Page 61 - 4567

The greatest improvements in petroleum exploration in the last several decades have
involved new seismic acquisition techniques and computer processing of digital seismic data.

Acquisition
The seismic method uses impulses of sound energy that are put into the earth. The energy
travels down through the subsurface rocks, is reflected off subsurface rock layers, and returns to
the surface to be recorded. Seismic exploration uses subsurface echoes to image the shape of
subsurface sedimentary rocks and locates petroleum traps. A source and a detector are used. The
source emits an impulse of sound energy either at or near the surface of the ground or at the
surface of the ocean. The sound energy is reflected off subsurface rock layers. The maximum
reflection energy occurs when the angle of incidence between the seismic source and reflector is
equal to the angle of reflection between the reflector and seismic detector. Only about 2 to 4% of
each sound impulse is reflected off each layer, and the remaining sound impulse goes further into
the rock, to be reflected off deeper and deeper layers. The reflected sound energy from each
layer returns to the surface, where the detector records it.
The detector on the surface records both the signal, wanted direct (primary) reflections
from the subsurface rock layers, and noise, unwanted energy. Noise can be caused by surface
traffic, wind, surface and air waves, and subsurface reflections that are not direct reflections from
subsurface rock layers. A high signal/noise ratio is desired. A noise survey, a small seismic
survey, can be run first to determine the nature of noise in that area and plan the optimum
seismic program to reduce noise.
The location of the seismic source is called the shot point. On land, the common seismic
sources are explosives and vibroseis. Dynamite was the first seismic source used and is still the
most common explosive source used today. Explosives are used today where the surface is
covered with loose sediments, swamps, or marshes. When using explosives, a small, truck-
mounted drilling rig often accompanies the seismic crew to drill a shot hole, usually 18 to 30 m
deep, to a point below the soil. The explosives are planted in solid rock on the bottom of the
hole. Primacord, a length of explosive cord, can be planted in a trench about 0.3 m deep or
suspended in air as a seismic source.
At sea, a common seismic source is an air gun. Other seismic sources used at sea include
water gun, sleeve gun, and sparker.
A seismic contractor is a company that owns and operates the seismic equipment and
runs the seismic survey. The seismic contractor can run the seismic survey under contract with
an exploration company. A spec survey can also be run by a seismic contractor. A limited
number of exploration companies then pay for and view the nonexclusive seismic records. In
another method, several exploration companies share the cost and results of a seismic survey run
by a seismic contractor in a group shoot.
Before seismic exploration is run on private land a permit person must obtain permission
from the surface rights owners of the land. A fee per shot hole or seismic line mile is paid, and
damage fees are negotiated. A survey crew then cuts a path through the trees and brush (if
necessary), accurately locates and flags the shot points and detector locations, and records them
in the survey log book. Members of the seismic crew called jug hustlers lay the cable and
arrange and plant the geophones. At sea, permitting is not necessary. The ship’s crew does the
navigation while the seismic crew runs the seismic equipment. Surveying on land and at sea
today is done by global positioning using navigational satellites.
Seismic exploration is most expensive on land, especially in rugged terrain. It is less
expensive and of better quality at sea.

Task 3. Answer the following questions, using the vocabulary from Task 1.
1. What is geophysics?
2. What kind of surface methods do geophysicists usually use to explore the subsurface?
3. Which of the methods is the most useful and advanced?