1,721,032 research outputs found
Comparison of Ground-Roll Attenuation Methods for Near Surface Investigations
No full textIn this work four methods for Ground-Roll attenuation in a near surface data case are compared. The methods presented are: simple band-pass filtering, time-offset variant filtering based on the Short-Time Fourier-Transform, f-k filtering and time-offset variant filtering in the wavelet domain computed by means of the Continuous Wavelet Transform (CWT). The experience carried out demonstrates that the wavelet filter performs better than the others methods. This is because the transformed domain provided by the CWT is able to offer an improved representation of the frequency components of the signal making it easy to identify the GR and to draw a time-offset-frequency filter. The outcomes of the methods are assessed on both shot and stack domain
The limits of narrow and wide-angle AVA inversions for high Vp/Vs ratios: An application to elastic seabed characterization
Full text linkSince its introduction in the oil and gas industry, amplitude versus angle (AVA) inversion has become a standard tool in deep hydrocarbon exploration. However, with the intensification of offshore construction activity, applications of this method have been extended to evaluate the elastic properties of seabed sediments and of the shallowest part of the subsurface. These regions are often characterized by undercompacted sediments with very low S-wave velocities (Vs) and high P-wave velocity to S-wave velocity (Vp/Vs) ratios. However, the importance of the Vp/Vs ratio is usually underrated in AVA inversion. In this study, we analyse the limits of the AVA method in cases of high Vp/Vs ratios and the benefits introduced by wide-angle reflections in constraining the inversion results. A simplified seabed model that is characterized by a high Vp/Vs ratio is used to study the influence of the elastic and viscoelastic parameters on the P-wave reflection coefficients and to compute the error function of the AVA inversion. In addition, a synthetic AVA inversion is performed on this simplified model, which enables us to apply the sensitivity analysis tools to the inversion kernel. These theoretical analyses show that in the case of high Vp/Vs ratios, the Vs contrast at the reflecting interface plays a very minor role in determining the P-wave reflection coefficients and that the viscoelastic effects can be neglected when only pre-critical angles are considered in the inversion. In addition, wide-angle reflections are essential to reducing both the cross-talk between the inverted elastic parameters and the uncertainties in the Vp and density estimations, but they are not sufficient to better constrain the Vs estimation. As an application to field data, we derive the elastic properties of the seabed interface by applying AVA inversion to a 2D seismic dataset from a well-site survey acquisition. The limited water depth, the maximum available source-to-receiver offset, and the high frequency content of the data allow two different ranges of incidence angles to be considered: 0-30° and 0-60°. The results of the field data inversion confirm the conclusions from the theoretical analysis
A two-step elastic full-waveform inversion applied to reflection seismic data for shallow hazard identification
No full textWe apply a two-step elastic full-waveform inversion (FWI) to well-site survey (WSS) marine seismic data to estimate high-resolution P-wave (Vp) and S-wave (Vs) velocity models. Our approach combines a first global, genetic-algorithm optimization and a subsequent gradient-based inversion. The broad-band frequency content of the available seismic data makes it possible to extend the frequency range considered in the inversion up to 70 Hz and thus to derive a high-resolution elastic characterization of the shallowest part of the subsurface. The lack of low frequencies and the limited maximum source-to-receiver offset of the WSS acquisition, make the GA inversion particularly crucial as it provides a starting model for the gradient-based FWI that contains the large-medium wavelengths of the seismic velocity field. The following gradient-based FWI yields Vp and Vs models characterized by an improved resolution with respect to the outcomes of GA-FWI. The match between the observed and the predicted seismic data proves the reliability of our predictions
Characterisation of shallow marine sediments using high-resolution velocity analysis and genetic-algorithm-driven 1D elastic full-waveform inversion
Full text linkWe estimate the elastic properties of marine sediments beneath the seabed by means of high-resolution
velocity analysis and one-dimensional elastic full-waveform inversion performed on twodimensional
broad-band seismic data of a well-site survey. A high-resolution velocity function is employed to exploit the broad frequency band of the data and to derive the P-wave velocity field with a high degree of accuracy. To derive a complete elastic characterisation in terms of P-wave and S-wave velocities (Vp, Vs) and density of the subsurface, and to increase the resolution of the Vp estimates, we apply a one-dimensional elastic full-waveform inversion in which the outcomes derived from the velocity analysis are used as a priori information to define the Vp search range. The one-dimensional inversion is done using genetic algorithm as the optimisation method. It is performed by considering two misfit functions: the first uses the entire waveform to compute the misfit between modelled and observed seismograms, and the second considers the envelope of the seismograms, thus relaxing the requirement of an exact estimation of the wavelet phase. The full-waveform inversion and the high-resolution velocity analysis yield comparable Vp profiles, but the
full-waveform inversion reconstruction is much more detailed. Regarding the full-waveform inversion results, the final depth models of P- and S-wave velocities and density show a fine-layered structure with a significant increase in velocities and density at shallow depth, which may indicate the presence of a consolidated layer. The very similar velocities and density-depth trends obtained by employing the two different misfit functions increase our confidence in the reliability of the predicted subsurface models
Studio, sviluppo ed applicazione di codici di inversione sismo-petrofisica: terzo rapporto annuale del progetto.
