- Time sequence analysis in geophysics
- Time Sequence Analysis in Geophysics by E. R. Kanasewich, Hardcover | Barnes & Noble®
- Geology & Geophysics
- 1st Edition
- 1. Introduction
Geological work of the sea-marine basins-relief features of the world, ocean floor. Temperature, salinity of seawater. Destructive work of sea-near shore accumulation forms-sedimentation in various zones of sea. Distribution of marine sediments. Morphometric analysis of drainage basins, water sheds. Elements of hill slopes-pediment, bazadas.
Time sequence analysis in geophysics
Landforms in relation to rock types, paleochannels, buried channels. Soils types and their classification. Evolution of major geomorphic process in India. The magnitude of the voltage output is proportional to the number of magnetic lines of force cut per unit time; thus, geophone response indicates the velocity of the geophone case, which, in turn, is proportional to Earth particle velocity at the geophone station.
The polarity of the geophone output voltage depends on the direction that the electrical conductors are moving as they cut across the magnetic lines of force. If an upward movement creates a positive voltage, a downward movement produces a negative voltage. Thus, a geophone is a vector sensor that defines not only the magnitude of Earth motion, but also the direction of that motion. Because geophones are directional sensors and can distinguish between vertical and horizontal Earth motions, they are used to record multicomponent seismic data.
Three-component 3C geophones are used to record compressional and shear seismic data onshore. Shear waves do not propagate in fluids. In marine environments, geophones have to be placed in direct contact with the Earth sediment on the seafloor, with data-recording cables connected to surface-positioned ships or telemetry buoys. Four-component 4C sensors used for this service are encased in large, robust, watertight enclosures that include a hydrophone and a 3C geophone. In this cable design, a 4C sensor station is positioned at intervals of 50 m along the m cable segment. A large number of these segments, each containing three receiver stations, are connected end to end to make a continuous OBC receiver line several kilometers long.
Time Sequence Analysis in Geophysics by E. R. Kanasewich, Hardcover | Barnes & Noble®
The exact length of the receiver line is determined by the depth of the target that is to be imaged. Seismic Wave Propagation The full elastic seismic wavefield that propagates through an isotropic Earth consists of a P-wave component and two shear SV and SH wave components.
Marine air guns and vertical onshore sources produce reflected wavefields that are dominated by P and SV modes. Much of the SV energy in these wavefields is created by P-to-SV-mode conversions when the downgoing P wavefield arrives at stratal interfaces at nonnormal angles of incidence Fig. Horizontal-dipole sources can create strong SH modes in onshore programs.
No effective seismic horizontal-dipole sources exist for marine applications. A principal difference among P, SV, and SH wavefields is the manner in which they cause rock particles to oscillate. A compressional wave causes rock particles to oscillate in the direction that the wavefront is propagating.
In other words, a P-wave particle displacement vector is perpendicular to its associated P-wave wavefront. In contrast, SV and SH waves cause rock particles to oscillate perpendicular to the direction that the wavefront is moving, with the SH and SV displacement vectors orthogonal to each other. A shear-wave particle-displacement vector is thus tangent to its associated wavefront. In a flat-layered isotropic Earth, the SH displacement vector is parallel to stratal bedding, and SV displacement is in the plane that is perpendicular to bedding.
Body Waves and Surface Waves Seismic wavefields propagate through the Earth in two ways: body waves and surface waves. Body waves propagate in the interior body of the Earth and illuminate deep geologic targets. These waves generate the reflected P, SH, and SV signals that are needed to evaluate prospects and to characterize reservoirs. Reflected or scattered body waves are the fundamental signals sought in seismic data-acquisition programs. Surface waves are noise modes that overlay the desired body-wave reflections. Surface waves can be a serious problem in onshore seismic surveys.
There are two principal surface waves: Love waves and Rayleigh waves Fig. Love waves are an SH-mode surface wave and do not affect conventional P-wave seismic data. Love waves are a serious noise mode only when the objective is to record reflected SH wavefields.
Geology & Geophysics
The more common surface wave is the Rayleigh wave, which combines P and SV motions and is referred to as ground roll on P-wave seismic field records. Love waves create particle displacements in the horizontal plane; Rayleigh wave displacements are in the vertical plane Fig. Seismic Impedance The concept of acoustic or seismic impedance is critical to understanding seismic reflectivity. Reflections Coefficients Seismic reflectivity is best explained with a simple two-layer Earth model in which Layer 1 is above Layer 2 Fig.
The seismic reflection coefficient, R , for a downgoing particle-velocity wave mode that arrives perpendicular to the interface between the two layers is The velocity parameters, V 1 and V 2 , are P-wave velocities if P-wave reflectivity is being calculated; they are S-wave velocities if S-wave reflectivity is to be determined. Seismic Attributes The fundamental properties of processed seismic data that are used in interpretation are temporal and spatial variations of reflection amplitude, reflection phase, and wavelet frequency. Complex Seismic Trace Fig. In this discussion, we ignore what a Hilbert transform is and how the function y t is calculated.
