Basic Knowledge Of Gun Mechanic/Source Mechanic On Seismic Survey Vessel
Background :
Seismic methods, as employed in
marine applications, differ little in theory from ground based seismic
surveys. A sound wave generating device
is used to transmit a sound wave, while receiving devices measure the amplitude
and arrival times of the returned (reflected /refracted) signals. However, practical applications of field
techniques, field equipment, and geographic control can vary greatly between
surface and marine surveys.
In the case of marine surveys,
the instrumentation is generally a towed transmitter and array of
geophones. In some shallow water
applications, a specialized array that rests on the ocean floor during
acquisition is used. Given the
difficulty associated with establishing straight survey lines while in a vessel
on a body of water, and with the necessity to image deep structures, the field
equipment can be cumbersome. Figure 3
demonstrates the scale of field equipment necessary for marine seismic
acquisition. Positional control is
generally provided by a global positioning system (GPS) where the GPS sensor is
mounted on the vessel towing the magnetometer, with a constant offset equal to
the distance from the GPS sensor to the geophone array.
A variety of seismic sources are
available for marine applications, including water guns (20-1500 Hz), Air Gun
(100-1500 Hz), Sparkers (50-4000 Hz), Boomers (300-3000 Hz), and Chirp Systems
( 500 Hz-12 kHz, 2-7 kHz, 4-24 kHz, 3.5 kHz, and 200 kHz). The greatest resolution of near surface
structure is generally obtained from the higher frequency sources such as the
Chirp systems, while the lower frequency tend to better characterize structure
at depth.
An seismic airgun is a
mechanical device that releases a high pressure bubble of air underwater, the
expansion of the air bubble generates seismic waves the water that are the
source waves of the seismic waves used in reflection seismology. The surveys
are conducted by trawling arrays of airguns behind a ship which explode every
10-15 seconds. A detailed knowledge of the radiation field of seismic sources
is essential to effective source design and source wavefield deconvolution. To
understand the principles of airgun theory and operation it is necessary to
follow the motion of the air bubble that is produced and released underwater by
the airgun. In seismic surveying, sound
waves are mechanically generated and sent into the earth . Some of this energy
is reflected back to recording sensors, measuring devices that record
accurately the strength of this energy and the time it has taken for this
energy to travel through the various layers in the earth’s crust and back to
the locations of the sensors. These recordings are then taken and, using
specialised seismic data processing, are transformed into visual images of the
subsurface of the earth in the seismic survey area.
Outline :
A.
Day 1 : What Is Seismic Surveying?
1.
The nature of
seismic data
2.
What is wave
propagation?
3.
What causes
seismic reflections and how they relate to rock properties including pore
filling material
4.
The wavelet in
the seismic data and its limit of resolution
5.
Seismic
velocities as they relate to rock properties and the imaging process
6.
The
relationship between seismic velocities and pore pressure
7.
Pore pressure
prediction
8.
Seismic data
processing and seismic migration
9.
Prestack,
poststack, time and depth imaging
10.
Direct
hydrocarbon indicators and AVO
11.
Seismic
inversion for rock and fluid properties
12.
Seismic
attributes
13.
Time lapse
reservoir monitoring (4D seismic surveys)
14.
Recent
developments in seismic acquisition, processing, and interpretation
B. Day
2 : Seismic Survey Vessel
1. Research Vessel Facilities and Equipment
2.
The Navigation And Positioning Equipment :
-
The Instrument Room,
-
The Back Dek,
-
Compressor Room
3.
Seismic Streamer
4.
The Seismic Source
5.
Vessel Configurations And Multi-Vessel
Operations
6.
Other Marine Seismic Techniques :
-
Seabed Recording Systems,
-
Ocean Bottom Seismometers,
-
Dragged Array,
-
Dual Component/Multi-Component Ocean
Bottom Cable,
-
Vertical Cable,
-
Vertical Seismic Profiling (VSP),
-
Site Surveys,
-
Transition Zone Acquisition,
7.
Operational Performance :
-
Survey
Location
-
Time of
Year
-
Survey
Size, particularly Sail Line Length
-
Technical
Acquisition Parameters
-
Vessel
Configuration
-
Line
Orientation and Prevailing Current Direction
-
Fishing
and Shipping Activity in the Survey Area – trawling especially
-
Other
Seismic Operations nearby
-
Marine Mammal
Activity
-
Drilling
and Subsea Equipment Maintenance, including diving
-
Technical
Equipment Downtime
C. Day 3 : Basic Principles Of
Airgun Operation
1. Air gun
history
2. A
single airgun
-
Principles
of airgun operation
-
Bubble
Motion
-
Theory of
bubble motion
-
The gas
behavior
-
The fluid
motion
-
Boundary
conditions
-
Airgun
parameters
-
The surface
ghost
-
The dipole
3. Arrays
of airguns
-
Airgun array
performance
-
Efficiency
of airgun arrays
-
Performance
specifications
-
Time
synchronization
-
Stability
-
The effects
of weather
-
Array
Simulation - Modeling in the computer
-
Peformance
specifications in the field
-
Interaction
between airguns
-
Clustered
airguns
4. Types
of airguns
-
The airgun
system
-
The port
type airgun
-
Sleeve airgun
-
The watergun
-
The Gas
Injector (GI) gun
-
Bolt Long
life gun
-
The Annular port Bun
Participants :
Geoscientists, engineers, team leaders, geoscience technicians, asset
managers, and anyone involved in using seismic data that needs to understand
and use this data as a communication vehicle.
