GeophysicalDataacquisition Offshore

8/12/2019 GeophysicalDataacquisition Offshore

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OFFSHORE SEISMIC DATA ACQUISITION

S. PANIGRAHI


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Planning&ManagingoffshoreOperations

GeophysicalRequirements

Positioningand

Navigation

in

offshore

LatestTechniquesandTechnologies

QHSE

Offshore Geophysical Surveys


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OFFSHORE DATA ACQUISITION

1. HOW IT IS DIFFERENT FROM LAND ACQUISITION

A. RECEIVERS

B. ENERGY SOURCEC. NAVIGATION & POSITIONING

2. PLANNING OF SURVEY ( 2D or 3D or 4D )

3. METHODS OF OFFSHORE ACQUISITION

4. RECENT DEVELOPMENTS


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OFFSHORE Seismic Data Acquisition


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Tail

Buoy

To Vessel

Tail Buoy

Vessel

Energy

source

CMP

Seismic Data Acquisition(OFFSHORE)


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Single Streamer Geometry


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MULTI Streamer Geometry


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A View from the Back Deck of the Boat


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RECEIVERS / STREAMERS

Hydrophones : Pressure sensitive piezo-electric material.

Produces an Electric analog signal of

pressure variations.

The pressure changes associated with a sound wave can be

Detected by a piezoelectric element. Under the pressure

of a sound wave, the piezoelectric element flexes and in

return gives off electrical signals.

These electrical signals can be recorded and later analyzed.


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STREAMER

Trade name for Offshore Data Acquisition Cables.

Contains groups of Hydrophones at pre-determined intervals.

Consists of many sections of cables of 100 mt length.

Standard Length can be 2.5 KM to 8 KM as per requirement

Payed out Streamer Streamers on winch


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STREAMER SECTIONS STORED ON DECK


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So l i d S t r e am e r s a r e r e c o g n i ze d fo r t h e i r c o n s i s t e n c y ,

r e l ia b i l i t y , h i g h s i g n a l -t o -n o i s e r a t i o a n d l ow f r e q u e n c yr e c o r d i n g .

-T h e i r lo w n o i s e c h a r a c t e r i s t i c s a l l o w t h e e x t e n s i o n o f

o p e r a t i o n s i n t o m o r e m a r g in a l w e a t h e r w i nd o w s .

- T h e c o n s is t e n t b u o ya n c y a l l o w s s t a b le t o w i n g a t

s h a l lo w e r d e p t h s t o o p t i m i ze h i g h -f r e q u e n c y a c q u i s i t i o n

w h e r e r e q u i re d .

SOLID STREAMERS


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PAYING OUT OF STREAMERS FROM BACK DECK OF THE SHIP


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ENGAGING OF BIRDS IN STREAMER

Provides:Advanced acoustic positioning,

Powerful streamer steering,

Automatic depth control

Resulting in spread regularization &

improved full streamer positioning


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BOAT TOWING A SINGLE STREAMER

THE STREAMER IS TOWED AT A DEPTH TO REDUCE NOISE


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MARINE ENERGY SOURCE

1. AIRGUN : Compressed air at high pressure is forced out.

2. WATERGUN : Compressed water is forced out. The expelled water

creates a vacuum in its wake, resulting in an implosion.

3. BOOMER : Two metal plates are shoved apart by electric current,

creating a void space.

4. SPARKERS : Sparkers send an electric spark between two electrodes,vaporizing the surrounding water and simulating an explosion.

5.AQUAPULSE : A submerged explosive source in which the waste gas

is ejected into the atmosphere.

6. VAPOURCHOC : which fires a bubble of superheated steam .

7. FLEXOTIR : Which detonates a charge in a porous steel cage.


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AIR GUN ENERGY SOURCE

CLUSTER OF AIR GUNS- Volume ranging from 50 to 700 cubic inch


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The most commonly used marine seismic source is the airgun. This gun injects high-pressure air into the water.

The air guns consists of two chambers and a piston-like

apparatus called the shuttle. Air moves from one chamber to the

other through the shuttle.

Eventually, the pressure forces the shuttle upward, releasing

air into the water through vents.

As with explosive sources, the bubble effect creates

oscillations, but due to the small energy release, the bubble

effect lengthens the initial pulse instead of creating conflicting

pulses.

Air guns are generally used in arrays.

