Prepared by

Mohamed Abdel-Baset & Mai El-Sayed

Agenda :-

What is marine seismic

Properties of seismic signal in marine

Characteristics of seismic Signal

Factors effecting Seismic wave propagation

Cables used in marine shooting

Survey plan design

Marine acquisition noises

What is Marine Multi-component data

Ocean bottom cables ( OBC )

Factors effecting marine acquisition

What Is Marine Seismic Acquisition ?

Recording the seismic signals in offshore surveys Using sensors (hydrophones) installed in long streamers behind a boat. Air guns serve as the energy source .

Properties of seismic Signals in marine :-

The properties of the seismic signals generated by all marine energy sources are strongly dependent upon the bubble oscillation , which in the case of dynamite & certain nonexplosive sources such as air guns is associated with an actual bubble in the water .

Characteristics of seismic Signal :-

Noise

Amplitude

Frequency

Phase

Definitions :-

Noise:- Any vibration that is not part of the signal is noise.

Noise is inevitable and coherent noise is generated by the shot itself.

Noise which is not generated by the shot is termed random. Movements of traffic, animals and people all generate random noise and can, to varying extents, be controlled.

Random noise is also produced by vegetation moving in the wind and disturbing the ground.

Factors effecting Seismic wave propagation :-

Seismic source energy

Geometric spreading

Earth absorption

Energy scattering

Near surface noise

Depth of target

Seismic sources energy :-

Explosive source .

-Dynamite

• Non Explosive Sources .

-Air Gun .

-Water Gun .

Sources using controlled charges of Dynamite .

Flexotir .

Flexotir used dynamite in a cage to try to diffuse the bubble .

Non Explosive source :-

Air Gun :-

The most widely used of all nonexplosive sources is the air gun .

Mechanism of air gun :-

High pressure air , which pass through a hose from the compressor to the towed submerged unit, centers through the connection at the upper lift .it flows into the upper chamber . Although the same pressure is developed in each chamber, the area of the triggering piston above is somewhat greeter than that of the firing piston below, and the net downward force on the shuttle cases it to move down until it is stopped by the base of the upper chamber. At the instant the gun is to be fired, a solenoid opens a valve that injected high-pressure air between the triggering piston & the base of the upper chamber though the opening on the right side of this chamber. The sudden introduction of the air through the solenoid-controlled valve upsets the equilibrium of the system , & the shuttle moves upward at a high velocity.

As the firing piston the four large ports, most of high-pressure air from the lower chamber is suddenly spilled out into the water, certain an air bubble quite similar to that from dynamite explosion & giving rise to repetitive bubble pulses at a rate determined by the oscillation period of the air mass thus generated .

Water Gun :-

Water guns are often used when sharp , clean , bubble-free impulses are needed and greater source power is not as important.

Cables used in marine shooting :-

Single streamer cable .

Multiple streamer cables .

Single streamer cables :-

In shallow reconnaissance exploration as well as in engineering surveys ,it is more expedient & more economical to record from a single channel than from a long multichannel spread .

On marine reflection surveys ,the cable used for single-channel surveys is of the streamer type. Because only one channel is used , it is economically feasible to introduce many more hydrophone elements that would be possible for each element of multichannel cables .

Multiple-channel streamer cables.

Most marine recording makes use of cables 3000 or more meters long, which contain 96 or 240 recording segments ,each feeding separate channel. Individual segments are 12 to 30 m. long , & each contain 6 to 15 hydrophones.

Survey plan Design :-

2-D Survey .

Most marine reflection surveys are carried out as a single-ship operations. The same ship tows both the energy source & the recording cable .

3-D Survey .

Use two ships simultaneously. Each ship is equipped with both a source array & recorded cable . The two ships travel abreast a few hundreds feet part. Typically the two ships alternate firing their source arrays.

Common Midpoint Method (CMP Method)

Is fundamental to deep reflection surveys. In shallow surveys this technique is normally used only to stack (enhance) results obtained with identical source and detector positions. If, however, the data are recorded digitally, NMO corrections can be made (although not in the field) to traces produced with different source–receiver combinations. The technique normally used is to collect together a number of traces that have the same mid-point between source and receiver (common midpoint or CMP traces), apply the corrections and then stack.

