Tuesday, September 15, 2015

Wavelet Polarity Convention


Minimum phase Ricker wavelet reverse polarity -> Increasing of Reflection Coefficient is located in peak (black)

Minimum phase Ricker wavelet normal polarity -> Increasing of Reflection Coefficients is located in trough (white)

Which one wavelet that is easy to interpret and correlate with increasing impedance/RC ? The answer is minimum phase Ricker wavelet reverse polarity, because it is consistent with increasing impedance and interval velocity values.

Saturday, August 1, 2015

Offset Range in Seismic Acquisition

The offset range is one of the most important attributes. Without an adequate offset range in the bin, the bin is useless for velocity analysis and probably will be a poor stack trace. Near traces represent the best approximation to the zero offset for depth conversion. The middle traces connect the near and far traces for defining the move out hyperbola. In some case, the middle traces contain the best image of the target. The far traces are absolutely essential. Without the far traces there is not enough moveout on the traces to determine the velocity. Other techniques, such as AVO, cannot be applied without the far traces.

Saturday, July 25, 2015

Normal & Reverse Polarity in Seismic



When a minimum phase wavelet  in normal polarity ('a' in picture) strikes a contrast impedance medium then it could be marked in near trough for increasing impedance medium in earth.

When a zero phase wavelet in normal polarity ('b' in picture) strikes a contrast impedance medium then it could be marked in centre of trough for increasing impedance medium in earth.

Diagenetic Processes in Limestone

Limestones are strongly affected by diagenetic processes that accompany lithification. Aragonite will be replaced by calcite (if the original carbonate mineral was aragonite), dissolution may be significant , and calcite may be replaced with dolomite.

Limestone is often impure. Calcium carbonate may be mixed with siliclastic material (clay, sand) in all proportions but calcium carbonate must form the majority. Otherwise, we have calcareous sandstone or marl instead of limestone. The term 'marl' is mostly used in the field and often somewhat muddy limestones, even if they contain more calcium carbonate than mud, are called that way.

Reference: Jackson, J.A. (1997). Glossary of Geology. American Geological Institute. 

Saturday, June 6, 2015

Seismic Facies Analysis

In certain circumstances, it can be a powerfull asset to seismic facies analysis to describe properties such as reflection continuity, frequency and amplitude. Unless there is high well density, these attributes are highly qualitative. Continuous reflections imply uniformity of lithology and discontinuous reflection imply laterally varying lithologies. Reflection continuity is highly prone to interference of noise and multiples, and it's easy to 'over-interpret' lateral and vertical variations in continuity and ascribe genuine lithological reasons for the variability when processing or acquisition artefacts are to blame. Peg-leg multiples generated in an overlying horizontal sequence can produce discontinuity in an underlying dipping continuous sequence simply by interference. These pitfalls also apply to the generation of false onlaps and downlaps particulary in sequences with low signal to noise ratio.

(Deltaic Sedimentation and Its seismic Expression, T. Elliot)

Tuesday, May 26, 2015

Shear Wave Propagation

When a seismic wave is reflected at a boundary , there is generally some conversion from compressional (P) waves to shear (S) waves and vice versa. The amount of conversion, which is zero at normal incidence , increases with increasing incident angle. A great deal of shear wave energy is generated when using surface sources on land. Therefore it is expected that some of the events which correspond to low velocity on the velocity analysis display represent not multiples but primaries that have travelled part of the path as shear waves.


Thursday, April 9, 2015

Gas in Seismic Reflections

The presence of gas in a reservoir often produces a detectable suite of responses in the seismic record; and it is obviously very important for the interpreter to be able to recognize these gas effects.

Acoustic-Impedance Effects


The way in which a reservoir responds to the presence of gas depends on the acoustic impedance of the gas-filled portion of the reservoir, the water-filled reservoir, and the cap rock; and the thickness of the gas-filled interval. If the gas column is thick enough and there is an acoustic-impedance contrast between the gas-/oil or the gas-/water filled portions of a reservoir, a reflection commonly called a flat spot will result.

