new introduction to seismic method

• Measurement of seismic-wave travel time is one of the most common geophysical method.

• The main objective of this method is to map the structures of subsurface formations in order to infer the existence of possible petroleum traps.

• In addition, the method can be used to identify lithology (rock type), fluid content (oil, gas, or water), and fine structures (fractures).• In this method, seismic energy is generated artificially at the near surface and the generated waves travel in the subsurface and get reflected and refracted off layer boundaries.• Seismic exploration is divided into refraction and reflection surveys, depending on

whether the predominant portion of the seismic waves travel is horizontal or vertical.

• Refraction seismic surveys are used in engineering geophysics and petroleum exploration.

• Seismic reflection surveys, on the other hand is widely used in petroleum exploration; e.g. to detect boundaries between different kinds of rocks, this detection assists in the mapping of geological structures.

• The reflected waves are recorded at the surface and the travel times and amplitudes analyzed to map the subsurface.• Almost all of petroleum exploration is done using the seismic method.• Seismic energy is detected on land by using devices called geophones, which react to

on-site ground motions.Compared to other exploration methods, the seismic method gives, by far, the best subsurface structural and lithological image.

SEISMIC METHOD

SEISMIC METHOD

SEISMIC SURVEY

• OBJECTIVES

• PREPARATION

• PLANNING & DESIGNING

• PARAMETER SELECTION

• DATA ACQUISITION

OBJECTIVES

REGIONAL EXPLORATORY INVESTIGATIONS• Delineation of structural and stratigraphic anomalies

FIELD DEVELOPMENT• To mark suitable locations for field development and for reservoir

studies

PROSPECT DELINEATION• To mark the depth of prospective horizon and trap closures

PREPARATION

AVAILABLE BUDGET & TIMEAVAILABLE DATA

Geological Data

Geophysical Data

Other Information

GEOLOGICAL DATA

Surface maps

Cross sections

Contour maps

Isopachs

Formation depths/thickness

Lithologies

Well history

Wire Line Logs

MAPS

WELL DATA

GEOPHYSICAL DATA

Time / Depth contour maps

Isochore / Isopach maps

Velocity maps

MAPS

SECTIONSField monitors

Migrated / Unmigrated sections

REPORTS

Isochore Map

A contour map that displays the variation in time between two seismic events or reflections.

Isopach Map

An isopach map illustrates thickness variations within a tabular unit, layer or stratum. Isopachs are contour lines of equal thickness over an area.

OTHER DATA

• Topographic Maps

• Arial Photographs

• Cultural Information

KEY INFORMATION

• Depth of Target Horizon• Dip of the Target Horizon• Arial Extension of Target• Accessibility in the Area• Noise Type in the Area• Thickness of the Weathered Layer• Structural Settings of the Area• Terrain• Climate

PLANNING & DESIGNING

1. ACQUISITION PARAMETERS

2. BID INVITATION

a. Minimum Offset b. Maximum Offset

c. Source Interval

e. No. of Channels

d. Receiver Interval

g. Source

PARAMETER SELECTION

• SPREAD TYPE

• RECEIVER LAYOUT

• SOURCE TYPE

SPREAD TYPE

The shot or the vibration point can be situated:

• At the extremity of the active geophones (End shot)• At the center of the geophones. (Split spread shot)• At a certain distance (offset) of first geophone

The last geometry allows to have a long distance of offset, The second is more adapted if the reflectors are tilted.

SYMMETRIC SPLIT SPREAD

Maximum

Offset

Minimum

Offset

Minimum

OffsetMaximum

Offset

Shot pointCH 1 CH 61CH 60 CH 120

For a symmetric split spread, equal number of channels are laid out on both sides of the shot

Minimum Offset: The distance between shot and very first live channel on either side of spread.

Maximum Offset: The distance between shot and last live channel on either side of spread

RECEIVER TYPE

GEOPHONES

HYDROPHONES

SEISMIC DETECTORS

• Land detectors (Geophone):

It is a device is used to detect the seismic vibrations. A geophone is a transducer which transform mechanical energy (seismic vibrations) into electrical energy. It consists of a moving coil and a stationary magnet. The movement of the coil due to vibration creates electromagnetic flux proportional to the magnitude of vibration.

