Also shown are the straight ray paths traveled by seismic waves from each of five different source positions down to the boulder and back up to. Inverse dmo impulse responses computed by the fourier method left and by. When there are errors in the velocity model, zerooffset condition and zerotime condition will. S187s193 july 2007 with 68 reads how we measure reads. This hyperbolic curve shown in the dotted red line is called the normal moveout curve or nmo and is related to travel time, offset and velocity of the medium as shown by. We will now sum different traces, also called stacked, to improve the signalto. Ultrasonic modeling frequencies of 100 khz to 1,000 khz are scaled down by the scaling factor of. Seismic reflection is the most widely used geophysical technique. Figure 3 similar to figure 2, this time using lowfrequency processed reflection data. In table 1, we show a summary of the cluster coordinates and the apollo stations for which each cluster is. E r e i i0 the square root of the above relationship is called reflection coefficient, r. Amplitude within the fresnel zone for the zerooffset case. Seismic reflection methods applied to engineering, environmental, and groundwater problems by don w. The input to seismic modeling is a representation of the earths reflectivity and a velocitydepth model.
A conventional vertical seismic profile in which the energy source is positioned directly above the receivers, typically very close to the wellbore. The data of a survey are the arrival times of the wave fronts at various distances from the source. In case of i0, the ratio of the reflected energy of pwave, e r, to the incident energy, e i, is given by. Applied geophysics seismic reflection iv anticlines and synclines migrated stack applied geophysics seismic reflection iv migration procedure for a zero offset seismic section ie stacked data. Major fields of application of seismic reflection include. Pdf estimation of reflection coefficients from zerooffset. Results from applying the method to a real seismic line and zero offset vsp data from the nanyang oilfield, central china, indicate that q from the tomographic inversion of reflection and transmission data contains useful information on medium properties, which can aid in reservoir appraisal. Therefore, geophysicists processing the data to reduce amplitude effect from another waves. The application of a macromodel independent approach in obtaining improved. Avo analysis amplitude variation with angle or offset ava avo behaviour in prestack seismic gathers is a classic direct hydrocarbon indicator dhi based on the principle that variations in amplitude of a seismic reflection with angle of incidence or sourcereceiver distance offset are caused by fluid andor lithology changes in the reservoir. If we can identify the seismic wavelet that propagated through the earth and reflected from the layer contrasts, we can then remove the effect of that wavelet and obtain a series of.
Common offset seismic reflection method a technique for obtaining onefold reflection data is called the common offset method or common offset gather cog. Using the above methods to generate a zerooffset trace, these parameters can be measured directly as the arrival time in the zerooffset trace is equal to the avo intercept time, and its amplitude is the zerooffset reflectivity. A true zerooffset field data set from a specially designed seismic experiment is then used as input to estimate reflection coefficients. Reflections recorded at zero sourcereceiver offset follow ray paths that are perpendicular to the reflector. Comparison to regional seismic reflection data the stacked zero offset vsp data should be directly comparable to the regional mcs data. As a result, the reflection from the point on the reflector beneath point p, for example, would be recorded by the geophone at location g, to the right. Estimation of reflection coefficients from zerooff set field data. Seismic attenuation estimation from zerooffset vsp data.
Processing of seismic reflection data 1 tu delft opencourseware. The diffracted phase shown as the curved heavy line. Illustration of down going seismic waves in zerooffset case. A conventional vertical seismic profile in which the energy source is positioned directly above the receivers. Since energy is proportional to the square of the amplitude, the signal amplitude drops off like 1r. Nmo correction transforms a reflection record at offset x into a normalincidence x0 record. Stack ing velocities from a conventional seismic survey were used to.