As a rule of thumb, flat spots are likely to be found in porous sandstones or carbonates down to about 2.5 km. Below this depth the effect of gas on velocity is less marked and the chance of getting a good reflection from a gas contrast is reduced.

Flat spots will always have a positive reflection coefficients, appearing as a trough on seismic sections displayed with SEG normal polarity or a peak on reverse polarity sections. Although gas contacts are usually horizontal in depth, they do not always appear horizontally in time due to the push-down effect of the lower velocity in the gas interval.

Dim spots due to a reduced reflection coefficient of the top-reservoir reflector are more common with less porous or well-compacted sands and carbonate reservoirs. The carbonate would usually have strong positive reflection coefficient. Gas in the reservoir reduces the reflection coefficient, causing the top-reservoir reflector to lose amplitude and dim. Amplitude anomalies are sometimes accompanied by corresponding polarity changes. A polarity reversal of the top-reservoir reflector at the gas-oil or gas-water contact is a common feature of bright spots.

Velocity Effects

If the gas column is sufficiently thick, a push-down may be observed on underlying reflectors.

Diffractions are developed where there is a significant lateral contrast in acoustic impedance and are often seen at the edges of bright spots.

"Gas chimneys" or "gas cloud' -poor data zones above gas-bearing structure are quite common and can be very characteristics. Poor data zones are thought to be caused by scattering of seismic energy by escaped gas penetrating the cap rock above a gas reservoir. Gas leakage into the cap rock can occur through a variety of mechanisms (leakage along fault planes; fractures; or overpressure exceeding the mechanical strength of seal rocks.

Reference : Practical Seismic Interpretation, Michael E. Badley 

Monday, April 6, 2015

Salt Structure chapter 2

Regardless of the mechanism responsible for salt movement, flow of salt into a growing structure creates a withdrawal basin that is a structural low and an isopach thick.

Three main growth stages of salt diapirs observed in the East Texas basin, this model can be used as a basis to interpret seismic-reflector configurations. Diagnostic configurations for the various stages are:


  1. Pillow stage : Syndepositional thnning of sediments over the pillow crests and flanks , developed in response to pillow growth, is the most diagnostic feature of this stage. Only minor thickening usually develops into the primary rim syncline.
  2. Diapir stage : Withdrawal of the salt into the growing diapir leads to a collapse of the flanking sequence that thinned toward the original pillow. A secondary rim syncline, its axis immediately adjacent to the diapirs's edge, develops above the collapsed area. The secondary rim syncline is usually more extensive than primary rim syncline and also accumulates a thicker sequence. 
  3. Postdiapir stage: During this stage, diapirs stay at or near the sediment surface (assuming there is sufficient salt for continued movement) despite continued subsidence. A small, often subtle , tertiary rim syncline flanks the diapir.



Reference : Practical Seismic Interpretation , Michael E. Badley

Salt Structure chapter 1

Salt and associated evaporites are quite common in many sedimentary sequences. Salt has a low density (2.2 gm/cm3), lower than that of most other commonly occuring sediments. When deposited in sufficiently thick layers, it becomes inherently unstable if it is buried; and a density inversion between the overburden and salt is achieved. In such circumstances, salt flowage is initiated and passes through threee widely recognized stages of pillowing, diapirism, and postdiaprism. Bishop (1978) discusses the complex interaction between depositional history of the surrounding sediments and growth of salt structure. Controversy surrounds the question of whether the dominant processes in development of a salt diapir involve intrusion or extrusion of the salt.

Wednesday, April 1, 2015

Carbonates in Seismic Reflections

Normally, reflections from the top boundary of a carbonate unit have a large positive reflection coefficient because carbonates usually have high velocity and density compared to other common sedimentary rocks.

Only in cases where the carbonates are very porous or fractured are reflection coefficients of upper boundaries likely to be negative.

The usually high interval velocity introduces a potential resolution problem. Not only do thick sequences appear thin in time on seismic sections due to high velocities but the minimum thickness required for adequate vertical resolution can be quite high. Interval velocities between 4500 and 6000 m/s are common for older carbonate units.

