Apds - Materi Prauts 2019.pdf

AKUISISI & PENGOLAHAN DATA SEISMIK REFLEKSI RUHUL FIRDAUS, MT.

Teknik Geofisika Institut Teknologi Sumatera TA. 2018/2019

Penilaian • Komponen Penilaian

UTS 20%, UAS 25%, Prakt 25%, Keaktifan 25% • Grading • A • AB • B • BC • C • D • E

75 - 100 70 - 75 60 - 70 50 - 60 45 - 50 20 - 45 0 – 20

• Tidak hadir > 3 UAS=0 • Kecurangan Nilai D

TEORI ELASTISITAS

Stress (σij) σij = Stress dengan arah -i bekerja pada permukaan -j

Strain (εij) εij = Strain dengan arah -i pada permukaan -j

x

Normal strain

Shear strain

Transformasi dan Dilatasi

Pertambahan volume dapat didekati ≈ 𝑣 = 𝑥. 𝑦. 𝑧

ln 𝑉 = ln 𝑥 + ln 𝑦 + ln 𝑧

∆𝑉 ∆𝑥 ∆𝑦 ∆𝑧 = + + 𝑉 𝑥 𝑦 𝑧

∆=

∆𝑉 = 𝜀𝑥𝑥 + 𝜀𝑦𝑦 + 𝜀𝑧𝑧 𝑉

Hukum Hooke Menyatakan hubungan Stress dan Strain

Konstanta elastik

E (Modulus Young) σ (Poisson’s Ratio) k (Modulus Bulk) μ (Modulus Shear)

Hubungan Konstanta Elastik

Persamaan Gelombang

Persamaan Gelombang P Diferensialkan terhadap x

Diferensialkan terhadap y

Diferensialkan terhadap z

Maka akan kita peroleh

Kemudian jumlahkan

Persamaan Gelombang S Diferensialkan terhadap z Kemudian kurangkan Diferensialkan terhadap y Kita akan peroleh

Solusi Persamaan Gelombang Gelombang menjalar pada ruang 1 dimensi

Propagasi Gelombang Elastik

KONSEP DASAR

Konsep Dasar • Definisi Gelombang

• Hukum Snell • Prinsip Huygens • Difraksi gelombang • Atenuasi Gelombang • Kecepatan rambat gelombang seismik • Gelombang seismik refraksi, refleksi, transmisi • Partisi gelombang di bidang batas • Koefisien refleksi dan koefisien transmisi • Gelombang permukaan dan dispersi • Gelombang langsung, refraksi dan refleksi

Definisi Gelombang Gangguan yang merambat melalui suatu medium dari satu lokasi ke lokasi lain atau transportasi energi dari satu lokasi (sumber) ke lokasi lain tanpa perpindahan materi.

Body Wave • Gelombang P

Compressional Wave (P-Wave)

• Gelombang primer, menjalar paling cepat • Gelombang kompresi-dilatasi • Gelombang longitudinal, arah osilasi partikel

searah dengan arah rambat gelombang • Dapat merambat pada medium padat maupun fluida seperti udara, air dan lapisan bumi yang cair • Vp = 0.3 – 8 km/s, • Propagation velocity Vp = [ ( K + 4m/3 )/r ] 1/2 Shear Wave (S-Wave) • Gelombang S

• Gelombang sekunder, gelombang geser, shear •

• • •

wave Gelombang transversal, arah osilasi partikel tegak lurus dengan arah rambat gelombang Kecepatan rambat lebih rendah dari gelombang-P Tidak merambat pada medium fluida Propagation velocity Vs = [ m/r ] 1/2

Surface Wave Rayleigh Wave (R-Wave)

• Gelombang Rayleigh (R-wave) • Gerak partikel eliptik searah dengan arah rambat gelombang • Amplitudo

berkurang seiring menjauhi bidang permukaan • Frekuensi yang lebih rendah dapat masuk lebih dalam

• Gelombang Love (L-wave) • Gerak

partikel transversal horizontal berarah tegak lurus arah perambatan tetapi sejajar dengan bidang permukaan bumi • Frekuensi lebih rendah menjalar lebih cepat

