Seismic Data AcquisitionLand
M.Hanuman SastryDGM(GP) ONGC
Hydrocarbon Exploration involves locating X, Y, Z of HC bearing reservoirs and inferring their characteristics
G & G surveys are carried out to converge from a larger area to a drilling location Geological surveys : To identify Sedimentary
Basins Geological study of Basin Evolution,
Depositional Environment, Tectonic Activity etc., to determine the possibility of HC reservoirs formation.
Introduction
Introduction Geological Surveys to converge from large
area to Sedimentary Basins
Introduction Geological study of Basin Evolution, Depositional
Environment, Tectonic Activity etc.,
Introduction Geophysical Gravity & Magnetic Surveys to converge from Basins to Major Tectonic Blocks
Introduction Geophysical GM & Regional 2D Seismic
Surveys to delineate Tectonic elements
Introduction Geophysical Semi Detailed 2D Seismic Surveys
to identify Geological structural features
GO
JALIA
BA
RA
MU
RA
TULAMURA
ATH
AR
AM
UR
A
N
BO
GHA
PA SYNC
LINE
Introduction Geophysical Detailed 2D Seismic Surveys to delineate
individual structural elements (HC Reservoirs)
Introduction Geophysical 3D Seismic Surveys to
delineate / develop identified reservoirs
Stages of Exploration
Gravity Magnetic exploration methods:Gravity Magnetic exploration methods:
To delineate the basin with a generalized picture of the To delineate the basin with a generalized picture of the subsub-- surfacesurface
2D seismic reflection method:2D seismic reflection method:To find structures within the basins which are favorable To find structures within the basins which are favorable for hydrocarbon explorationfor hydrocarbon exploration
Close Grid 2D seismic reflection:Close Grid 2D seismic reflection:To map the structures preciselyTo map the structures precisely
Stages of Exploration 3D Seismic Reflection methods3D Seismic Reflection methods
To delineate To delineate stratistrati--structural features and to structural features and to understand reservoir characterizationunderstand reservoir characterization
4D Seismic4D Seismic
For identification of bypassed oil as well as thermal For identification of bypassed oil as well as thermal front movementfront movement
MultiMulti--component Seismic component Seismic
To detect the gas clouds, to identify the To detect the gas clouds, to identify the faciesfaciesvariation within reservoir sequence and to variation within reservoir sequence and to understand fracturesunderstand fractures
Semi-detailed 2D
Detailed 3D
Coarse Grid 2D(reconnasance)
3D Seismic Imaging
High resolution-3D
Viewing 3D data in many ways
Xline
Secti
on
Inline Section
Time SliceChair Display
2D SEISMICTWO DIMENSIONAL IMAGE OF THE SUB SURFACE
FIRST DIMENSION : X (DISTANCE ALONG THE LINE)
SECOND DIMENSION : T (TRAVEL TIME)
RECONNAISANCE SEISMIC SURVEY
GRID : 8 KM X 12 KM
SEMI DETAILED SEISMIC SURVEY
GRID: 4 KM X 4 KM
DETAILED SEISMIC SURVEY
GRID : 1KM X 1KM
Final Processed Data(2D)
Provides some geological meaning. Is not it ?
3D SEISMIC
THREE DIMENSIONAL IMAGES OF THE
SUBSURFACE
FIRST DIMENSION : X (ALONG THE LINE)
SECOND DIMENSION : Y (ACROSS THE LINE)
THIRD DIMENSION : T (TRAVEL TIME)
Final Processed Data(3D)
CYOSN2 3D (MIGRATION)SEISCHAIR
The 3D extentionof wedge out within paleocene is clearly brought out
Paleocene wedge out
Lower cretacious top Basement
But how do we get here ?
Objective of Seismic SurveyObjective of Seismic Survey To understand geological structure and stratigraphy
at different Target depths.
In the oil industry - to reduce the risk of drilling dry wells
Reserve AccretionReserve Accretion Discovery Of New Oil Pools Discovery Of New Oil Pools More Oil From Known FieldsMore Oil From Known Fields Enhanced Oil RecoveryEnhanced Oil Recovery
Principles of Seismic Reflection SurveyPrinciples of Seismic Reflection Survey
- Seismic reflection profiling is an echo sounding technique.
- A acoustic impulse is issued into the Earth at a surface location and the sub-surface response is recorded by a group of receivers placed on the surface surrounding the source location for some duration of time.
- The interface is often a geological boundary.
- Travel-time to the reflectors and the velocity of propagation is required to construct the geometry of the reflecting interfaces which are interpreted in terms of geological structures in depth.
ENERGY PARTITIONING AT INTERFACES.REFLECTION (Rc) AND TRANSMISSION (Tc) COEFFICIENTS
Rc = Ar / Ai Tc = At / Ai Tc = 1 - Rc
SEISMIC REFLECTION SURVEY1. Introduction of a seismic
pulse into the ground.
2. Pulse spreading as a down going seismic wave front.
3. Reflection at a boundary between rock layers.
4. The pulse travels upwards as a reflected wave front.
5. Recording at a receiver on the earth's surface.
Arrival
time
0surface
Layer-1
Layer-2
Layer-3
Shot Receiver
Reflection Survey
Seismic Trace
Seismic eventsSeismic events Travel time curves are fundamental to any understanding and
discrimination of seismic events.
