredging on an oil field channel. In 

 the past, selection of dredge type has been largely determined by economic 

 factors. However, particular requirements such as spoil placement may dictate 

 use of one type of dredge over another. 



The typical drilling structure in shallow coastal waters is the self- 

 contained submersible drilling barge. It generally requires a depth of 2.5 to 

 3 m or 8 to 10 ft (Davis, 1973), for navigation and placement; typically the 

 canal is dredged to a width of 21 m (70 ft). Since the bays along the gulf 

 coast are generally shallow, particularly near the marshes, channels must be 

 dredged to approach the drilling sites. Spoil may be placed either on one or 

 both sides of the excavated channel. Usually the spoil is submergent. 



If entrance is being made from the bayside to the marsh, a channel is 

 dredged right to the m.arsh edge. Spoil is placed adjacent to the channel on 



90 