Love Wave (L-Wave)

Seismic Wave Propagation Wavefront Perspective

Raypath Perspective

http://www.geophysics.rice.edu/department/faculty/zelt/faeroe/faeroe.html

Prinsip Huygen

Hukum Snell* (Akustik)

*Fermat’s Least Time Principle

METODA AKUSTIK SUMBER

DETECTOR

REFLECTION

ACOUSTIC WAVE

RAYPATH

BOUNDARY BETWEAN ROCK LAYERS

TRANSMISSION

Hukum Snell (Elastik)

sin f1 = sin f2 = sin f3 Vp1 Vp2 Vs3

Critical Refraction

sin ic = sin (90º) V1 V2

sin ic = V1 V2

Seismic Refraction Waves passing from slow to fast medium

Snell’s Law

Snell’s Law

METODA SEISMIK REFLEKSI

The Basic Process of Seismic Exploration  Excite a seismic wave  Record the seismic wave  Reconstruct the process of the wave propagation

From the recorded wave, including its reflection point and ray paths. The geological information can then inferred from the reconstructed images.

Signal and Noise Signal : desired Noise : not desired So for reflection seismology: - Primary reflections are signal - Everything else is noise ! - Direct wave through first layer - Direct air wave - Surface wave

- Refraction - Multiple reflections,

Aim for Signal, Get over Noises Signal Noise

=

Primary Reflected Energy The Others

Goal of Seismic Reflection Survey : Maximize reflected energy and suppress all other type of waves

Goal of Processing : Remove effects of all-the-others while maintaining the primary reflection energy and focus the energy to where it comes from. In other word, maximize Signal to Noise Ratio (SNR).

Reflection Seismology

Basic Principles - Land

Land Seismic Survey  Shooter Truck  Shooter-man  Recorder Truck  Drill Truck  Radio / Communicatio n Line  Geophone Cable Line  Shot Point  Seismogram

Desain Survey (2D and 3D)

Basic Principles - Marine

Marine Seismic Survey

3D Marine Seismic Acquisition

Image courtesy CGG Veritas http://gcaptain.com/spotd-cggs-geo-coral-drags-full-spread/

Marine data

Land data

Split shot gather

ACQUISITION OBJECTIVES

Managerial Consideration • Cost

Acquiring seismic data is more expensive by a factor of 210 times than processing • Sustainability

Once data are acquired correctly can be processed and interpreted a number of times • Planning

The choice of acquisition parameters is very important

Improvement in the quality of seismic section

Objectives • Maximize the recording of primary reflections and

minimize the recording of noise • Maximize signal to noise ration (SNR) within the

constraints of the cost and the recording environment

Technical Considerations 1. Geological Objectives • Depth to target horizon • Maximum dip of horizon • Lateral & vertical resolution of the layering

2. Transmissivity of the Earth Amplitude, phase and band-width • • • •

Geometrical spreading / Spherical Divergence Reflection interference Absorbsion (intrinsic attenuation) Scattering (extrinsic attenuation)

3. Recording Environment • Convolution model • Noises

4. Recording System

Geological Objectives Depth to Target Horizon

Rule of thumb: the seismic reflection spread length should be about the same as the depth to the horizon target

Reason: to obtain enough move-out on our target reflection to differentiate between signal and noise

Geological Objectives Maximum Dip Need to have additional coverage at the edge of the region of interest to: • Obtain full-fold of coverage of area of interest (dip independent) • To account for the migration aperture

Geological Objectives Vertical Resolution Vertical Resolution answers the question as to what controls the thickness of the bed we can resolve using seismic reflections.

