Polarity

The nature of the positive and negative portions of the seismic wavelet, the positive and negative aspects of electrical equipment, or the north and south orientations of magnets and the Earth's magnetic field.
Wavelet
                A one-dimensional pulse, usually the basic response from a single reflector. Its key attributes are its amplitude, frequency and phase. The wavelet originates as a packet of energy from the source point, having a specific origin in time, and is returned to the receivers as a series of events distributed in time and energy. The distribution is a function of velocity and density changes in the subsurface and the relative position of the source and receiver. The energy that returns cannot exceed what was input, so the energy in any received wavelet decays with time as more partitioning takes place at interfaces. Wavelets also decay due to the loss of energy as heat during propagation. This is more extensive at high frequency, so wavelets tend to contain less high-frequency energy relative to low frequencies at longer travel times. Some wavelets are known by their shape and spectral content, such as the Ricker wavelet.

Geomagnetic Polarity Reversal
                                                       The periodic switching of the magnetic north and south poles of the Earth throughout time, probably as a result of movement of fluid within the Earth's core. The onset and duration of the many episodes of reversed polarity have been documented by examining the polarity of magnetic minerals within rocks of different ages from around the world, particularly in basalts or igneous rocks of the oceanic crust. Oceanic basalts record the Earth's magnetic field as they solidify from molten lava symmetrically on each side of the midoceanic ridges. These data have been compiled to create a time scale known as the geomagnetic polarity time scale (GPTS). In the oil field, borehole recordings allow direct correlation to GPTS and well-to-well correlations.

synthetic seismogram
                                           The result of one of many forms of forward modeling to predict the seismic response of the Earth. A more narrow definition used by seismic interpreters is that a synthetic seismogram, commonly called a synthetic, is a direct one-dimensional model of acoustic energy traveling through the layers of the Earth. The synthetic seismogram is generated by convolving the reflectivity derived from digitized acoustic and density logs with the wavelet derived from seismic data. By comparing marker beds or other correlation points picked on well logs with major reflections on the seismic section, interpretations of the data can be improved. The quality of the match between a synthetic seismogram depends on well log quality, seismic data processing quality, and the ability to extract a representative wavelet from seismic data, among other factors. The acoustic log is generally calibrated with check-shot or vertical seismic profile (VSP) first-arrival information before combining with the density log to produce acoustic impedance.

Seismogram
                       Traces recorded from a single shotpoint. Numerous seismograms are displayed together in a single seismic section.

Trace
          The seismic data recorded for one channel. A trace is a recording of the Earth's response to seismic energy passing from the source, through subsurface layers, and back to the receiver

Common depth point
                                      In multichannel seismic acquisition where beds do not dip, the common reflecting point at depth on a reflector, or the halfway point when a wave travels from a source to a reflector to a receiver. In the case of flat layers, the common depth point is vertically below the common midpoint. In the case of dipping beds, there is no common depth point shared by multiple sources and receivers, so dip moveout processing is necessary to reduce smearing, or inappropriate mixing, of the data.

Synonyms: common reflection point

Common midpoint
                                 In multichannel seismic acquisition, the point on the surface halfway between the source and receiver that is shared by numerous source-receiver pairs. Such redundancy among source-receiver pairs enhances the quality of seismic data when the data are stacked. The common midpoint is vertically above the common depth point, or common reflection point. Common midpoint is not the same as common depth point, but the terms are often incorrectly used as synonyms.



Fresnel zone
                        A frequency- and range-dependent area of a reflector from which most of the energy of a reflection is returned and arrival times differ by less than half a period from the first break, named for French physicist Augustin-Jean Fresnel (1788 to 1827). Waves with such arrival times will interfere constructively and so be detected as a single arrival. Subsurface features smaller than the Fresnel zone usually cannot be detected using seismic waves.

Seismic modeling
                              The comparison, simulation or representation of seismic data to define the limits of seismic resolution, assess the ambiguity of interpretation or make predictions. Generation of a synthetic seismogram from a well log and comparing the synthetic, or modeled trace, with seismic data is a common direct modeling procedure. Generating a set of pseudologs from seismic data is the process known as seismic inversion, a type of indirect modeling. Models can be developed to address problems of structure and stratigraphy prior to acquisition of seismic data and during the interpretation of the data. As Sheriff (1991) points out, agreement between data and a model does not prove that the model is correct, since there can be numerous models that agree with a given data set.

Resolution
                    The ability to distinguish between separate points or objects, such as sedimentary sequences in a seismic section. High frequency and short wavelengths provide better vertical and lateral resolution. Seismic processing can greatly affect resolution: deconvolution can improve vertical resolution by producing a broad bandwidth with high frequencies and a relatively compressed wavelet. Migration can improve lateral resolution by reducing the size of the Fresnel zone.

