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SEMINAR
PERANAN FISIKA DALAM EKSPLORASI
MINYAK DAN GAS
METODE EKSPLORASI MINYAK & GAS
METODE GEOFISIKA
UNIVERSITAS MATARAM 23-24 SEPTEMBER 2011
OLEH:
WAHYU TRIYONO
PERTAMINA PHE ONWJ
Lingkup Kerja Geofisika
1. PENGUKURAN GRAVITY/MAGNETIK UNTUK STUDY
AWAL
2. PENGAMBILAN DATA SEISMIK
3. PENGOLAHAN DATA SEISMIK
4. INTERPRETASI DATA SEISMIK
5. PEMETAAN BAWAH PERMUKAAN
6. PENENTUAN LOKASI PENGEBORAN
7. PENENTUAN KEDALAMAN PENGEBORAN
8. PENGAMBILANG DATA KECEPATAN SUMUR
(CHECK SHOT/VSP)
Seismic Acquisition
Akuisisi data seismik terdiri dari:
1. Land Acquisition
2. Marine Acquisition
3. Transition
Seismic• sources and recek/ers On land On water
Sources:
Impact
Sledpi e harnmer
Drop-weight
Accelerated \veiailt
Impulsive Dynamite
Detonating cord
A irgun
Shotgun
Borehole sparker
Airoun Pinger
Gas gill] Boomer
Sleevo gun Sparker
Water gun Steam gun
\ ibrator Vibroseis
Vibrator plate
Rayleigh wave generator
Multipulse
Geochicp
Receivers: Kieophonc. accelerometer I lydrophones (streamers)
IL Arff PERTAMINA
PHE ONWJ bpmigas
Land Acquisition
• Sumber gelombang (source): Dinamit, vibroseis
• Receiver: Geophone
• Recording unit menggunakan truk atau mobile unit
Source
Dynamite
Land Acquisition Jenis-jenis Vibroseis
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.
Jenis-jenis Geophone
Kondisi Seismik Akuisisi Darat
Schematic Land Acquisition
Marine Acquisition
•Sumber gelombang (source): air gun
•Receiver: Hydrophone
•Recording unit: menggunakan kapal
• Reciever dibentangkan menggunakan
streamer
Marine Streamers
• ~6 km length
• Groups of hydrophones every 25
meters
“birds” receivers
Marine Acquisition
Seismic Acquisition
MARINE SEISMIC DATA ACQUISITION
Transition Zone Acq
•Sumber gelombang (source): air gun &
Dynamite
•Receiver: Hydrophone & Geophone
•Reciever dibentangkan menggunakan kabel
•Area di sekitar pantai atau laut dangkal yang
tidak bisa dilewati kapal
TEORI SEISMIK
HUKUM SNELL
KECEPATAN GELOMBANG SEISMIK
Faktor-faktor yang mempengaruhi :
Bulk density
Porositas
Perbedaan tekanan
Fluida pori
Litologi
KECEPATAN GELOMBANG & LITOLOGI
GL-5044 INTERPRETASI SEISMIK GEOLOGI
Waves
fvvelocity
kwavenumber
fperiod
.
2
21
WAVELET
Definisi :
gelombang dengan durasi waktu (t) yang pendek
yang dihasilkan oleh suatu impuls
Dalam pengolahan data seismik biasa digunakan
dua jenis wavelet
GL-5044 INTERPRETASI SEISMIK GEOLOGI
Minimum phase and zero-phase wavelet
Energy is maximally front-loaded
(Energy does not arrive before zero time) Symmetric with respect to zero time
and peaks at zero time
(Energy arrives also before zero time)
Instrument Balance
Noise
Source Strength-9 Interference and Coupling of Different Events Array Directivity
Scattering
Tran,missivity
Peg—Leg Multiples from Thin Reflectors
Spherical Divergence Absorption
Reflection Coefficient Reflection Curvature
Variation of Reflection and Rugosity
Coefficient with Incident Angle
Superimposed Geophone Sensitivity and Coupling
IllfatiVer V V
FACTORS WHICH AFFECT AMPLITUDE
PERTAMINA PHE ONWJ
Exam & OF r0.110CtiOn CO.ISrnO9rarn
Morriant of clischar9E
First arrivirlEi icripulses
0 100 200 300. 41X1 1:o4) 600 703
Vibrator Apcoming trkr.K.
Arm PERTAMINA PHE ONWJ bpm igas
Shot geometry variables
Stacking
CMP Stack a CMP gather
Stack Repeat separately for each gather
• Improve Signal—to—Noise
• Cancel Multiples ▪ One Trace Per Reflection Point • Easy—to--interpret Times
IL Arff PERTAMINA
PHE ONWJ
Example of stacking to attenuate multiple
........ ...... •01■ ,1/4.