• Marine detectors (Hydrophone):

It is a device used to detect the pressure waves.A hydrophone is a transducer which convert a sound signal into an electrical signal, since sound is a pressure wave.Most hydrophones are based on a piezoelectric transducer that generates electricity when subjected to a pressure change.

Hydrophone

RECEIVER LAYOUT

Linear Array

Weighted Array

Parallel array

G1 G2 G23

G24

G1 G12

G24G13

G1 G10 G11 G12

G13 G14 G15 G24

Weight drop

Dynamite

SOURCE TYPE

Vibroseis

Hammer

LAND SURVEY

MARINE SURVEY

Air Gun

SEISMIC SOURCES

• Land Sources:

• A Impulsive sources: ‐ which are divided to Explosive sources such as Dynamite(common in Petroleum exploration), and Non Explosive such as Weight drop &Hammers (common in shallow seismic investigation).• B Non impulsive sources: ‐ the main common is Vibroseis which is a designedvehicle lift its weight on large plate in contact with ground surface in sweeps.• Up Sweep: Frequency begins low & increase with time.• Down Sweep: Frequency begins high & decrease with time.

• Marine sources:

• Air gun: the common in offshore survey (first produced in 1960). This gunreleases highly compressed air into water. It uses a compressed air at 2000‐5000PSI to produce an explosive blast of air. Several air guns with different sizesare fired to enhance their initial pulses & reduce their bubble effects.

DYNAMITE

• SHOT HOLE DEPTH• CHARGE SIZE• SHOT POINT INTERVAL• NUMBER OF HOLES• SOURCE ARRAY PATTERN

SP Interval

Hole Depth

Charge Size

PARAMETERS:

• The blast of explosives produces a very strong energy,

• Placing of the explosives to the bottom of a hole from 1 to 35 meters,)and the energy is better distributed.

• In order to make the holes, a hammer perforator, an auger or drilling rig is used.

• In the sand, a simple metallic tube is very efficient.

• In some desert regions, where the drilling is impossible, explosives can be suspended and exploded in the air about 1.5 m from the ground

• This method, very noisy & dangerous

VIBROSEIS

SOURCE POINT

Due to different types of sources the source point is referred as:• Shot Point (SP) for Dynamite Sources• Vibroseis Point (VP) for Vibroseis Sources

It is the generation and recording of seismic data. Acquisition involves many different receiver configurations, including laying geophones on the surface of the Earthor seafloor, towing hydrophones behind a marine seismicvessel to record the seismic signal. A source, such as a vibrator unit, dynamite shot, or an air gun, generates acoustic or elastic vibrations that travel into the Earth, pass through strata with different seismic responses and filtering effects, and return to the surface to be recorded by seismograph as seismic data.

SEISMIC DATA ACQUISITION

INSTRUMENT FOR SEISMIC SURVEYING

1. Geophone/Hydrophone2. Energy Source3. Seismic Crew4. Seismic Cables5. Recorder/Seismograph

REFLECTION FROM A HORIZONTAL REFLECTOR

Offset

CMP

Surface

Reflector

Source

Detector

SEISMIC DATA ACQUISITION ON LAND

Shot Hole

Geophones

Recording Truck

Seismic Waves

Reflector

SEISMIC DATA ACQUISITION IN MARINE

Hydrophones

Seismic ship

Seismic Waves

Shot

Reflector

Streamer

Sea surface

• Survey Crew• Drilling Crew• Loading Crew• Layout Crew• Recording Crew• Shooting Crew• LVL Crew• Safety Crew (HSE)

SEISMIC CREWS

Trimble Man Pack - 4700 GPS

• GPS used by the Survey Crew for locating the Source & Receiver Points of the Seismic lines, carried on their shoulders

External Batteries and Receiver

• Its is connected with two external batteries and a receiver which receives the signal from the satellite.