A carbonate 100 m thick, with an interval velocity of 5500 m/s, would be represented by only 36 ms two-way time. Alternatively, for a seismic frequency of 25 Hz and interval velocity of 5500 m/s, the half wavelength thickness for no interference between reflections from top and base is 110m and the quarter wavelength tuning thickness is 55 m.

From a seismic viewpoint, carbonates can be conveniently divided into three groups :


  1. sheet-like deposits : These are often extremely extensive laterally and consist of fine-grained carbonate particles or calcareous microfossils deposited from suspension (e.g, , micritic limestones, chalk, calcareous claystone, etc). These deposits show characteristics similiar to those of other fine-grained deposits but can usually be recognized by their high amplitudes, good continuity, and if thick enough -by high interval velocities, which are rarely less than 3500 ms/s. Mistaking volcanic ash or tuff beds for carbonates is a potential interpretation pitfall. Tuff beds have high interval velocities and are laterally extensive, producing a seismic response similiar to that of bedded carbonates.
  2. Bioclastic deposits : Consisting of sand-sized carbonate grains transported and deposited by high-energy currents, these will have the same form and depositional setting as other noncarbonate clastics. Bioclastics may possibly be identified by their expected higher interval velocity and higher reflection amplitudes. Other considerations, such as the paleogeography and other recognizable associated lithlogies may aid an identification. In many cases, however, it may be impossible to differentiate between bioclastics and noncarbonate clastics.
  3. Buildups, reefs, bioherms, banks, mounds, etc. This type of deposit has a large biological element comprising the skeletal remains of living organism. These deposits are usually characterized by shape and high interval velocity. 


Bubb and Hatledid (1977) subdivided carbonate buildups into four major types 
  1. Barrier buildups, tending to be linear with relatively deep water on both sides during deposition.
  2. Pinnacle buildups-roughly equidimensional features surrounded by deep water during deposition.
  3. Shelf margin buildups- linear features with water on one side and shallow water on the other.
  4. Patch buildups-usually formed in shallow water, eiter in close proximity to shelf margins, or over broad, shallow seas.
Reference : Michael E. Badley, Practical Seismic Interpretation.

Tuesday, March 31, 2015

Clays and Silts in Seismic Reflection

Clays and silts include sediments settled from suspension, whatever the depositional environment. Such sediments tend to be thin bedded and produce closely spaced reflections (relative to other reflection spacings for a particular seismic section).

If the depositional area is extensive, the reflections generally show moderate to good continuity.

Amplitudes tend to be moderate to poor, but is very dependent on bed spacing (interference effects) and lithology. Divergent reflection patterns are diagnostic of fine-grained sediments , as they indicate deposition under conditions where subsidence and sedimentation rates are of similiar magnitude.

Not uncommonly, acoustic impedance contrasts are so low that the interval appears reflection free. Alternatively, destructive interference by beds of a thickness 1/30 wavelenght or less, can also produce reflection free intervals. Chaotic reflection patterns can result from deep-sea current activity or slumping, and from flowage due to loading, elevated pore pressure, or slope instability.


Reference : Practical Seismic Interpretation, Michael E. Badley 

Monday, March 30, 2015

Seismic Reflection Configuration

Reflection configuration is the shape of a reflection or surface. Some depositional environments produce characteristic reflection configurations. For example, a prograding delta produces a characteristics reflection package consisting of subparallel topset reflections, inclined foreset or clinoforms, and subparallel bottomset reflections in the basinal area. Lithology can be inferred from the location and the individual reflection attributes. For example, sigmoidal prograding configurations are more likely to be shale prone than are oblique prograding configurations; slumps tend to occur in shale-prone and so on.

Reference : Practical Seismic Interpretation, Michael E. Badley. 

Seismic Facies Parameters

Seismic facies parameters and Geologic Interpretation
----------------------------------------------------------------


  • Reflection configuration  --->  - Bedding patterns , depositional processes, erosion and paleotopography, fluid contacts
  • Reflection continuity ---> Bedding continuity, depositional processes
  • Reflection amplitude ---> Velocity-density contrast, bed spacing, fuid content
  • Reflection frequency ---> bed thickness, fluid content
  • Interval velocity ----> Estimation of lithology, estimation of porosity, fluid content

Reference : Mitchum, R.M, Vail, P.R and Thompson. S. 1977

Sunday, March 29, 2015

Shale Sequences

In basinal clay or shale sequences, deposition is predominantly from suspension load. Conditions of slow but variable, and not necessarily continous, sediment accumulation typify such sequences.