No full textThe Limits of Narrow and Wide Angle AVA Inversions in Case of High Vp/Vs Ratios - Application to Seabed Characterization
No full textSince its introduction in the oil and gas industry, amplitude versus angle (AVA) inversion has become a standard tool in hydrocarbon exploration. However, with the intensification of off-shore construction activity, applications of this method have also been extended to evaluate the elastic properties of seabed sediments. These sediments are often undercompated and characterized by very low Vs and high Vp/Vs ratios. The importance of Vp/Vs ratios is usually underrated in AVA inversion which is often applied with few misgivings in case of high Vp/Vs ratios. In this work, we derive the elastic properties of the seabed interface by applying AVA inversion to a 2D well-site survey acquisition. The limited water depth, the maximum offset of the seismic acquisition and the high frequency content of the data make it possible to consider two different ranges of incidence angles: 0-30 and 0-60 degrees. The limits of AVA inversion in case of high Vp/Vs ratios are discussed together with the benefits introduced by wide angle reflections in constraining the inversion. In particular, these reflections are essential to decrease both the cross-talk between the inverted parameters and the uncertainties in the Vp and density estimations, but they do not guarantee a reliable Vs estimation
Elaborazione di dati sismici marini ad alta risoluzione e stima del campo di velocità delle onde di pressione
No full textSeismic and well log characterization of fractures for geothermal exploration in hard rocks
Full text linkWe study the well log and seismic responses of intensively fractured portions of deep intrusive/metamorphic rocks in southern Tuscany (Italy), which constitute the main drilling targets of the geothermal exploration in the Larderello–Travale area. In particular, the target we consider is located near the contact between a deep Pliocene granitic intrusion and the overlying Palaeozoic metamorphic basement. Sonic, density and borehole image logs are analysed together with post-stack reflection attributes and reflection amplitude versus source to receiver azimuth (AVAZ) responses. It turns out that the intense fracturing in the contact zone causes significant decreases in the density and P-wave velocity, and that fracture planes exhibit very high dips and a common preferential direction. The fractured zone found by the well coincides with peculiar alignments of high-amplitude signals in the 3-D seismic stack volume, which are particularly visible on the reflection strength and instantaneous phase time slices. The normal incidence synthetic seismogram based on the log data matches the observed stack trace nearest to the well and confirms that the high-amplitude reflection occurs at the fractured zone. We then consider the pre-stack domain to study the same reflections on bin gathers that are close to the well and coincident with the anomalies in the 3-D volume. In particular, we perform AVAZ analysis to detect possible anisotropic features in the reflected amplitudes due to the preferential orientation of the fractures, and we study the effect of crack density on the seismic responses and on velocity and density values. To this end, we build simplified models where a level with vertical fractures is encased in tight isotropic rocks. Notwithstanding the suboptimal quality of the seismic data, we estimate the overall matching between the borehole information and the seismic response as fair. In particular, the azimuthal amplitude variation of the reflections from the studied fractured zone has a sinusoidal trend that is quite consistent with the fracture planes’ orientation as indicated by the image logs. Moreover, the comparison between the actual AVAZ response and the AVAZ responses of synthetic seismograms generated on models with different crack densities suggests that it may be feasible to estimate crack density values from the azimuthal amplitude variation of the observed reflections, within the resolution of the seismic data
Igneous body characterization by means of seismic reflection attributes and wavelet transform
No full textIntroduction. The aim of this work is the detection and characterization of several igneous bodies observed in a series of 2D seismic sections. The studied seismic reflection survey has been acquired in the offshore Senegal and is located on the Western African margin, South-West of Dakar.
The identification, description and characterization of the igneous bodies is carried out by means of complex seismic attributes. The considered complex attributes are the Reflection Strength and its first and second derivative, the Instantaneous Frequency, the Instantaneous Phase, the Cosine of the Instantaneous Phase and the Sweetness. Moreover, in order to improve the visualization of the entire seismic dataset, the Continuous Wavelet Transform (CWT) is applied on the data. The CWT helps to better discriminate the geometrical features and the morphologies of the igneous bodies.
