The U.S. Bureau of Commercial Fisheries has also studied the

effect of a geophysical energy source on marine planktonic organisms. Results of these tests showed: no apparent damage to the developing embryos in fish eggs; no injury to directly exposed jellyfish and comb jellies; no physical damage to copepods, amphipods, and crustacean larva; and no injury.

to juvenile fish.

In addition to the deep penetration CDP seismic reflection data, some companies purchase and interpret shallow penetration high resolution geophysical data to locate potential geologic hazards. such as unstable bottom sediment conditions and fault zones. A

typical high resolution data acquisition system is illustrated in Fig. 37. A discussion of geologic hazards present in the proposed sale area is presented in Vol. 1, Section II.A.4.

Figure 37--Geophysical and Seismic Data Collecting

(from exhibits of Henry A. Hill, OCS

Public Hearing at New Orleans, La. Aug. 23 1972)

b. U.S. Geological Survey

The U.S. Geological Survey, Conservation Division, has acquired from industrial sources more than 28,000 miles of modern CDP seismic data offshore Texas in support of the Federal offshore leasing program. These data begin at the Three League Line and extend seaward to the 1000-meter water depth line. The data provide definitive information on the size, shape, type, and depth of the prominent structural features in the area of the proposed general lease sale. The structural attitude of subsurface beds from depths of -1500 feet to over -20,000 feet can be mapped to show the relative merits of potential structural traps for oil and gas.

The structural information derived from seismic data is used as a

basic input in selecting tracts to be offered for leasing.

Regional seismic interpretations show that salt and/or shale piercement domes, and deepseated salt and/or shale domes, together with their associated faulting will be the types of structures most likely to be encountered. These structural features are considered favorable for

the accumulation of oil and/or gas.

Detailed seismic interpretations of each proposed sale prospect are being prepared by the U.S. Geological Survey to support the economic evaluation of the acreage.

Within the East Half Texas OCS Area, the USGS has no seismic data available in a 25-mile-wide belt which trends NE-SW through the

approximate center of the Federal acreage. This belt is recognized as being underlain by a trough of shale deposition, almost totally devoid of sands. Because of this acknowledged lack of reservoir beds, and a consequent lack of seismic reflections, there has been little or no interest in exploration or drilling in this belt for some years. Company sale tract nominations reflect this

lack of interest.

High resolution geophysical data which show the shallow geologic structures, sediments, and faults are used to predict, and thus minimize, hazards to drilling operations and possible consequent dangers to the environment from pollution. These are used as guides to regulate platform and well locations as well as drilling procedures. This will be discussed in Vol. 2, Sec. V.D.5. "Mitigating Measures".

Approximately 14,000 line miles of high resolution sparker data, comprising a 2x2-mile survey grid over the Texas OCS have been collected. There is an additional 3500 line miles of high resolution data, comprising a 1/2x1/2-mile data grid over specific areas. In addition, the USGS has initiated procurement action to acqure another 5000 miles of high resolution data covering all of the prospective leases with a 1/2x1/2-mile data grid.

2. Exploratory Drilling

a. Drilling Phase

Most offshore exploratory drilling is accomplished with

the use of mobile drilling rigs that can be moved from one location to another with relative ease. These mobile rigs include those that are

bottom-supported while drilling and those floating rigs that are held in position over the site by anchors.

Shallow (less than 200 feet) water exploratory drilling is commonly conducted using bottom-supported rigs of the "jack-up" (Fig.38) and submersible (not pictured) types. The submersible drilling rigs normally operate in water depths to approximately 70 feet. Most "jackup", bottom-supported drilling rigs are towed by tugboats from one location to another while in a floating position, whereas some are self-propelled and do not require tug assistance.

The semi-submersibles (Fig.38) are large, advanced-design floating rigs that can work in water depths up to 1,000 feet and beyond, and have better motion characteristics in rough seas than do ships or barges. These rigs are floated to the site, partially submerged, and held in place by anchors. The major problem encountered during the use of floating and semi-submersible drilling vessels is keeping them properly aligned with the drill hole on the sea floor. Winds, waves, and ocean currents tend to push them off location regardless of how good the mooring system. One solution has been to connect the wellhead (on the sea floor) with the drilling rig by a drilling riser pipe which is tensioned at the top to maintain its structural integrity. The tension requirements can be reduced by attaching bouyant material to the riser. High seas, strong currents, and heavy weather, however, can still cause the rig to drift off location, putting excessive stresses on the riser. One company's solution to this problem is the use of an acoustic posi