Raw zero offset section 512 traces with 25cm trace spacing containing three reflectors acquired in balaton lake 86 fig 4. It states that the angle of incidence is equal to the angle of reflection. Seismic reflection methods environmental geophysics us epa. Pdf zerooffset seismic amplitude decomposition and migration. In reality, we have to record at source receiver offsets, and over complex. The old and the new in seismic inversion cseg recorder. The cancellation occurs due to the different sign i. The optimum offset technique was designed with the aim of keeping equipment and computing requirements to a minimum, and providing a reflection technique that could be applied by small geophysical contracting companies hunter et. Within the fresnel zone, the diffraction energy constructively builds the reflection energy.
The most widespread example of such procedure is the socalled map migration, in which normalincidence, zerooffset stacked seismic data are employed. Avo analysis is a technique that geophysicists can execute on seismic data to determine. The aim of seismic reflection measurements is a zerooffset seismic section where source and receiver points coincide fig. Seismic displays visualization is key to seismic data analysis 3d displays are mostly done interactively using workstations in this plot.
The upper part of the figure depicts a zero offset survey conducted over a subsurface medium that is homogeneous constant pwave velocity with the exception of an isolated boulder at some depth. Miller kansas geological survey the university of kansas lawrence, kansas abstract the seismicreflection method is. In this zero offset model, the energy travel path from source to reflector is identical to that from reflector to lx 4 fig. If the velocity model is accurate, the effective zero offset will occur at zero time imaging condition. This process is necessary to overcome the limitations of geophysical methods imposed by areas of complex geology. When there are errors in the velocity model, zero offset condition and zero time condition will. Seismic lowfrequency effects from oilsaturated reservoir. The reflector itself can be modeled as the coherent sum of the diffraction hyperbola from individual point scatterers. The result is the reflector is imaged by the seismic data with an apparent dip of. If the velocity model is accurate, the effective zero offset will occur at zerotime imaging condition.
Seismic modeling essentially is a simulation of a recorded seismic wavefield, seismic amplitudes, or seismic traveltimes. Seismic reflections at zero offset result from contrasts in acoustic impedance, involving just the pwave velocity and density of the layers at the interface. Commonoffset seismic reflection method a technique for obtaining onefold reflection data is called the commonoffset method or commonoffset gather cog. Seismic lowfrequency effects from oilsaturated reservoir zones. Estimation of geological dip and reflector curvature from. Zero offset vsp s providing precise velocity information, as well as true seismic wavelet welltie with phase, frequency, and timedepth, a zero offset vsp is the only tool that can identify events target, over pressure, casing point, base salt, etc. Bancroft abstract reflection energy from a linear reflector comes from an integrant over an aperture often described by the fresnel zone. A direct method of estimating depth to a reflector from seismic wideangle reflection times prakash kumar. In this zerooffset model, the energy travel path from source to reflector is identical to that from reflector to lx 4 fig. Because of the different deconvolution schemes applied to the vsp and mcs data, the vsp. Zerooffset seismic trace construction schlumberger. Seismic migration is the process by which seismic events are geometrically relocated in either space or time to the location the event occurred in the subsurface rather than the location that it was recorded at the surface, thereby creating a more accurate image of the subsurface.
Accuracy, limitation, and acquisition abdulfattah al. All of the models had 2d zerooffset seismic reflection surveys acquired as a quality check. Diffraction hyperbolae are identified by scanning adjacent traces and collapsed back to the point of origin kirchhoff migration uses hyperbola. A seismic survey is setup with a source in a borehole, 100m below the surface, and an array of geophones on the surface.
Reflection seismic method university of saskatchewan. Amplitude within the fresnel zone for the zerooffset case shuang sun and john c. Pdf reducing seismic uncertainty by using zero offset vsp. Results from applying the method to a real seismic line and zerooffset vsp data from the nanyang oilfield, central china, indicate that q from the tomographic inversion of reflection and transmission data contains useful information on medium properties, which can aid in reservoir appraisal. All of the models had 2d zero offset seismic reflection surveys acquired as a quality check. Stacking as seismic energy moves away from the source, there is a decrease in signal strength as the energy spreads out. Velocity analysis and nmocorrection until now we have only discussed data processing methods that improve the signal of each separate trace. The upper part of the figure depicts a zerooffset survey conducted over a subsurface medium that is homogeneous constant pwave velocity with the exception of an isolated boulder at some depth. The model and seismic acquisition measurements are scaled, where 1 mm in the physical modeling world is equivalent to 10 m in field. Seismic lowfrequency effects figure 2 standard processed seismic sections for westsiberian oil field showing well locations. From cmp to crs an overview of stacking techniques of. The zero offset section from multicoverage seismic reflection data is obtained without explicit knowledge of the macrovelocity model i.