• Trimble Survey Controller is connected with the GPS, it has the designed coordinates of each source & receiver points, given by the Operating Company

Trimble Survey Controller

• When it gets the coordinates from the satellite it compares the designed and actual coordinates and gives the original values

Shot Points Receiver Points

• After locating the points, the crew mark the places of source and receiver points finally marking a seismic line

Source Points with red paint Receiver Points with white paint

Drilling Jackhammer

• Drilling Crew uses Jackhammers for shallow holes

Man Portable Drilling Rig

• For deep holes, Man Portable Drilling Rigs are used

Dynamite Stick for Deep Shots

• After drilling shot holes dynamite is loaded in them, using dynamite sticks

Detonator

• Detonators are placed in the dynamite stick, used to initiate the blasting of dynamite

• Lowering of Charge in a Drilled Hole with detonator in them

• Hole is loaded and filled with Sand

• Loading is done and place is marked

Other then dynamite, Vibroseis is also a source in Seismic Acquisition Surveys

Geophone Stings

Takeout Point of a Takeout Cable

• This crew first of all layout the Geophones on the seismic lines as spread.

• These geophones are connected to each other by the Takeout Cable

CABLE & GEOPHONE

Link cableGeophone string

LAYOUT(FRONT CREW)

CABLE LAYOUT

GEOPHONE PLANTATION

ROAD CROSSING

LINE

SOURCE

RECEIVER

Station Unit Box

• Takeout cable is then connected to Station Unit Box

Crossing Station Unit Box

• All the Station Unit Box is further connected with one Crossing Station Unit Box (of an array)

• Then yellow cable called Jumper Cable is finally connected to Recording Truck through which all the information goes into it

RECORDER

RECORDING

Color Line Monitor

Recorder

Tape Driver

• Monitor on which the Observer control all the activities on a seismic line

• The recorder which records all the data coming through cables

• The tape driver records all the data on the magnetic tapes or cartridges

Camera

Seismic Source Synchronizer

• The thermal plotter used to make a hard copy of the recorded data

• Device which synchronizes the blaster. Before shooting, recorder presses ARM button to charge the detonators, at the same time the blaster operator presses the ARM button in the blaster

• Then recorder presses GO button to blast the dynamite

Blaster

• Bag carried by the shooting crew is called The Blaster, it works in coordination with the Seismic Source Synchronizer

• “Blast view” on seismic line

Portable recording system

SEISOMOGRAPH

A recorded result of shot point after the blast

SEISOMOGRAM

Camera For LVL

Blaster used For LVL

• Camera used for Low Velocity Layer showing the status of spread, it has built in floppy drive, printer and hard disk

• Refraction blaster which is connected with the camera for blasting the dynamite

LVL Spread

• A view of a spread for the Refraction Survey

• Recorded result of an Low Velocity Layer shot

• Safety Caps• Foot Wears• Coveralls• Loose Clothing• Water Bottles• Fire Extinguishers, Fire Alarms, Smoke Alarms• Sign Boards

• The job of this crew is to provide the following things to all the crews

• After all the shots are recorded every thing is wounded

• The data is send to the Operator Company

• The place is restored by the Green Team and inspected by the Client Company

• Then Operator Company sends data to DGPC

Seismograph:It is the instrument that measure motions of the ground, including those of seismic wavesgenerated by earthquakes, nuclear explosions, and other seismic sources.

Seismogram:It is a graph output by a seismograph. It is a record of the ground motion at a measuring station as a function of time.

Two dimensional survey (2 D):‐ ‐

Seismic data or a group of seismic lines acquired individually such that there typically

are significant gaps (commonly 1 km or more) between adjacent lines. A 2D survey

typically contains numerous lines acquired orthogonally to the strike of geological

structures (such as faults and folds) with a minimum of lines acquired parallel to

geological structures to allow line to line tying of the seismic data and interpretation‐ ‐

and mapping of structures.

The seismic data recorded by 2 D survey is seismic line.‐

Three dimensional survey (3 D):‐ ‐

The acquisition of seismic data as closely spaced receiver and shot lines such that

there typically are no significant gaps in the subsurface coverage.

The seismic data recorded by 3 D survey is seismic cube.‐

LAND SURVEY

The 2-D method, which can be used to map structures along widely spacedtraverses.

Geometry

2 D SURVEY‐

Result

The 3-D method, which can be used to accurately map subsurface structures in athree-dimensional sense.The majority (> 80%) of seismic exploration is done using the 3-D method.