Often deposition of clay-sized grains is interrupted by "rare event" deposits (turbidite flows induced by storms, earthquakes, etc) . Bed thickness in this type of sequence generally ranges from a few millimeters to a few tens of centimeters with the exceptional bed being more than a meter thick.

In view of the thin-bedded nature of this type of sequence, seismic reflections from this depositional setting are almost always an interference response from several acoustic-impedance boundaries. The reflections will follow the attitude of the bedding planes. Since bedding planes in this type of environmental typically parallel time lines, the seismic reflections will not be precise time lines, but will certainly parallel them.

Reference : Practical Seismic Interpretation, Michael E. Badley.

Thursday, March 26, 2015

Seismic Sequence Stratigraphy chapter 3

If the clastic unit is followed by fine-grained deposits, an onlap relationship may be inferred.

In fine-grained deposits , bedding planes are generally subparallel and exteremely laterally extensive compared to bed thickness. Deposition is almost entirely from suspension load, and bedding planes and lithological change result from lithification, variations in sediment accumulation rate, etc. In these cases bedding planes, approximate to, or parallel, time lines. Gaps, even if geologically significant, will also tend to parallel time lines. From the relationship of thickness to lithology we can also conclude that time lines will be more closely spaced in fine grained sequences than in sand-sized grain deposits.


Reference : Practical Seismic Interpretation, Michael E. Badley

Seismic Sequence Stratigraphy chapter 2

There are lessons here for the ways in which we correlate wells. The dependence of the depositional process on varying current velocites dictates that sands must be accumulated episodically. The time gap between successive depositional episodes will depend on the process causing the current velocity ( tides, storms, earthquakes, etc). Whatever the process, it is clear that sands cannot be deposited in the continual manner that is theoretically possible for clay-sized grains.

Clays-sized grains, on the other hand, do have the potential to be deposited continually from suspension load in areas of the basin where current activity is low, although only modest currents may be required to retain nall clay-sized grains in suspension.

Seismic Sequence Stratigraphy



An example of lateral relationships between sands and clays in a deep-water setting. Well A drilled a sandy sequence, Well B a clay sequence of similiar thickness and overall age. It is impossible for sands and clays of similiar thickness to be equivalent in age. There must be gaps in the sand sequence. Onlap relationships can be used to locate the gaps and assess their magnitude.

Fundamental shape relationships between deposits of sand-sized and clay-sized grains.
(a) A current transporting sand-sized grains flows into a deep water body from the left and slows down. A tapering bed is deposited. 
Further in the basin clay-sized grains are deposited out from the suspension load in (b) 

Sketches showing how the shape of a clastic body can influence subsequent sedimentation. 


Reference : Practical Seismic Interpretation, Michael E. Badley 









Tuesday, March 24, 2015

Seismic Reflection Amplitude

Amplitude is the height of a seismic reflection peak (or trough) and is dependent on the reflection coefficient, but this direct relationship may be lost during processing. Frequently, amplitudes on seismic sections are balanced during processing to produce what is thought to be more easiliy interpretable sections. Usually, however, this makes it difficult and in many instances no longor possible to determine the relative strenghts of reflection coefficients. However, where amplitudes can be differentiated, the quantifying terms of high, medium, and low are used. Vertical changes in amplitude can be used to locate unconformities, whereas lateral changes can be used to help distinguish seismic facies. Great cautions must be exercised, however, as itnerference patterns from tuning, multiples, etc. , are responsible for many amplitude changes observed in seismic sections.

Reference : Practical Seismic Interpretation, Michael E. Badley.

Velocity Anomalies Associated With Channels

Channels often produces a velocity anomaly. If the channel cuts down into consolidated material, there is a good chance that the channel fill will have lower velocity than the adjacent rock. Such a channel would cause a velocity push-down. Channels or canyons in the sea bed have the same effect.