Stacking velocities from a conventional seismic survey. Seg international exposition and 72nd annual meeting main menu. A true zero offset field data set from a specially designed seismic experiment is then used as input to estimate reflection coefficients. Stacking charts normal moveout and correction for it stacking zerooffset reflection section migration.
In geophysics and reflection seismology, amplitude versus offset avo or amplitude variation with offset is the general term for referring to the dependency of the seismic attribute, amplitude, with the distance between the source and receiver the offset. As shown in the previous section, the dip will not be measured correctly. Nmo correction transforms a reflection record at offset x into a normalincidence x 0 record. Amplitude within the fresnel zone for the zero offset case shuang sun and john c. Since the late seventies and early eighties, under the. Figure 4 shows two duplicate traces of the stacked vsp compared to the migrated seismic reflection data from line or8905. For a pair of source and receiver with any offset, during the downward continuation process, its effective offset is reduced toward zero. National geophysical research institute, uppal road, hyderabad 500 007, india. Stacking velocity is determined from the data, as a measure of the reflection hyperbola best aligned of with the reflection. Cmp processing greatly enhances the signal to noise ratio and allows coherent reflections to be visible. Refraction timedistance plots the raytracing algorithm in surface seismic surveys the point source is located on the surface and detectors of the resulting seismic waves are located on the surface. Depth conversion of selected seismic reflections is a valuable procedure to position key reflectors in depth in a process of constructing or refining a depthvelocity model. The reflection from the flat interface produces a curved series of arrivals on the seismic traces since it takes longer to travel to the far offsets than the near offsets.
Zerooffset seismic amplitude decomposition and migration article pdf available in geophysics 724. The model and seismic acquisition measurements are scaled, where 1 mm in the physical modeling world is equivalent to 10 m in field equivalents 1. We use prestack data obtained from a purpose designed true zero offset seismic experiment as input to estimate reflection coefficients. Introduction traditionally, migration has meant constructing an image of the earth from seismic reflections recorded at its surface robinson, 1983. It can be used to derive important details about the geometry of structures and their physical properties. Physical seismic modeling of a nearvertical fault zone. Migration of seismic data university of california, davis. Poststack data analyze seismic data with zero offset amplitude, while prestack data analyze seismic data with non zero offset amplitude. Seismic reflection survey with offset, singlefold coverage record traces from several geophones spaced away from source shot display traces sidebyside, so distance between traces proportional to geophone spacing display increasing time downward time approx.
In addition, since rocks are not truly elastic anelastic, some energy is lost to heat with every cycle. Processing of seismic reflection data tu delft opencourseware. Depths and average velocities calculated by this method are close to the actual values see the table in fig. Cmp sorting, velocity analysis and nmo correction, stacking, zerooffset migration. Comparison to regional seismic reflection data the stacked zerooffset vsp data should be directly comparable to the regional mcs data. Vertical axis is the twoway reflection travel time, horizontal axes give the collocated sourcereceiver coordinates, color represents reflection amplitudes the colorcoded upper surface is the water bottom. Estimation of reflection coefficients from zerooffset. The seismic data after processing can be classified to be poststack and prestack data. Reflection imaging of the moons interior using deep. However, the data that goes into the cmp processing is often contaminated with noise. Pdf zerooffset seismic amplitude decomposition and. This law is utilized in the seismic reflection method. We use prestack data obtained from a purpose designed true zerooffset seismic experiment as input to estimate reflection coefficients. Reflection imaging of the moons interior using deepmoonquake seismic interferometry.
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