Geometry

3 D SURVEY‐

Result

Seismic cube

2‐D and 3‐D survey in marine differ from land survey by:

‐The seismic source is Air gun & not dynamite or Vibroseis

‐The seismic detector is Hydrophone & not a geophone

‐The sources and detectors are always at depth below the sea level & the depth

of the cable is controlled by Streamer‐Birds

‐The receivers are connected together by streamer.

MARINE SURVEY

2 D marine‐

3 D marine‐

Tail buoy:

A floating device used in marine seismic acquisition to identify the end of streamer. It allows the seismic acquisition crew to monitor location and direction of streamers.

Streamer Bird: A device connected with the streamer to control the depth of streamer.

Streamer Bird

REFRACTION SURVEY

Advantages

– Amplitude of refractor are large.

– Best for exploration of shallow structures.

– Easy to process and interpret.

Disadvantages

– Exploration for deeper structures is difficult because of large XCrit distance,

– Blind to low velocity layers.

SEISMIC REFLECTION SURVEY

Advantages

– Geophone spread needed is relatively shorter.– Not blind to low velocity layers. Since acoustic impedance exist for

every layer.– Convenient for deeper exploration.

Disadvantages

– Difficulty in recognizing reflected waves.– Small amplitude.– Obscured by overlapping refracted and surface waves.

SEISMIC NOISE

Noise is the undesired information contained on a seismic record which one does not wish to use and these are to be filtered. There are two types of seismic noises encountered in seismic survey are;

(1) Coherent Noise. (2) Incoherent Noise or Random Noise.

(1)COHERENT NOISE

• It displays some regular patterns on seismogram.

• It is seismic energy which aligns from trace to trace or record to record on seismic record .

• Often, it is very similar to the signal and usually more difficult to overcome than the incoherent/random noise.

• By examining the patterns of coherent noise, we can

devise field procedures to reduce it.

It has various sources as discussed below Ground Rolls Direct arrivals Refracted events Multiples reflections Diffraction events

• Head waves are always the first events seen on a record. • They are linear and often very visible and strong. • Sometimes they can disappear with large offset. • Many geophysicists try to remove head wave using

• surgical mute, • FK filtering• adapted NMO stretch mute value,

• but in some case, uppermost reflection and refraction can be very close and difficult to distinguish.

Direct and Refracted waves

Refracted Waves

In Seismic reflection survey the refracted events comes as first arrival.

Ground Roll

Ground rolls are Rayleigh waves which appear as second arrival on seismic section.

• Surface waves (Rayleigh, Love) primarily Rayleigh are – low velocity, – low frequency signal,– With relatively higher amplitude seen below shots.They override the useful reflections.

• They can be reduced using – frequency filtering.

Diffracted Events

At faults and some unconformities diffracted waves are generated on seismic section.

• The seismic reflection takes an interest essentially to the P compression waves, in the setting of seismic studies for the geo-technical; the recording of the S waves can to prove out to be useful for the shearing modules calculation.

• In knowing the velocity of the P waves and S, it is possible of calculating the Poisson’s ratio:

S Waves

(2)INCOHERENT NOISE OR RANDOM NOISE

It is the seismic energy that does not align up from trace to trace or record to record on seismic record. It displays no systematic pattern. This noise is uncorrectable. We can overcome random noises by recording more than one traces from the same location. It has various sources such as given below.

i. Wind Noiseii. Water Flow Noiseiii. Small Movement With in the Earthiv. Local Noisev. Bad Geophone Noise

FILTERING

A process or algorithm using a set of limits used to eliminate unwanted portions of seismic data, commonly on the basis of frequency or amplitude, to enhance the signal-to-noise ratio of the data.The common use of digital filter in data processing is to filter out unwanted frequencies.

Types of filters:

• Band-Pass filter:A filter used to pass a defined band of frequencies.Any high or low frequencies outside this range will be attenuated.• Low-Cut filter (High pass):A filter that pass only the high frequencies and eliminate the low frequencies. • High-Cut filter (Low-Pass):A filter that pass only the low frequencies and eliminate the high frequencies. • Notch filter:It is used to filter out narrow band of frequencies within frequency range ofdataThe most common use of this filter is to attenuate noises caused by powerlines.