In some channels, however, the channel fill can have a higher velocity than the surrounding sediments and produce a velocity pull-up.


Reference : Michael E. Badley, Practical Seismic Interpretation

Monday, March 23, 2015

Velocity Model Beneath Reefs


(a) A reef composed of high-velocity limestone encased in lower-velocity shales produces a velocity pull-up on underlying reflections.

(b) Same as (a), except the reef has a more rounded form.

(c)A reef encased in an evaporitic sequence with high interval velocity push-down on underlying reflections.

(d) A reef with low-velocity basinal shales to its left and high-velocity backreef evaporites to its right produces a push-down beneath the shales and pull-up beneath the evaporites. 

Reference & Image Courtessy: Michael E. Badley, ; Practical Seismic Interpretation

Tuesday, February 24, 2015

Biodegradation

Biodegradation involves microbes which occur at shallow depths (Connan, 1984), near faults or near unconformities, degrade hydrocarbons (consuming parafins and lighter ends) leaving behind napthenes and asphaltenes (gas chromatograms show parafin spikes missing). Surface (meteoric) water is the main vehicle that brings microbes in contact with pooled hydrocarbons.
Microbes that can metabolize oil include fungi, yeast, and both aerobic and anaerobic bacteria, with aerobic bacteria considered to be the most effective of the group.

Superseals

Superseals are those rocks that have a high ductility (ability to deform without fracturing) under pressure (great depths), and for long time periods. Examples of superseals in order of quality are

  1. Salt (halite). This is the only true superseal.
  2. Anhydrite -containing 30% or more gypsum (water)
  3. Kerogen-rich shales (plastic shales). Usually deep water pelagics.
  4. High pressure shales. May be effective at great depths. 

Growth faults sealing capacity

Growth faults, at shallow depths in soft ductile sediments, may have a better capacity to trap hydrocarbon than post depositional faults at depth in more indurated (brittle sediments).

In Nigeria, hydrocarbons are trapped in the downthrown side of growth faults when they are juxtaposed against an overpressured shale zone on the high side.

Wednesday, February 11, 2015

Instantaneous Velocity

Instantaneous velocity is the speed at a given moment of a wavefront in the direction of energy propagation (Sherrif, 1984) . It is also used to refer to velocities that are derived from sonic or acoustic logs obtained in a borehole, since these are typically taken every few centimeters in the borehole.
In its simplest form, a sonic tool consists of a tranmsitter that emits a sound pulse and at least two receivers at some distance from the transmitter.
The sound propagates from the transmitter through the mud , the formation and through the mud again to the receivers where it is detected as a pressure pulse.
Sonic tools are designed such that sound travels slower through them than through most formations.

The difference in arrival times at two receivers , divided by the distance separating these receivers , provides a measurement of the interval transit time, delta t , or slowness . The velocity of the formation is the inverse of the slowness.

Tuesday, January 27, 2015

Reservoir Simulation Model

The distribution of porosity, permeability, and other properties over the area of a reservoir are incorporated in a reservoir simulation model. The models are updated as additional information becomes available. The validity of a model is judged by how closely actual results (fluid-flow rates, bottom-hole pressuress, and so on) match predictions based on the model.

A general rule of thumb is that a model can be used to predict reasonably well for about the same length of time as it has matched past history. Scenarios of production programs and strategies (for example, varying the number of wells and their locations, preferentially producing different wells, injecting water or gas to enhance the drive, and so on) are run on the model to determine how to optimize the economic return. Obviously, very realistic maps and data are required to achieve this end result.

(Sherrif. 1992. Exploration Seismology)


Monday, January 26, 2015

VSP Vertical Seismic Profile

The traces of the upgoing VSP are often stacked together to yield the pattern of primary reflection for correlating to conventional seismic data.

Corridor stacks are usually better than synthetic seismograms made from well-log measurements for relating reflections to interfaces because the measurements are made at seismic frequencies and are not sensitive to logging uncertainties.