The analysis through the seismic attributes and the CWT allows to identify the amplitude anomalies related to the igneous bodies, to describe the different seismic facies and to comprehend the mutual relationship and the behaviour between them. Furthermore, it allows the evaluation of the geometrical properties of the intrusive bodies such as the thickness, lateral extent, depth of emplacement and it also permits to delineate the deformation structures, that affected the overburden due to the igneous intrusion.
Finally, the changing in the attribute response allows to divide each observed seismic section in three seismofacies and permits to classify the intrusive bodies on the basis of their age, shape and level of emplacement. Moreover, a comparison between the seismic stratigraphy and the real geological outcrops of the Senegal onshore has been attemped.
Seismic Reflection Data. This study is carried out interpreting eighteen 2D seismic lines extracted from a 3D data volume. In particular the entire dataset is composed by eleven in-lines and seven cross-lines. In Figure 1 is shown the map of the survey with the position of the seismic lines analysed; in Table 1 the most important acquisition and recording parameters are reported. All the sections are time migrated with zero phase conversion applied.
A line by line description of each amplitude anomaly is carried out. The seismic lines are characterized on the basis of their amplitude, phase and frequency by means of the complex seismic attributes (Taner et al., 1979; White, 1991; Satyavani et al., 2008; Barnes, 2001; Yushun, 2011; Subrahmanyam et al., 2008) Reflection Strength and derivatives, Instantaneous Phase, Instantaneous Frequency and Sweetness.
The Reflection Strength attribute permits to visualize the data taking into account the seismic reflectivity and, in particular, it highlights the strong impedance contrasts related to the lithological changes and thus allows to discriminate the limits between sequences and to localize the amplitude anomalies related to the igneous bodies.
Therefore, according to the observed amplitude response, each seismic section is divided into three different seismofacies (Figure 2). The seismofacies A is located in the upper part of the sections and is characterized by alternating reflectors with medium and high reflection strength response; the seismofacies B (below A) is constituted by a package of reflectors that have the maximum amplitude values, alternated by thin reflectors with medium amplitude values; the seismofacies C, located in the lower part of the sections, presents discontinuous reflectors with amplitude values much smaller than those of the seismofacies A and B. In Figure 2 is shown an example of the result obtained through the application of the Reflection Strength attribute for the inline1560 and the limits between the seismofacies A, B and C and the amplitude anomalies L, M and N are highlighted.
The Instantaneous Phase attribute is applied in order to emphasize the lateral continuity of strong as well as weak events, to estimate the geometrical properties of the amplitude anomalies and to highlight the discontinuities, such as faults.
The Instantaneous Frequency attribute is useful to discriminate the different range of frequency that characterized the data. In particular, the three seismofacies exhibit distinct Instantaneous Frequencies behaviours: the seismofacies A and B are characterized by medium to high frequency reflectors, while the seismofacies C shows a low frequency content. Furthermore, also the amplitude anomalies highlighted by the Reflection Strength attribute correspond to low Instantaneous Frequency anomalies.
The Sweetness attribute is calculated by the combination of Instantaneous Frequency and Reflection Strength (Hart, 2008; Riedel, 2010; Yushun, 2011) and it is useful to better distinguish the events characterized by high values of acoustic impedance. This attribute clearly highlights the limits of the three seismofacies as well as the anomalies. The seismofacies A presents dominant Sweetness values around zero and, within it, few reflectors with higher Sweetness values are observed. The seismofacies B is made up of packages of laterally continuous reflectors with medium-high Sweetness values. Below, the seismofacies C is characterized by the presence of less continuous reflections, compared with the seismofacies A and B. They exhibit quite high Sweetness values and are enclose in a background, that presents Sweetness values comprised between 0 and 1000.
Therefore, the detection of the anomalies related to the igneous bodies are performed through the application of the seismic attributes. In particular, the Reflection Strength and the Sweetness attributes permit to isolate the high amplitude value anomalies from the low reflective seismofacies C; the Instantaneous Phase allows to identify the bodies, that are masked by chaotic reflections, as sills with antiformal junctions, T- and F-shaped morphologies and saucer-shaped geometries. The Instantaneous Frequency permits to characterize the sills as bodies with a frequency range of about 25-30 Hz. Especially, the amplitude anomalies within the seismofacies C show values between 10 and 25 Hz, those within the seismofacies B are not resolved due to the chaotic facies with too high frequencies and the anomalies in the seismofacies A show values up to 40 Hz.
