Basic Knowledge Of Gun Mechanic/Source Mechanic On Seismic Survey Vessel

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.