Geological Objectives Vertical Resolution

Geological Objectives Vertical Resolution

Geological Objectives Vertical Resolution • Resolution is the ability to distinguish distinct

events • Thin bed response occurs below tuning

thickness • Short-duration seismic pulses are preferred • Broad bandwidth, zero-phase pulses are best • Pulses with minimal side-lobe energy enhance

interpretability • To Improve Resolution • Bandwidth can be increased by deconvolution • Frequencies to be included must have adequate S/N

Geological Objectives Vertical Resolution

Input Parameters:

Pulse

Peak Frequency of the pulse at the zone of interest Computations: Period = 1/Peak Frequency

Period (ms)

Velocity at the zone of interest

wavelength = period X velocity

Wavelength = Period * Velocity

Limit of Vertical Resolution = Wavelength/4

Geological Objectives Lateral Resolution Lateral Resolution answers the question what controls the accurately with which we can determine the termination of a bed using seismic reflections. • Migration enhances lateral resolution

• Large aperture (receiver cable length) is needed for high

lateral resolution • Fine spatial sampling is needed for high lateral resolution • Prestack migration provides better lateral resolution than

poststack migration • Depth migration provides better resolution than time

migration

Geological Objectives Lateral Resolution What is the minimum horizontal distance between two subsurface features such that we can tell them apart seismically? Reflections from Reflector with Gaps

Geological Objectives Lateral Resolution – The Fresnel Zone • An event observed at a detector is reflected from a zone of points • The raypaths from source to detector which differ in length by less than a quarter wavelength can interfere constructively • The portion of the reflector from which they add constructively is the Fresnel zone

• Changes that occur within this zone are difficult to resolve • The size of the Fresnel zone depends upon the wavelength of the pulse and the depth of the reflector

Migration Reduces Lateral Smearing Ideal / Model Response 800 m

Stack No Migration

Image After Migration

Fresnel Zone Equations Pre-Migration

Post-Migration

Fd = Vavg T/F

Fd = λ /4 = Vavg /4 F

where:

Fd = Fresnel Diameter Vavg = Average Velocity T = Time F = Frequency of Pulse λ = Wavelength

TIME-OFFSET (T-X) RELATIONSHIP

Direct, Reflection & Head Waves

Direct, Reflection & Head Raypath from Source to Receivers

𝑋𝑖 = 𝑜𝑓𝑓𝑠𝑒𝑡 (𝑘𝑚)

𝑧

𝑇𝑑1 𝑇ℎ1 𝑇𝑟1

𝑇𝑑2 𝑇ℎ2 𝑇𝑟2

𝑇𝑑3 𝑇ℎ3 𝑇𝑟3

𝑇𝑑𝑖 𝑇ℎ𝑖 𝑇𝑟𝑖

Direct Wave

Refracted Wave at critical angle

Head Wave

Head Wave

Reflected Wave

𝑻𝒙=𝟎 , 𝒗𝟏 , 𝒛 ?

Reflection Hyperbola

All the waves together

T-X modeling as function of depth and velocity

Discussion

offend

What the difference of the two data? How we get them? Can you describe each waves in the data ? (some of them already marked)

split spread

Data with Groundroll Noise

Data with Attenuated Groundroll-Noise

Latihan Seabed Reflektor Reflektor 1

Reflektor 2

Reflektor 3

What the depth dan velocity of seabed and reflector 2 ?

Tentukan Vrms & z reflektor yang Anda pilih

Interval Velocity How to define the velocity for each interval ?

Multiple Horizontal Reflector Layers

Latihan Seabed Reflektor Reflektor 1

What the depth dan velocity of seabed and reflector 2 ?

Reflektor 2

Reflektor 3

Compute interval velocity bertween marked-reflectors. Interval:  Seabed-R1  R1-R2  R2-R3

DATA PROCESSING

Basic Processing Flow • Insert geometry into raw •

• • •

•

data (CSG) Sorting of seismic data Normal Move-out and Velocity Analysis Stacking Zero-offset Migration Time-Depth Conversion

Raw Data (Aux and Trace)

The Main 4 Processing Steps

Koreksi Semua posisi shot dan receiver seolah-olah sudah dipindah ke Common Midpoint

Zero Offset (ZO)

