Geophysical data acquisition-land.pdf

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


*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


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


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


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

Documents

Geophysical data acquisition-land.pdf