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October 4, 2006

SEISMIC BASICS

Filed under: Basic knowledge — haris @ 9:25 am

Basic PrincipleSeismic surveying is that sound is reflected at an interface materials that have sufficiently different acoustic properties. Reflectivity is determined by density and acoustic velocity, which greatly with lithology and less so with pore fluid composition.

SOURCEAt sea, à an explosive or a burst of air bubbles from an air gun.On land à an explosive charge or a vibrator (Farmer et.al., 1993)

PROCESSINGDemultiplexing à edit and organize dataStatic correction à different datum, different arrival timeDeconvolution à compressing waveletVelocity analysis à bring in same timeStacking à combine data from different recording (CMP, NMO gather)Migration à placing reflector in its true position (Badley, 1985)

Velocity increases with depth, because rocks are older and more compacted.Predominant frequency decrease with depth, because the higher frequencies in the seismic signal are more quickly attenuated

Wavelength increases significantly with depth à making resolution poorer. (Brown, 1999)

MIGRATION Is principle technique for improving horizontal resolution, and in doing to perform three distinct functions?Migration Process· Repositions out-of-place because of dip· Focuses energy spread over a freshnel zone· Collapse diffraction patterns from points and edge

Migrated benefits· Intersected events will be gone· Confusion of diffraction patterns will be gone· Dipping events will be moved to their correct subsurface position

Amplitude more accurate

Velocity, depend on Depth à increasing with depth Density à increasing with density Grain size à increasing with grain size External pressure à increasing with external pressure Rock age à increasing with age Porosity à decreasing with porosity Temperature à decreasing with temperature Gas saturation à decreasing with gas temperature

RESOLUTIONA minimum space of two objects that can be separated by seismic wave.Seismic = 5-10 meters or 15-30 feetLog = 0.15 m or ½ feet

Continuity : menggambarkan kemenerusan horisontal suatu refleksi (Badley, 1985)Amplitudo : ketinggian dari puncak (peak/trough) reflektor dalam satu trace yang tergantung kepada koefisien refleksi (Badley, 1985).Frekuensi : banyaknya refleksi per satuan waktu (Badley, 1985).

TECHNOLOGY THROUGH TIMESeismic à structureSeismic à lithology from attributeAVO à Offset seismic for fluid prediction (qualitative)Inversion à lithology from Acoustic ImpedanceEI à elastic impedanceSADI à simultaneous impedance rock properties fluidGeostatistic à Resolution and varianceAVOis a seismic technique that uses pre-stack seismic data, instead of the more normally used post-stack data, to detect the presence of hydrocarbons in the reservoir.Three physical parameters of seismic data are fundamental to seismic interpretation-density, P-wave velocity and S-wave velocity. Understanding these is necessary to application of AVO technology. Density, P- and S-wave velocity data examples were used to demonstrate how to construct a reservoir model and show the AVO response of the model.

From Well Logs to SEISMIC INVERSION· There are properties measured in the wells can be derived from seismic, like AI, EI, Poisson Ratio, etc· Lateral information from well is poor, vertical is good· Lateral information from seismic is good/better, vertical is poor· Basic ideas of inversion: start characterizing reservoir from well and continue with seismic to get full information on reservoir and fluid distributionSeismic Inversion Benefits· Look more like geology· Easier to interpret horizon, fault, stratigraphic unit· Forces well ties to be made and understood· Areally extension· Possibility of quantitative predictions of reservoir properties

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