ZO-Stack Section

Time-Migrated Stack

Processing Goal

https://www.geoexpro.com/articles/2015/07/exploring-the-mid-north-sea-high

Final Presentation Product

Depth-Migrated Seismic Section

Interval Velocity Model

Processing Tools • Correlation

• Deconvolution • Frequency Filtering (1D FT) • Velocity Filtering (2D FT, f-k transform, tau-p transform) • Wave Modelling

KONSEP EKSPLORASI MIGAS

Basin (From Surface to Basement) • Weathering layer – upper most

• Bed rock – upper most hard matter (stone) underground • Stratified rock – sedimentary rock • Basement rock – crystalline rocks (igneous rock or

metamorphic rock)

Petroleum System – Target Eksplorasi • Source Rock : Kitchen – Temperature – Burial Depth -

Age • Carrier Bed : Migration Path – Permeable Rock • Seal Rock : Soft Rock • Trap : Stratigraphic (pinch out) or Structural (fold, fault)

Tahapan Eksplorasi Migas • Survey Permukaan – Pemetaan Geologi • Mendapatkan peta geologi permukaan (litologi, struktur, stratigrafi) • Mendapatkan Kolom Stratigrafi • Survey Awal Geofisika – Gaya Berat & Magnet • Mengetahui geometri cekungan (basin) bawah permukaan • Menentukan arah lintasan seismik 2D yang optimal

• Reconaissance – Seismik 2D • Konfirmasi data geologi • Membangun konsep geologi regional hingga lokal • Penentuan lokasi sumur eksplorasi (basement test) • Survey Detail – Seismik 3D • Stratigrafi detail • Monitoring – Seismik 4D • Penentuan sisa cadangan (reserve)

BASIC TECHNIQUES

Basic Concepts  Provide the main source of data about the subsurface at (or

approaching) reservoir scale  The main principle of seismic is to send a vibration (elastic wave) down to our target, and allow this energy to interact with the target.  Some of this energy propagates back to the surface and is recorded. Unfortunately we also record many other interactions.

“Exploration seismology deals with the use of artificially generated elastic waves to locate mineral deposits (including hydrocarbons, ores, water, geothermal reservoirs, etc.), archeological sites, and to obtain geological information for engineering.” Sheriff & Gerdart, 1995

Basic Concepts • A source is generated such as an explosion

• The waves propagate through an elastic medium • At layer interface, the wave reflected, refracted and •

• • •

converted Waves reflected back to the surface along with other type of waves (direct wave, refracted wave, surface wave) are recorded at a receiver The arrival time of waves represent Earth’s layering structure The intensity (amplitude) hold information about physical properties of rocks Seismic interpretation estimates geometry and properties of the Earth.

Basic Concepts We try to extract the relatively small part relating to our target interaction. • A complex task (non-unique, not an objective solution) • Interpretation (Expected Geology) is highly non-unique

• Survey design & Data processing involves subjective

interpretation

Seismic Acquisition / Survey Design • Each shot is recorded by a number of receiver

• 2D land or marine • Pattern • Off-end (marine and land survey) • Split spread (land survey only)

http://www.oilinuganda.org/features/environment/ugandapioneers-3d-seismic-surveys.html

https://krisenergy.com/company/about-oil-andgas/exploration/

Land Seismic Survey

Marine Seismic Survey

Basic Techniques Receivers Source

Target reflector

Expected Geology

Basic techniques: measuring travel times from seismic time series

Raw Data

Raw Data

Data Sorting • Common Shot Gather

• Common Receiver Gather • Common Offset Gather • Common Midpoint Gather • Common Depthpoint Gather

Basic Flow Seismic Data Processing  Shot gather (raw data)

 CMP gather (data sorting)  NMO (zero offset data)  Stacking (higher SNR data)  Time Migration

Advance Technique Seismic Data Processing • Groundroll Filtering

• Multiple Attenuation • Static Correction • Deconvolution • VSP application • Depth Migration • Full-Waveform Inversion • WEM • Reverse Time Migration (RTM)

• Pre-stack migration (PSDM) • Seismic Attribute

Image Goal

Interpreted       

Structure Stratigraphy Lithofacies Depositional Environment Paleogeography Petroleum System Hidrocarbon Indicator (DHI, AVO, EEI)