The number of traces gathered together in a CMP stack defines the fold of coverage.

Common Midpoint Method (CMP Method)

Common Midpoint Method (CMP Method)

Shotpoint # 1 Hydrophone groups

#1 #2 #3 #4 #5 #6

Midpoints

Common Midpoint Method (CMP Method)

Shotpoint # 2 Hydrophone groups

#1 #2 #3 #4 #5 #6

Midpoints

Common Midpoint Method (CMP Method)

Shotpoint # 3 Hydrophone groups

#1 #2 #3 #4 #5 #6

Midpoints

Common Midpoint Method (CMP Method)

Shotpoint # 4 Hydrophone groups

#1 #2 #3 #4 #5 #6

Midpoints

Common Midpoint Method (CMP Method)

Shotpoint # 5 Hydrophone groups

#1 #2 #3 #4 #5 #6

Midpoints

Common Midpoint Method (CMP Method)

Shotpoint # 6 Hydrophone groups

#1 #2 #3 #4 #5 #6

Midpoints

Common Midpoint Method (CMP Method)

Shotpoint # 7 Hydrophone groups

#1 #2 #3 #4 #5 #6

Midpoints

Common Midpoint Method (CMP Method)

Shotpoint # 8 Hydrophone groups

#1 #2 #3 #4 #5 #6

Midpoints

Common Midpoint Method (CMP Method)

Shotpoint # 8 Hydrophone groups

#1 #2 #3 #4 #5 #6

Midpoints

Data Transmit :-

Recorded data can be transmitted via satellite

Principles of position location from satellite data :-

The receiver on broad the ship, which is completely passive , picks up the signals from satellite that provide the information necessary to determine the height , latitude , & longitude of the satellite at any time of day.

Picture show data transmit via satellite :-

Common Marine Acquisition Noises :-

There are two types of noise in marine survey :-

Swell noise .

Multiples noise .

Swell Noise :-

Rough seas are the most important source of noise in marine acquisition. This noise is called swell noise. It corresponds to ocean swells during rough weather. The level of swell noise is actually the principal factor determining the acceptability of marine seismic data in rough weather.

Swell noise occurs predominantly at low frequencies, below 20 Hz.

The swell noise above 20 Hz is generally known as break-out noise because it is due to a hydrophone either breaking out from the sea to the atmosphere or being close to breaking out.

Figure show swell noise :-

(a) shows a shot gather from the Atlantic Ocean

(b) shows a Break- out noise

Swell Noise figure :-

shows a shot gather from the Atlantic Ocean

Multiples noise :-

Multiples noise is a wave reflected upwards with high amplitude within a low-velocity zone, such as between the sea surface and sea bottom, or between the earth’s surface and the bottom of a layer of unconsolidated rock . Multiples can appear as later arrivals on a seismic section, and are easy to confuse with deep reflections . They can be removed through processing techniques, called demultiple processing .

Figure show Multiples noise :-

There are 3 types of multiple reflection :-

Simple multiple ..

A wave reflected upwards from a subsurface interface can be reflected down again from the ground surface and then back from the same interface..

Two strong reflectors can generate other two types :-

Peg-leg multiple ..

intraformational multiples ..

What is Marine Multi-component data ?

Hydrophones have the ability of recording X,y,Z components

We have two types of wave recorded by multi-component method (PP wave , PS wave)

Marine multi-component 3D recording geometry are (3D Patch , 3D Swath )

Ocean Bottom cable Acquisition :-

Ocean Bottom cable (OBC) Acquisition :-

Detectors are stationary for set of shots before they re-deployed

Sail lines can be parallel or perpendicular to the cables

Usually smaller surveys are acquired due to huge efforts of the OBC

Can record P-wave and/or converted PS waves by using hydrophones and geophones

Factors effecting marine acquisition :-

Winds

Water current

Wave action

Depth of streamers

Positioning

Repeatability

Shooting around platforms

Spacing between streamers

Thank you & Good luck