Primary Reflector 2 STACK --A,
S I G NALIT 0-NOISE AIIPUTUDE IMPROVEMENT
Arrival Time
1 Multiple ) liN
sec 6—CMP Input Traces Stacked after NMO Correction Trace
where NI a Immalhot off mak= tow two% %WWI
EMII, Ar‘k PERTAMINA PHE ONWJ
Definisi Fold
Sampling
By measurements using a digital system, the data is not continuously measured,
but at a specific time interval measured and transported to the AD-converter.
Analogue representation
of a sinusoidal function Digital representation of a
sinusoidal function
NOISE
Definisi : gangguan yang sering ditemui pada rekaman data seismik
Noise dapat dibagi dua :
Noise koheren
Noise tak-koheren
GL-5044 INTERPRETASI SEISMIK GEOLOGI
GL-5044 INTERPRETASI SEISMIK GEOLOGI
Noise
The S/N ratio “Signal-to-noise ratio” gives the ratio between
the amplitude of a
signal (e.g.Reflections) to background noise(“random noise”)
or noise sources
(“coherent noise”).
One aim of the dataprocessing is to increase the S/N ratio.
Coherent Noise
• Sided-scattered noise
From irregularities or the point scatters
• Multiples (repeated pattern)
Ghost, Simple multiple, Water-bottom multiple,
Peg-leg multiple, Inter-bed multiple
• Vibration noise
• Interface wave
• Ground roll (Rayleigh surface waves on land survey)
Low frequency
Strong amplitude
Low group velocity
• Guided waves (refractions and head waves on shallow marine survey)
Strong velocity contrast with the substratum
Early arrivals
Multiples
• Long-path multiples
– (occur when exceptionally large reflectioncoefficients are present):
-> Ghost reflections, where rays from a buried explosion on land
(or an airgun in water) are reflected back from the ground surface
(or sea surface) to produce a reflection event, known as a ghost
reflection, that arrives a short time after the primary.
-> Water layer reverberations, where rays from a marine source
are repeatedly reflected at the sea bed and sea surface
• Short-part multiples (“peg-leg multiple”):
– Involve only a short additional path length to arrive so soon after
the primary event that they extend the overall length of the pulse.
(Multiples between two interfaces of a layer)
Ray paths of some common multiple families
Source ghost Receiver ghost Simple multiple
Water bottom multiple Peg-leg multiple Inter-bed multiple
96 bpmigas
PERTAMINA PHE ON\NJ
Random Noise
• Instrument noise
• Machinery
• Power line
• Cable noise
A, B, C, D are guided waves
trapped (refracted waves) at the sea
floor. They have a dispersive
character with low frequencies C
arriving first, higher frequencies B
arriving next, and moderate
frequencies D arriving last
A = weak reflectors
B = strong reflectors
C suggests a fault in spread
D = ground roll
Deep water shot record
• A is direct arrival
• B is water-bottom reflection
• C is shallow reflection
• M is 1rst water-bottom multiple, note double
time of B
• PL is water-bottom and peg-leg multiples of C
Resolution of two boundaries depends on
wavelength
Decreasing image separation
Resolution
Resolution refers to the minimum separation between two features such that we
can tell that there are two features rather than only one.
Comparison between the wavelength of a 30-Hz Signal, Big Ben and a Log of a drilling.
Geological model, reflection series and seismogram
Contoh Hasil Field Record
Shot Gather
Shot gather dari
seismik data darat:
A. Direct wave
B.Head wave
C. Ground roll (surface
wave)
D. Reflection events
Mann et al. (2004)
Representation
The traveltime is in general shown with increasing time along the vertical Increasing downwards
(larger traveltime corresponds to a larger depth). There are several ways to represent seismic
traces. The sort of representation depend on the processing used, but also on the number of traces.
Reflectivity
series
Wiggle
only
Variable
area
Variable
density Variable
area+wiggle
Different representations of a seismic trace.
Seismogram
Geological
section
Accoustic
impedance
log
Refflectionc
coefficient
log
Reflectivity
coefficient
Input
pulse
seismic
trace
From a geological subsoil to a seismic trace.
Alur Pemrosesan Data Seismik
• Demultiplexing/Reformatting
• Geometry
• True Amplitude Recovery
• Surface Consistent Amplitude Correction
• Statics Correction
• Velocity Analysis
• Koreksi Normal Move Out (NMO)
• Brute Stack
• Residual Statics
• Dip Move Out
• Velocity Analysis
• DMO Stack
• Migration
Various ways to display seismograms:
Wiggle trace/
Variable trace Variable trace Wiggle trace
Top Formasi
Reef
Sesar/patahan
TWT (Two Way Time) Seismik section Variable area wiggle trace yang sudah
diinterpretasi
Penampang Seismik dengan Variable Density