The slope of the first breaks VSP (direct-wave traveltimes) gives the velocity. Reflections have a slope opposite to the first breaks. By using this difference, it is possible to separate downgoing waves (which consist of direct waves and multiples involving an even number of reflection) from upgoing waves . The upgoing waves may be 30dB below the downgoing waves.

(Sherrif. 1995. Exploration Seismology)

Thursday, January 22, 2015

Instataneous Frequency Seismic Attribute

A lowering of instataneous frequency is often observed immediately under hydrocarbon accumulations. Such "low-frequency shadows" seem to be confined to a couple of cycles below (not at) accumulations.  No adequate explanation is available, proposed explanations generally involve either the removal of higher frequencies because of absroption or other mechanisms, or improper stacking because of erroneous velocity assumptions or raypath distortions.

(Sherrif. 1995. Exploration Seismology. )

Seismic Reflection Characters

Parallel reflections suggest uniform deposition on a stable or uniformly subsiding surface, whereas divergent reflections indicate variation in the rate of deposition from one area to another or else gradual tilting. Chaotic reflections suggest either relatively high depositional energy, variability of conditions during deposition, or disruption after deposition, such as can be produced by slumping or sliding or turbidity-current flow.

A reflection-free interval suggests uniform lithology such as a relatively homogenous marine shale, salt , or massive carbonates ; however, distinguishing reflection-free patterns from multiples and noise that obsecures reflections may be difficult.

(Sherrif. 1995. Exploration Seismology)

Erosional Truncation & Toplap

Erosional truncation indicates that the sediment package formerly extended higher than it does today but that portions were removed by erosion,

whereas toplap indicates deposition near sea level and that the sediment package never extended significantly higher in the section.

With good seismic data quality, toplap sometimes can be distinguished from erosional truncation because there were changes in the depostional environment near toplap and consequently reflections are changeable in attitude and character, whereas no such changes occured at erosional truncation terminations.

Erosional truncation is the primary evidence for a sea-level fall. Where sediment packages are thick enough and noise sufficiently low, reflections showing these features can be seen in seismic data and used to determine the sea-level changes.

(Sherrif. 1995. Exploration Seismology)

Onlap & Downlap

Onlap indicates locations that are proximal (deposition close to the source of sediments, that is, on the landward side of a sediment package) and downlap locations that are distal (deposition distant from the sediment source).


(Sherrif. 1995. Exploration Seismology)

Tuesday, January 20, 2015

Classification of Stratigraphic Traps


  • Not adjacent to unconformities

Facies-change traps involving current-transported reservoir rock


  1. Eolian (dunes or sheet)
  2. Alluvial fan
  3. Alluvial valley (braided stream, channel fill, point bar)
  4. Deltaic (distributary mouth or finger bars, sheet, channel fill)
  5. Nondeltaic coastal (beach, barrier bar, spit, tidal delta or flat)
  6. Shallow marine (tidal bar, sand belt, washover, shelf edge, shallow turbidite or winnowing)
  7. Deep marine (marine fan, deep turbidite or winnowing)

Seismic Facies Characteristics



Picture above is the seismic facies characteristics (Fontane et al. 1987)

Sunday, January 18, 2015

Consistency of Seismic Interpretation

It is rare that the correctness or incorrectness of an interpretation can be ascertained because the actual geology is rarely ever known in adequate detail. The test of a good interpretation is consistency rather than correctness (Anstey, 1973). Not only must a good interpretation be consistent with all the seimic data, it also must be consistent with all that is known about the area, including gravity and magnetic data, well information and surface geology, as well as geologic and physical concepts.

One can usually be consistent and still have a choice of interpretations, the more so when data are sparse. The interpreter should explore various possibilities, but usually only one interpretation is wanted, that which offers the greatest possibilities for significant profitable hydrocarbon accumulation (assuming this is the objective).

An interpreter must be optmistic, that is, he must find the good possibilities.whereas a nonoptimistic interpretation may result in abandoning the area. Management is usually tolerant of optmistic interpretations that are disproven by subsequent work, but failing to recognize a possibility is an "unforgivable sin."


(Sherrif. 1995. Exploration Seismology)

Thursday, January 15, 2015

Static Analysis

The statics-analysis program looks for systematic variations such as would be expected if time shifts were associated with particular source activations, particular geophones, and so on.