3D Survey for Detail Structures & Rock Properties Comparison to 2D data  Smaller coverage  Higher image resolution  More realistic image  Better interpretation

SURVEY PARAMETER

• Source Parameter • Source type : dynamit, gel, spiker, vibroseis, hammer, airgun • Energy • Waveform (Wavelet) • Hole Depth • Coupling • Location (coordinate, station) • Receiver Parameter • Receiver type : geophone, hidrophone • Respon Function • Band-type (broadband, narrow band, high frequency, low frequency) • Location (coordinate, station) • Radial or Vertical receiver • Source – Receiver Relationship (Geometry) • Offset • Pattern (off-end, split spread)

Acquisition Setup • Single channel measurements (profiling)

Only one source and receiver are used with often an equal distance between the source and receiver. This is repeated for several positions along a line. • Multi channel measurements

• Multi channel systems use one source and several receivers, which

measure at the same time. Several spreads are possible to orient the sources and receiver:

Types of reflection spreads. The symbol o and + represent source and geophone-group center locations, respectively.

CDP, CMP and Zero-Offset, Common Offset There are different possibilities to sort the data: • Common shot - all traces, that belong to the same shot • Common midpoint (CMP) - all traces with the same midpoint • Common receiver - all traces, recorded with the same geophone • Common offset - all traces with the same offset between shot and geophone

Difference between CMP and CDP: For a horizontal Reflector all traces that have the same midpoint, have also the same reflectionpoint in the subsurface. Is the layer inclined than the traces have a different reflection point.

Acquisition Steps

Acquisition Steps (non-geophysical jobs) • Acquisition designing

• Bench marking or tail bouy (total station GPS) • Determining position of receiver and Shot point • Environmental analysis, social issue, Base camp, etc

Acquisition Steps (geophysical jobs) • Field Test – Noise test (land only) • To determine wavelength of noise (e.g. groundroll) for optimum receiver

array. – Offset test • To determine optimum distance of near-offset and far offset.

– Charge test • To determine optimum source power

– Charge Depth • To determine optimum source depth. This due to prevent blow up and

minimize static correction.

• Shot Geometry • Off End Spread • Split Spread • Alternating Spread

• Length of Line and Direction of Line Primary direction of line have to perpendicular with target zone D = L + 2Z D>Z

• Far Offset • Very dependent on stretch-mute, multiple attenuation, and velocity resolution (see text book for detail)

Far Offset

Xmax > Z

• Near Offset. Near Offset

Near offset very dependent on shallow target horizon. Most likely, near offset distance is 1,5 times or same distance with receiver interval.

• Group Interval Group interval can be determined from number of channel and length of line.

q = 1 for off end q = 2 for split-spread

This equation used to prevent angle aliasing in migration process Group Interval

• Fold Coverage

Fold can be defined as one point that mapped by n amount of seismic ray. Higher fold means higher S/N ratio.

• Source Interval

source interval

The most optimum distance for source interval is The same distance as Group Interval

• Number of Shots

Number of shots can be calculated from ratio of Length of Line with Source Interval

• Sampling Rate

Alliasing must take account into sampling rate

• Array Configuration

Array Configuration must be arranged according to the wavelength of groundroll, so that signal will be amplified and groundroll will be attenuated

Geophone-Group (Array) • To amplify the wanted signals and to suppress the unwanted signals like

surfacewaves • The signal of the separate geophones are added to one signal for the whole geophone group. • The directional response of any linear array is governed by the relationship between the apparent wavelength λa of a wave in the derection of the array, the number of elements n in the array and their spacing Δx.

SEISMIC RESOLUTION

Seismic Resolution Vertical Resolution • Resolution vs Detection • Thin Bed Response and Tuning

Lateral Resolution • Fresnel Zone

• Migration and Lateral Resolution

Resolution vs. Detection Detection:

Ability to identify that some feature exists

Resolution: Ability to distinguish two features from one another • Detection limit is always smaller than the resolution limit • Detection limit depends upon Signal-to-Noise

Lateral Resolution Would we image the narrow horst?