AIR GUN ENERGY SOURCE


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TYPICAL AIRGUN

Most widely used of allnonexplosive sources is

air

gun,

due

to

its

simplicity, robustness andreliability.

An

air

gun

releasescompressed air into the

water at preciselycontrolled time. This

rapidly/instant

expandingair bubble produces the

seismic shock.


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-in

11 meter separation rope

With sliding collar attached to the lead

but fixed to the gun bulkhead

9 meter separation rope between sub-arrayWith sliding collars both end.

M/V SAGARSANDHANI

POSITION OF GUN ARRAY TOWED BY THE SHIP


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OFFSHORE NAVIGATION & POSITIONING


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It is a "constellation" of twenty-four 20,000km

high GPS satellites.

The satellites are distributed on 6 orbits, 4 per

orbit.

Every satellite makes a complete rotation ofthe Earth every 12 hours.

Gives position in Lat , Long ( WGS-84 )

GLOBAL POSITIONING SYSTEM


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D t i i P iti


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Determining Position



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Position is Based on TimePosition is Based on Time


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PositionisBasedonTimePositionisBasedonTime

T + 3Distance between satelli te

and receiver = 3 times thespeed of l ight

T

Signal leaves satelliteat time T

Signal is picked up by the

receiver at time T + 3

Sources of Signal InterferenceSources of Signal Interference


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Sourcesof

Signal

InterferenceSources

of

Signal

Interference

Earths Atmosphere

Solid Structures

Metal Electro-magnetic Fields


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DifferentialGPS

(DGPS)

Real Time Differential GPSReal Time Differential GPS


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DGPS Site

x+30, y+60

x+5, y-3

True coordinates =

x+0, y+0

Correction = x-5, y+3

DGPS correction = x+(30-5) and

y+(60+3)

True coordinates = x+25, y+63

x-5, y+3

RealTimeDifferentialGPSRealTimeDifferentialGPS

DGPS ReceiverReceiver


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GPS RECEIVERS

Leica GPS1200+


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PLANNING OF SURVEY

1. 2D or 3D or 4D

2. TRANSITION ZONE OBC

3. SHALLOW WATER OR DEEP WATER

4. MULTI-COMPONENT ACQUISITION

ACQUISITION GEOMETRY


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ANTENNA

50.55m

VESSEL / STREAMER OFFSETS

CFGCOS

158.95 m

50 m

10m

STREAMER 1 (4600 mtrs )

STREAMER 2 (4600 mtrs)

90..5 m

300 m

100m

3 STRETCH

10m


Streamer 1, 6000 mts.


3 STRETCH

FOUR STREAMER G t


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FOUR STREAMER Geometry

O O O


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Inordertoacquire3Dseismicinasafeandeffectivemannerin shallowand

congestedareas,asforinstanceinGulfofMexico,OceanBottom Cable.

(OBC)is

the

preferred

acquisition

technique.

OBCalsooffersthebenefitofwideazimuthcoverageandadvancedrock

propertyanalysisduetomulticomponentrecording.

OCEAN BOTTOM SURVEYS


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Data Acquisition at Sea


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CONVENTIONAL METHOD OF OFF SHORE


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CONVENTIONAL METHOD OF OFF-SHORE

DATA ACQUISITION

THE ACQUISITION PATH FOLLOWS A STRAIGHT LINE ( BLUE LINE )

THEN TURNS 180 DEGREES TO ACQUIRE DATA IN OPPOSITE

DIRECTION ( ORANGE ARROW )


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RECENT DEVELOPMENTS & TECHNIQUES

1. WIDE AZIMUTH ACQUISITION

2. COIL SHOOTING

3. Q-MARINE ( High Density 3D )

4. GEO-STREAMER

WAZ WIDE AZIMUTH DATA ACQUISITION


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WAZ - WIDE AZIMUTH DATA ACQUISITION

A FOUR VESSEL WIDE AZIMUTH ACQUISITION CONFIGURATION

COIL SHOOTING


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COIL SHOOTING

SHOOTING SEISMIC SURVEYS IN CIRCLES

DIFFFERENT AZIMUTH SURVEYS


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DIFFFERENT AZIMUTH SURVEYS

COMPARISON OF AZIMUTH

OFFSET DISTRIBUTION PLOTS

CALLED ROSE DIAGRAMS.

