100 miles off the U.S. shores and in waters up to 
1,800 feet deep. About 100 core holes have been 
drilled beyond the U.S. Continental Shelves, some 
in waters nearly 5,000 feet deep in the Atlantic 
Ocean and Gulf of Mexico. 
2. Phases 
Offshore activities are conducted in three 
phases: 
Exploration consists of geophysical surveys to 
locate subsurface structures favorable for the 
accumulation of hydrocarbons, followed by ex- 
ploratory drilling to determine the presence or 
absence of oil or gas under the ocean floor. 
Production involves development drilling fol- 
lowed by installation of equipment for produc- 
tion, well service, and maintenance. 
Storage of the product and transportation to 
shore is the final phase of offshore activity. 
C. Exploration 
1. Geophysical Surveys and Geological Analysis 
Exploration encompasses the broad reconnais- 
sance surveys followed by more detailed surveys 
that actually delineate the geological structures 
that may contain oil or gas deposits (i.e., explora- 
tion activities involve locating promising areas for 
drilling activities). Exploration begins with geolo- 
gists making a general study of the structure of the 
earth to select an area with characteristics possibly 
favorable for oil or gas recovery. 
After selecting a promising area, tests pinpoint 
the site to probe further for possible reserves. 
These can be simple magnetometer readings taken 
from an airplane or ship. By showing a variation in 
the earth’s magnetic field, the tests indicate 
geologic structures below the ocean floor. In 
addition, towed marine gravimeters can determine 
very small variations in the earth’s gravity field. 
Both types of geophysical surveys can be made in 
any depth of water and at any distance from land. 
However, in themselves, they usually do not 
provide information of sufficient accuracy to 
permit siting an exploratory well. 
The most successful technique to locate test 
drilling sites is seismic profiling. Such surveys 
require much more expensive equipment but 
because of the high speed at which the surveys can 
VI-164 
be conducted, the actual cost per mile is about 
one-third as much as on land. A sound pulse is 
generated, a portion of which is reflected from the 
layers of sediment and rock under the ocean floor. 
The reflections when received at the surface are 
recorded on a graph showing an approximation of 
the depth and characteristics of underlying geolog- 
ical structures. In earlier surveys black powder or 
dynamite was used to generate the sound pulse. 
Today, electrical sparking systems, air guns, con- 
tained-gas explosions, mechanical boomers, and 
other nonexplosive energy sources are used. Seis- 
mic data are recorded routinely on magnetic tape 
and processed by digital computers, enhancing 
quality and reliability. 
The above-mentioned geophysical techniques 
are indirect methods for examining structures 
under our continental margins. The most satisfac- 
tory method to date for obtaining geologic sam- 
ples of rocks on or at shallow depths under the sea 
floor has been with small coring devices operated 
from a surface ship. These devices drill a hole 
several hundred feet into the sea floor and recover 
samples of rock for further study. Similar holes 
have been drilled in the U.S. continental margin 
beyond the shelves to 1,000 feet beneath the sea 
floor in waters from 600 feet to 5,000 feet deep. 
Coring in such depths has been accomplished from 
floating, dynamically positioned vessels. As long 
ago as 1961, several experimental core holes were 
drilled in 11,700 feet of water as part of the early 
phase of Project Mohole. 
Exploration technology has made rapid strides 
in new seismic energy sources and receiving sys- 
tems. Computers permit analysis of the data while 
under way at sea. The Navy Navigation Satellite 
System will permit seismic teams to determine 
more accurately survey locations in remote areas. 
2. Exploratory Drilling 
Determining the presence of oil or gas requires 
full scale drilling operations at the site, a much 
more difficult and expensive task than drilling 
shallow core holes. Multiple strings of casing must 
be set in the hole to keep it open. To control 
drilling fluids or fluids in the rock, large blowout 
preventers must be installed should high-pressure 
oil or gas be encountered. The drilling rig must 
maintain position at the wellsite for many weeks 
or months. More than 10,000 wells already have 
been drilled into the U.S. Continental Shelves. 