The Continuous Wavelet Transform (CWT). In order to improve the geometry interpretation, the Continuous Wavelet Transform (CWT) is applied (Debauchies, 1988; Mallat, 1989; Zhang et al., 2006). The CWT provides a method for displaying and analysing signals as a function of time and scale. The scale and the frequency are related to an inverse relationship of proportionality, which means that the higher the scale the lower the frequency. Therefore, the wavelet transform allows to decompose the seismic data into a time versus scale (frequency band) domain and subsequently, if needed, to reconstruct them in the scales of interest. The analysis of the reconstructed common scale volumes permits to obtain a representation of the frequency information, which is contained in a seismic section.
In particular, the CWT is defined as follow (Debauchies, 1988; Zhang et al., 2006):
It can be considered the cross-correlation between the seismic trace x(t) and the dilated and scaled versions of an user defined wavelet .The mother wavelet is the analysis wavelet function, a is the location parameter of the wavelet so that (t-a) define the shift of the wavelet along the trace and b is the scaling (dilation) parameter.
By shifting and scaling the mother wavelet, the Wavelet Transform is able to capture information of short duration (high frequency) or information of long duration (low frequency), at the same time.
Finally, the CWT can be interpreted as a bank of band pass filters process, but it is more flexible than other time-frequency transform such as the Short Time Fourier Transform (STFT) and it is particularly useful to locate and identify signals with exotic spectral characteristics (Sadowsky, 1996).
As well as for the seismic attributes, a line by line CWT analysis is carried out.
The CWT is tested using three different mother wavelets: the Haar wavelet, the order 2 Daubechies wavelet (db2) and the Morlet wavelet (morl).
Although the best result was expected by the application of the Morlet wavelet, that is the commonly used wavelet if the transform is aimed at improving the quality of the seismic section (Sadowsky, 1996), the db2 is adopted. As an example, the reconstruction of the inline 1560 through the sum of the coefficients of the chosen db2 scales is shown in Figure 3.
To sum-up, this analysis shows that the anomalies related to the igneous bodies can be approximated in a frequency ranges from 20 up to 30 Hz. This is in agreement with the Instantaneous Frequency response. Furthermore, the application of the CWT is useful to discriminate the igneous bodies within the seismofacies B, that were masked due to the presence of a chaotic facies.
Conclusion. The Senegal Basin is characterized by the presence of discordant low frequency high amplitude reflection anomalies, that are interpreted as Miocene intrusive bodies. This interpretation is supported by the data obtained from the on-land survey of the Senegal area. The main result of this survey is the identification of magmatic activity ranges from Oligocene to Quaternary.
From the line by line analysis of the eighteen 2D seismic lines results that the detected nineteen sills can be classified on the basis of their age, levels of emplacement and geometries. In particular, most of the sills develop within the seismofacies C at the first level of emplacement (TWT >3.5 s). These anomalies show a flat inner saucer geometry, often bounded by an inclined sheet that cross-cuts upwards the seismofacies C, and deform the overburden reflectors in a gently symmetrical antiformal fold.
The amplitude anomalies which develop in the second level of emplacement (3.5s < TWT < 2.7s) show a concave-upwards saucer-shaped morphology and are characterized by the formation of two inclined sheets, often one steeper than the other. Moreover, some of these bodies show T- and F-shaped or antiformal morphology. At the third level (TWT <2.7s), only two sills develop within the seismofacies A, cross-cut the seismo-stratigraphic reflectors, obliterate the underlying seismofacies B and deform the overlying reflectors in a domal structure.
Furthermore, by applying the seismic attributes, it is possible to better distinguish the sills on the basis of the strong Reflection Strength and Sweetness response, which allow to isolate the seismic anomalies from the low amplitude background. Also, the Instantaneous Phase allows to distinguish, particularly within the seismofacies B, the seismic bodies characterized by a composite shape and masked in a chaotic facies. The Instantaneous Frequency is useful to differentiate the igneous bodies on the basis of the frequency values, that result to be comprised between 20 and 30 Hz.
According to the seismic attributes, the results obtained by the application of the CWT permit to classify the anomalies as bodies with a frequency range about 25-30 Hz. Moreover, this tool permits to characterize the bodies on the basis of their geometrical features and is fundamental to compute thickness, lateral extent, depth of emplacement, diameter-to-depth ratio and the related-fold amplitude of the sills. Furthermore, from the obtained measurements it is possible to understand the relations between the thickness and the depth of emplacement, to compare the thickness and the related-fold amplitude and to compute the relationship between these parameters.
This work can supply new inputs that can be used to improve the knowledge of the stratigraphy, the geological setting and the evolution of the investigated area. The achieved characterization of the igneous bodies can be of crucial importance for the hydrocarbon exploration since the presence of igneous intrusions in a petroleum system could cause important effects on the maturation of the source rocks, on the creation of structural and stratigraphic trapping structures and, finally, could affect the oil migration pathway
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