ADVANTAGE OF WIDE AZIMUTH ACQUISITION


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ADVANTAGE OF WIDE AZIMUTH ACQUISITION

AZIMUTH OF OBSERVATION IMPACTS THE RESULTS

OF SEISMIC IMAGING THROUGH GEOLOGIC MEDIAWITH COMPLEX GEOMETRIES.

HIGH DENSITY 3D


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HD3Ddeliversoptimumseismicimagequalityandresolution,virtuallyfreeof

processingartifacts. HD3Ddataproductsaddressanentirespectrumof

seismicexploration,production,andtimelapse(4D)reservoirmonitoring

challenges.

SOLID STREAMER


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Proprietary solid fill material ( BVF ) provides both reducedself-noise and consistent buoyancy benefits.

BVF is a hydrocarbon-based material that cures into a solid

with specifically engineered properties.

The BVF material retains

its shape, demonstrating its

resistance to deformation.

GEO STREAMER


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GeoStreamer TwoSensorsareBetterthanOneGeoStreamer represents the most significant advance in towed marine

streamertechnologysince itsoriginal invention61yearsago.Therehave

beenmany

attempts

to

implement

aviable

dual

sensor

streamer

and

all

of

themhavefailed,withthenotableexceptionofthislatestonebyPGS.

IncreasedExploration

Success

GeoStreamer isdesignedtoprovidebetterimagingofbothlowandhigh

frequencies, recording both pressure and velocity fields during marine

seismic acquisition. In addition, the data is sampled in the quieter and

operationallymore

efficient

environment

of

adeeper

tow.

GEO-STREAMER

GEO-STREAMER SENSORS


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A wave is recorded by a hydrophone and a vertical velocity sensor.

The up-going wave is seen as a positive by both sensors while the

down-going wave is negative for the hydrophone and positive for the

velocity sensor.

Since the two sensors are collocated, they have identical ghost

period and can be summed to produce the up-going wave whilst the

difference produces the down-going wave.

GEO STREAMER SENSORS


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AA

MV SAGAR SANDHANI

(DEPARTMENTAL SEISMIC SURVEY VESSEL)


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MV SAGAR SANDHANIMV SAGAR SANDHANI

1. DUAL STREAMERS OF MAXIMUM LENGTH 6000M EACH

2. DUAL SOURCE (1500 LL BOLT AIR GUN) 2538 CU. INAND 2000 PSI

3. ACQUISITION SYSTEM OF HANDLING 1008 CH

IN EACH STREAMER

4. ONLINE QC MONITORING SYSTEM

5. NAVIGATION SYSTEM

6. ONBOARD PROCESSING SYSTEM




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ANTENNA

50.55m


CFGCOS

158.95 m

50 m

10m

STREAMER 1 (4600 mtrs )

STREAMER 2 (4600 mtrs)

90..5 m

300 m

100m

3 STRETCH

10m


Streamer 2,6000 mts.

3 STRETCH

2D VS 3D DATA ACQUISITION


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2D SURVEY 3D SURVEY

2D VS 3D DATA ACQUISITION


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EXCLUSIVE ECONOMIC ZONE


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The control of the oceans is currently regulated by the 1982 Law of the Sea

Convention that went into effect on November 16, 1994. This law definesoceanic jurisdiction for all nations. It establishes the principle of a 200-

nautical-mile limit on a nation's exclusive economic zone (EEZ) whereby a

nation controls the undersea resources, primarily fishing and seabed

mining, for a distance of 200 nautical miles from its shore.

EXCLUSIVE ECONOMIC ZONE


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The diagram above shows a very simplified, basic outline of the important

distance markers for the international zones of jurisdiction over theadjacent sea. The letter B is the low-water line along the coast and it

represents the baseline from which all seaward measurements are

determined. Be aware that international systems of measurement are

according to the metric system. This also includes all internationalcommerce.

The original metric system was based upon commonly found Earth

systems. The nautical mile was developed during the age of sailing and

has become the international system for measuring ocean (nautical)distances. It is defined as one minute (1') of latitude or 1.15 miles. (A

degree of latitude is approximately 69 miles; a minute of latitude is 1/60th

of that.) That makes a nautical mile about 1,852 meters (6,076 feet) in

length or 18% longer than a statute mile, the measurement commonlyused in the English system (5,280 feet or 1,609 meters). Few Americans

realize that their "frequent flyer" miles are calculated in nautical miles and

not statute miles.


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