Most modern seismic data-processing software packages provide Hilbert transform algorithms and allow processors to create the function y t shown in Fig. These two data vectors are displayed in a 3D x, y, t space in which t is seismic traveltime, x is the real data plane, and y is the imaginary plane. In this complex trace format, the actual seismic trace, x t , is confined to the real x plane, and y t , the Hilbert transform of x t , is confined to the imaginary y plane.
When x t and y t are added vectorally, the result is a complex seismic trace, z t , in the shape of a helical spiral extending along, and centered about, the time axis t. The projection of this complex function z t onto the real plane is the real seismic trace, x t , and the projection of z t onto the imaginary plane is y t , the calculated Hilbert transform of x t.
Applying the second equation of Fig. The phase behavior at times t 1 , t 2 , t 3 is critical to understanding the geologic significance of anomalous frequencies. Applications of Seismic Attributes All instantaneous seismic attributes amplitude, phase, frequency can be used in interpretation. Seismic Interpretation A stratal surface is a depositional bedding plane: a depositional surface that defines a fixed geologic time.
Structural Interpretation The original use of seismic reflection data circa through was to create maps depicting the geometry of a subsurface structure.
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When the seismic industry converted from analog to digital data recording in the mids, digital technology increased the dynamic range of reflected seismic signals and allowed seismic data to be used for applications other than structural mapping, such as stratigraphic imaging, pore-fluid estimation, and lithofacies mapping. These expanded seismic applications have led to the discovery of huge oil and gas reserves confined in subtle stratigraphic traps, and seismic exploration is now no longer limited to just "mapping the structural highs.
When 3D seismic data are interpreted with modern computer workstations and interpretation software, structural mapping can be done quickly and accurately. Different seismic interpreters use different approaches and philosophies in their structural interpretations. The technique described here is particularly robust and well documented. Coherency is a numerical measure of the lateral uniformity of seismic reflection character in a selected data window. All modern seismic interpretation software can perform the numerical transform that converts 3D seismic wiggle-trace data into a 3D coherency volume.
This figure is also discussed in the chapter on reservoir geophysics in the Emerging Technologies volume of this Handbook. The narrow bands of low coherency values that extend across this time slice are created by faults that disrupt the lateral continuity of reflection events.
atdervadal.tk Fault mapping is a major component of structural mapping, and this type of coherency display can be used to create fast, accurate fault maps. Coherency technology has evolved into the optimal methodology for detecting and mapping structural faults in 3D seismic image space. Imaging Reservoir Targets Fig.
These data include a good-quality reflection peak labeled "reference surface. This particular reflection peak satisfies the fundamental criteria required of a reference stratal surface used to study thin-bed sequences: the event extends over the total 3D image space and has a high signal-to-noise character; the event is reasonably close to the targeted thin-bed sequences that need to be studied i. The third criterion is the most important requirement for any seismic stratal surface that is to be used as a reference surface.
Because this reference surface follows the apex of an areally continuous reflection peak, the basic premise of seismic stratigraphy is that this reference surface follows an impedance contrast that coincides with a stratal surface. Three-Dimensional Seismic Survey Design Stacking Bins The horizontal resolution a 3D seismic image provides is a function of the trace spacing within the 3D data volume.
As the separation between adjacent traces decreases, horizontal resolution increases. At the conclusion of 3D data processing, the area spanned by a 3D seismic image is divided into a grid of small, abutted subareas called stacking bins.
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Each trace in a 3D seismic data volume is positioned so that it passes vertically through the midpoint of a stacking bin. In Fig. Inline coordinates increase from west to east as shown. Crossline refers to the direction that is perpendicular to the orientation of receiver cables; thus, the crossline coordinates increase from south to north.
The dimension of the trace spacing in a given direction across a 3D image is the same as the horizontal dimension of the stacking bin in that direction. As a result, horizontal resolution is controlled by the areal size of the stacking bin.
This stratigraphy, in turn, is also known as a function of VSP reflection time. This fixed relationship between stratigraphy and the VSP image results because VSP receivers are distributed vertically through the seismic image space. This data-recording geometry allows both stratigraphic depth and seismic traveltime to be known at each receiver station. The dual-coordinate domain depth and time involved in a VSP measurement means that any geologic property known as a function of depth at the VSP well can be accurately positioned on, and rigidly welded to, the time-coordinate axis of the VSP image.
VSP data are unique in that they are the only seismic data that are recorded simultaneously in the two domains critical to geologic interpretation: stratigraphic depth and seismic reflection time Fig. As a result, specific stratigraphic units, known as a function of depth from well log data, can be positioned precisely in their correct VSP-image time windows Fig. Each numbered stratigraphic unit shown in Fig. When the VSP image is shifted up or down to correlate better with a surface-recorded seismic reflection image that crosses the VSP well, the VSP-defined time window that spans each stratigraphic unit should be considered to be welded to the VSP data.
This causes the stratigraphy to move up and down in concert with the VSP image during the VSP-to-surface seismic correlation process.