Would we image all three channel sands?

Fresnel Zone

KB&H, 2002

Rf = (lz/2)1/2 Rf = (Vavg/2)(t/f)1/2

S&D, 1995

Amplitude & Reflector Curvature

“Brighten Up” Ratio

3D

S = Sflat

1 1 - rw/ri

2D

S = Sflat

1 1 - rw/ri

S = amplitude from curved reflector Sflat = amp from flat reflector rw = radius of curvature of wavefront ri = radius of curvature of reflector

“focussing”

Anstey 77

Fresnel Zone in 3D

“Sideswipe” Line 1

Line 2

Migration Increase Signal Focus and Resolution Ideal / Model Response 800 m

Stack No Migration

Image After Migration

Good Migration Enhances Resolution Better

Standard Migration

High-end Migration

Fresnel Zone Equations Pre-Migration

Post-Migration

Fd = Vavg T/F

Fd = λ /4 = Vavg /4 F

where:

Fd = Fresnel Diameter Vavg = Average Velocity T = Time F = Frequency of Pulse λ = Wavelength

Summary: Lateral Resolution • Migration enhances lateral resolution

• Large aperture (receiver cable length) is needed for

high lateral resolution • Fine spatial sampling is needed for high lateral

resolution • Prestack migration provides better lateral resolution

than poststack migration • Depth migration provides better resolution than time

migration

Shale Baseline

For Example: Based on seismic data, could you determine that there is a thin shale layer between the two sands?

Sand

Sd

What is the minimum vertical distance between two subsurface features such that we can tell them apart seismically?

Gamma Ray

Shale

Vertical Resolution

Seismic Resolution v.s. Log Resolution

Gas “lens”

Wedge

Waveform Interference (thin beds)

Reflection Points

Amplitude & Tuning

Amplitude & Tuning

S&G 95

Summary: Vertical Resolution • Resolution is the ability to distinguish distinct events • Thin bed response occurs below tuning thickness • Short-duration seismic pulses are preferred • Broad bandwidth, zero-phase pulses are best • Pulses with minimal side-lobe energy enhance interpretability

• To Improve Resolution • Bandwidth can be increased by deconvolution • Frequencies to be included must have adequate S/N

SEISMIC AMPLITUDE

Common-shot Gathers

Seismic Amplitude Decay

Factors Affecting Amplitude

Factors Affecting Amplitude • Source magnitude & Coupling

• Geophone sensitivity & Coupling • Wave-Medium Interaction • Attenuation • Spherical Divergence

• Scattering • Absorbtion

• Layer Interface curvature and rugosity • Reflectivity Coefficient • Transmisssivity Coefficient • Raypath from Source to Geophone • Multiple reflection (e.g. Shortpath multiples)

Spherical DivergenceAnstey (1977)

A = a 1/r = 1/(Vt) >>> 1/(V2t)

Vertical (Normal) Incidence Zero offset Ai

rv= acoustic impedance

Ar

𝒗𝟏 𝝆 𝟏 𝒗𝟐 𝝆𝟐

At RC = Ar Ai TC = At Ai

= =

r2v2 – r1v1 = Z2 – Z1 r2v2 +r1v1 Z2 + Z1 2 r1v1 r2v2 + r1v1

=

2 Z1 Z2 + Z1

Transmission Loss 𝑻𝑳 = ? A0 = 1

RC1 (1-RC1) RC2 (1-RC1) = ... (1-RC1)2 RC2 = (TL) RC2

?

RC1 (1-RC1)

(1-RC1) RC2

(1-RC1)

(1-RC1) RC2

RC2

(1-RC1) (1-RC2)

(1-RC1) RC2 RC1

Non-Vertical Incidence Amplitude variation with offset

Zoeppritz’s Equations

Anelastic Attenuation A = G A0 e - ar a = pf QV a = attenuation coefficient f = frequency Q = quality factor V = velocity

Factors Affecting Amplitude

THANKS Good luck for your midtest