Deconvolution
                           A step in seismic signal processing to recover high frequencies, attenuate multiples, equalize amplitudes, produce a zero-phase wavelet or for other purposes that generally affect the waveshape. Deconvolution, or inverse filtering, can improve seismic data that were adversely affected by filtering, or convolution that occurs naturally as seismic energy is filtered by the Earth. Deconvolution can also be performed on other types of data, such as gravity, magnetic or well log data.
                                                                 With reference to induction logging, a method of removing shoulder-bed effects from an induction log. The term refers to early 6FF40 and deep induction logs in which the standard method of deconvolution was based on three measurements separated by 78 in. [198 cm] in depth. The three measurements were weighted by an amount calculated to reduce the effect of shoulder beds on the readings in a high-resistivity bed. Originally, the resistivity of the shoulder beds could be input, but in later usage this resistivity became standardized at 1 ohm-m. The deconvolution was not effective in high-contrast formations. In modern tools, the shoulder effect is corrected by using an inverse filter or an automatic inversion.

Amplitude
                    The difference between the maximum displacement of a wave and the point of no displacement, or the null point. The common symbol for amplitude is a. 

Bright spot   
                     A seismic amplitude anomaly or high amplitude that can indicate the presence of hydrocarbons. Bright spots result from large changes in acoustic impedance and tuning effects, such as when a gas sand underlies a shale, but can also be caused by phenomena other than the presence of hydrocarbons, such as a change in lithology. The term is often used synonymously with hydrocarbon indicator.

Synonyms: amplitude anomaly, hydrocarbon indicator

Antonyms: dim spot

Dim spot
                  A type of local seismic event that, in contrast to a bright spot, shows weak rather than strong amplitude. The weak amplitude might correlate with hydrocarbons that reduce the contrast in acoustic impedance between the reservoir and the overlying rock, or might be related to a stratigraphic change that reduces acoustic impedance.




Amplitude anomaly
                                   An abrupt increase in seismic amplitude that can indicate the presence of hydrocarbons, although such anomalies can also result from processing problems, geometric or velocity focusing or changes in lithology. Amplitude anomalies that indicate the presence of hydrocarbons can result from sudden changes in acoustic impedance, such as when a gas sand underlies a shale, and in that case, the term is used synonymously with hydrocarbon indicator.

Synonyms: bright spot, hydrocarbon indicator


True amplitude recovery
                                            Steps in seismic processing to compensate for attenuation, spherical divergence and other effects by adjusting the amplitude of the data. The goal of TAR is to get the data to a state where the reflector amplitudes relate directly to the change in rock properties giving rise to them.

Polarity standard
                                 The convention adopted by the Society of Exploration Geophysicists (SEG) for the display of zero-phase seismic data. If the signal arises from a reflection that indicates an increase in acoustic impedance, the polarity is, by convention, positive and is displayed as a peak. If the signal arises from a reflection that indicates a decrease in acoustic impedance, the polarity is negative and is displayed as a trough. There is another standard for minimum-phase data. In order to interpret seismic data acquired at different times within a region, to model data, or to assess bright or dim spots, some knowledge of the polarity of the data is essential to correlate or tie data properly.

Zero phase
                       Pertaining to seismic data whose wavelet is symmetrical about zero time. Deconvolution during seismic processing can convert data of mixed phase to zero-phase data, but is not always successful. Zero-phase data tend to provide sharper definition and less distortion between stratigraphic features in the subsurface, such as sand and shale layers.

Phase
              Pertaining to seismic data whose wavelet is symmetrical about zero time. Deconvolution during seismic processing can convert data of mixed phase to zero-phase data, but is not always successful. Zero-phase data tend to provide sharper definition and less distortion between stratigraphic features in the subsurface, such as sand and shale layers. 

Ricker wavelet
                            A zero-phase wavelet commonly convolved with a reflectivity trace to generate a synthetic seismogram       

Amplitude variation with offset
                                                        Variation in seismic reflection amplitude with change in distance between shotpoint and receiver that indicates differences in lithology and fluid content in rocks above and below the reflector. AVO analysis is a technique by which geophysicists attempt to determine thickness, porosity, density, velocity, lithology and fluid content of rocks. Successful AVO analysis requires special processing of seismic data and seismic modeling to determine rock properties with a known fluid content. With that knowledge, it is possible to model other types of fluid content. A gas-filled sandstone might show increasing amplitude with offset, whereas a coal might show decreasing amplitude with offset. A limitation of AVO analysis using only P-energy is its failure to yield a unique solution, so AVO results are prone to misinterpretation. One common misinterpretation is the failure to distinguish a gas-filled reservoir from a reservoir having only partial gas saturation ("fizz water"). However, AVO analysis using source-generated or mode-converted shear wave energy allows differentiation of degrees of gas saturation. AVO analysis is more successful in young, poorly consolidated rocks, such as those in the Gulf of Mexico, than in older, well-cemented sediments
Convolution
A mathematical operation on two functions that is the most general representation of the process of linear (invariant) filtering. Convolution can be applied to any two functions of time or space (or other variables) to yield a third function, the output of the convolution. Although the mathematical definition is symmetric with respect to the two input functions, it is common in signal processing to say that one of the functions is a filter acting on the other function. The response of many physical systems can be represented mathematically by a convolution. For example, a convolution can be used to model the filtering of seismic energy by the various rock layers in the Earth; deconvolution is used extensively in seismic processing to counteract that filtering.

 

posted by Geology on 04:10

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