Preliminary statics, as determined in the field office from first-break information and from the elevation of geophone stations, is usually input before the static analysis, so that the statics analysis determines residual statics errors. The result of this analysis are output on a control plot.

(Sherrif, Exploration Seismology. 1995)

Seismic Depth Migration

Migrated time sections may be simply stretched according to a vertical velocity function to give sections where the vertical scale is linear in depth rather than in time. However, where velocity varies apreciably in the horizontal direction, raypath bending introduces additional complications that depth migration attempts to accomodate.

Hubral (1977) observed that the apex of a diffraction curve is where the image ray, a ray that approaches the surface at right angles, emerges. Therefore, if we follow the image ray as it refracts according to Snell's law down through the earth, it will lead to the correct position of the diffracting point even if velocity surfaces are not horizontal. This concept is the heart of depth migration, migration that accomodates horizontal changes in velocity. Conventional migration collapses diffractions to the image-ray positions, so an additional step is needed to move elements to their correct subsurface locations.

The velocity model defines the major velocity surfaces where significant raypath bending occurs; key horizons on a conventionally migrated time section are mapped assuming that these are the major velocity interfaces. Cleary, defining the velocity model adequately is the key to succesful depth migration. Specifying velocities is a very difficult task because choices are not obvious. Detailed knowledge of the velocity distribution is often not available, especially in the structurally complex areas where depth migration is most needed. However, even though velocity errors create depth and location errors in the final product, the improved structural clarity often makes the procedure worthwhile and an appreciable amount of depth migration is being done today.

Subsalt imaging is important in several areas to locate hydrocarbons trapped beneath salt. Appreciable raypath bending occurs at the large contrast between the salt and sediments and the surfaces of the salt may be quite irregular. Migration is usually done in steps: conventional migration frist defines the top of the salt, then the base-of-salt reflection is defined using the salt veloctiy, and finally migration is completed with sediment velocities. Subsalt imaging provides a severe test of migration accuracy and requires very reliable data, which are usually 3-D data, and processing, often prestack migration.

(Sherrif, Exploration seismology. 1995)

Pada tahapan ekplorasi dimana konsep play menggunakan primary objektif carbonate, depth migration menjadi sangat dibutuhkan. Kontras dan distorsi velocity secara lateral menyebabkan imaging reef menjadi kurang sempurna bila processing hanya pada time migration. 


Sunday, January 11, 2015

Events on a seismic record

CMP gather

a = direct wave, V=650 m/s
b= refraction at base of weathering , V=1640 m/s
c= refraction from a flat refractor, Vr=4920 m/s
d= reflection from the refractor in c , V=1640 m/s
e= reflection from a flat reflector , V=1970 m/s
f= reflection from a flat reflector , V=2300 m/s
g= reflection from a dipping reflector V=2630 m/s
h= multiple of d
i = multiple of e
j= ground roll, V= 575 m/s
k= air wave, V=330 m/s

Reference : Sherrif, 1995. Exploration Seismology.

Quick QC Seismic Processing

To do quick QC of Seismic processing, assumed that we are a geoscientist in Oil Company reviewing the progress of processed seismic by vendor, is to look the prestack seismic sorted into CMP gathers.

CMP gather is one very powerful technique for distinguishing between reflections, diffractions, reflected refractions, and multiples . The CMP gather to do QC must through worfklows like
(a) Weathering and elevatation static corrections, because the correction is the same for all arrival times on a given trace.
(b) Normal moveout.

Provided the correct normal moveout has been removed, reflections appear as straight lines, whereas diffractions and multiples still have curvature, becuase their normal moveouts are larger than those of primary reflections, and refractions have inverse curvature.

(Sherrif, 1995. Exploration Seismology)

Tuesday, January 6, 2015

Sonic Logs effect on Synthetic Seismogram

Sonic logs may indicate sand velocities that are too high because they measure an invaded-zone velocity that exceeds the velocity of uninvaded sand. The editing of sonic-log data for synthetic seismogram manufacture attempts to correct for this. (Sherrif, 1990)