Different kinds of events may be recognized by their travel timecharacteristics, amplitude variations, and relationship to related events:
- Air wave- Direct wave- Surface wave- Refracted wave- Reflection- Head wave- Diffraction- Ghost- Multiples
Different seismic eventsDifferent seismic events
2D Seismic RecordHigh velocity Direct Arrival
Reflection from sub-
surface
Low velocity Ground Roll
Refraction
Low velocity Air waves
Geophysical Surveys : Natural Source Methods :
Gravity & Magnetic, Magneto Telluric, Passive Seismic etc. Induced Source Methods :
Seismic , Electrical etc.
Seismic Methods : Refraction Reflection
Four Major Components of Seismic Surveys : Source (Acoustic energy) Earth Reflectivity System Receivers (Geophones Land, Hydrophones Marine) Recording (Digital) System & Ground Electronics
Geophysical Exploration Methods
Energy Source : To induce Acoustic Energy
Ideally to Generate Infinite Energy in Infinitesimally Small time The time domain graph is a Spike (zero to infinite frequencies) Dynamite, Vibroseis, Airgun etc.,
Earth Reflectivity System : Assumed to have layers of Varying Velocity & Density Reflection takes place at the layer boundaries
(Acoustic Impedance Contrast) Earth acts as a Low (Frequency) Pass Filter (1-150 Hz)
Receivers (Geophones Land) Electro Mechanical Transducers
Voltage Output proportional to the velocity of the vertical motion of particles (Multi-Component Geophones also exists)
Should be responsive to all the useful frequency range
Seismic Data Acquisition - Land
Seismic Data Acquisition - Land
*Energy Source
ReceiverV
EARTH SUBSURFACE Black Box
Recording Equipment
Ground Electronics
Energy Source : Where to keep ? Dynamite : (Sub-Surface Source)
In shallow sub-surface (5-50 mt.) by drilling Shot Holes In a medium having compact formation (Sticky Clay) having
Higher Velocity for proper energy transmission This medium is identified by conducting Up-Hole or SR
surveys and making a Near Surface Model Source Wave form can not be controlled
Vibroseis : (Surface Source) Used when dynamite can not be used Inhabited areas and
Sandy areas A metallic plate overlaid with heavy weight is vibrated in a pre-
set frequency pattern (Sweep) Controlled Source
Seismic Data Acquisition - Land
VIBRATOR
T-D Curve From Uphole Surveys
Up-Hole Survey Pulse Amplitude Study
Near Surface Model
OD
DD
Receivers : How many ? Where to place ? Geophones : (Group of sensors connected in Series)
Planted on the Surface coupled Vertically to the surface At many places (100s in 2D to 1000s in 3D) in a regular grid
surrounding the Source (Shot Point) Location to catch reflected signals from Dipping Layers
A pair of source-receiver generates a Seismic Trace The Distance between Source and Receiver is Called Offset Offsets ranging from zero to 5000-6000 mt. (or more) at
intervals of 30 / 40 mt. are covered Thumb rule : Maximum offset equal to the Target depth
Seismic Data Acquisition - Land
Geophone
Recording System : The signals (Seismic Traces) received by the
Geophones are : Amplified Digitized / Sampled Delta Sigma Technology Formatted in SEG standards Analyzed for Quality Recorded on Storage media
The Trace Data along with Meta Data viz., Positional Information, Observers Report, XYZ of Sources and Receivers is sent to Processing Centers
Seismic Data Acquisition - Land
FIELD LAYOUT
Split Spread: Minimizes shadow zones Ensure up dip shooting for half of ray paths Cost effective Easy to correct positioning of shot location in processing
centre In-between the pickets provides better offsets.
End On: Limited number of channels and required far offset is large.
ASS: Interest is shallow as well as deep
TYPE OF RECEIVER SPREAD
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
Surface diagram or Surface stacking chart ( shot/receiver and shot/midpoint coordinate system )
Xr
Xs
Xs
Xm
Common shot (S-7)
Common offset (7GI)
Common receiver (R-12)
Common Midpoint (CDP-15.5)
Common Midpoint (CDP-15.5)Common receiver (R-12)
Common offset (7GI)
Common shot (S-7)
#1 #2 #3 # 4 . #47 #48
PKt 1 2 3 4 47 48 49 50 51 52 53 54 55 56
3
4
5
67
8
9
1011
12
12
SWATH-1
SWATH-2
Different gather type
Red denotes shot point position
Green denotes receiver position
Yellow denotes coincidence of short and receiver position
CMP/CDP GatherCMP
CDP/CMP Gather
Seismic Data AcquisitionLandIntroductionIntroductionIntroductionIntroductionIntroductionIntroductionIntroductionStages of ExplorationStages of Exploration3D Seismic ImagingViewing 3D data in many ways2D SEISMICFinal Processed Data(2D)3D SEISMIC Final Processed Data(3D)Objective of Seismic SurveyPrinciples of Seismic Reflection SurveySeismic events2D Seismic RecordDifferent gather typeCMP/CDP GatherCDP/CMP Gather