Compensation of Absorption Effects in Seismic Data

The frequency content of seismic data is changing with propagation depth due to intrinsic absorption. This implies that the higher frequencies are highly attenuated, thus leading to a loss in resolution of the seismic image. In addition, absorption anomalies, for example, caused by gas sands, will further dim the seismic reconstruction. It is possible to correct for such absorption effects by employing so called inverse Q filtering (IQF). This is a filtering technique that tries to restore the loss of the higher frequencies due to propagation. Newer developments within IQF can be regarded as a migration type of algorithm, and such classes of techniques are studied in this paper. As seismic waves travel through the earth, the visco-elasticity of the earth's medium will cause energy dissipation and waveform distortion. This phenomenon is referred to as seismic absorption. In explaining the propagation of seismic wave in a given medium we explore the relationship between the pressure and displacement stresses. Therefore, by introducing an absorption function into the stress and strain relationship we derived a non-linear wave equation. We, then, employed a layered earth model to solve the non-linear wave equation

Seismic resolution enhancement in shale-oil reservoirs

We developed a case study of seismic resolution enhancement for shale-oil reservoirs in the Q Depression, China, featured by rhythmic bedding. We proposed an innovative methodology for the resolution enhancement, a full-band extension method, and implemented this method in three consecutive steps: wavelet extraction, filter construction and data filtering. First, we extracted a constant-phase wavelet from the entire seismic data set. Then, we constructed the full-band extension filter in the frequency domain using the least-squares inversion method. Finally, we applied the band extension filter to the entire seismic data set. We demonstrated that this full-band extension method, with a stretched frequency band from 7-70 Hz to 2-90 Hz, may significantly enhance the 3D seismic resolution and distinguish reflection events of the rhythmite groups in shale-oil reservoirs

INVERSE ATTENUATION-FILTERING

When seismic waves propagate through the Earth, they are affected by numerous inelastic effects of the medium. These effects are usually characterized by the concept of the Q-factor and lead to variations of spectra of the signal and shapes of the waveforms, which further affect the results of reflection seismic imaging. Attenuation compensation, also often called the inverse Q filtering is a signal-processing procedure broadly used to compensate both of these effects of attenuation in reflection sections or volumes. The objective of this thesis is to present and investigate a new attenuation-compensation approach that is much more general than the conventional inverse Q filtering

Seismic Ray Impedance Inversion

This thesis investigates a prestack seismic inversion scheme implemented in the ray parameter domain. Conventionally, most prestack seismic inversion methods are performed in the incidence angle domain. However, inversion using the concept of ray impedance, as it honours ray path variation following the elastic parameter variation according to Snell’s law, shows the capacity to discriminate different lithologies if compared to conventional elastic impedance inversion. The procedure starts with data transformation into the ray-parameter domain and then implements the ray impedance inversion along constant ray-parameter profiles. With different constant-ray-parameter profiles, mixed-phase wavelets are initially estimated based on the high-order statistics of the data and further refined after a proper well-to-seismic tie. With the estimated wavelets ready, a Cauchy inversion method is used to invert for seismic reflectivity sequences, aiming at recovering seismic reflectivity sequences for blocky impedance inversion. The impedance inversion from reflectivity sequences adopts a standard generalised linear inversion scheme, whose results are utilised to identify rock properties and facilitate quantitative interpretation. It has also been demonstrated that we can further invert elastic parameters from ray impedance values, without eliminating an extra density term or introducing a Gardner’s relation to absorb this term. Ray impedance inversion is extended to P-S converted waves by introducing the definition of converted-wave ray impedance. This quantity shows some advantages in connecting prestack converted wave data with well logs, if compared with the shearwave elastic impedance derived from the Aki and Richards approximation to the Zoeppritz equations. An analysis of P-P and P-S wave data under the framework of ray impedance is conducted through a real multicomponent dataset, which can reduce the uncertainty in lithology identification.Inversion is the key method in generating those examples throughout the entire thesis as we believe it can render robust solutions to geophysical problems. Apart from the reflectivity sequence, ray impedance and elastic parameter inversion mentioned above, inversion methods are also adopted in transforming the prestack data from the offset domain to the ray-parameter domain, mixed-phase wavelet estimation, as well as the registration of P-P and P-S waves for the joint analysis. The ray impedance inversion methods are successfully applied to different types of datasets. In each individual step to achieving the ray impedance inversion, advantages, disadvantages as well as limitations of the algorithms adopted are detailed. As a conclusion, the ray impedance related analyses demonstrated in this thesis are highly competent compared with the classical elastic impedance methods and the author would like to recommend it for a wider application

ENHANCEMENT OF MARGRAVE DECONVOLUTION AND Q ESTIMATION IN HIGHLY ATTENUATING MEDIA USING THE MODIFIED S-TRANSFORM

We evaluate the performance of the Margrave deconvolution and spectral ratio methods using the Gabor, S-, and modified S transforms in highly attenuating media, where the quality factor changes with depth dramatically. Our results substantiate that the modified S-transform deconvolution is more robust in terms of generating fewer artifacts and providing better estimates of reflectivities than the Gabor transform. The results also show that using the modified S-transform in the spectral ratio method produces better Q estimates than the S-transform and the Fourier transform that is conventionally used in the spectral ratio method. This improvement in the estimates of reflectivities and Q using the modified S-transform is due to the enhancement of the time-frequency decomposition obtained by substituting the frequency in the Gaussian window with a linear frequency function. The coefficients of this linear function control the time frequency localization by expanding the Gaussian window at low frequencies and tightening it at high frequencies, in turn providing a better time-frequency decomposition. We demonstrate the efficiency of the modified S-transform deconvolution and Q estimation through the analysis of both synthetic and field data

Signal processing of GPR data for road surveys

Effective quality assurance and quality control inspections of new roads as well as assessment of remaining service-life of existing assets is taking priority nowadays. Within this context, use of ground penetrating radar (GPR) is well-established in the field, although standards for a correct management of datasets collected on roads are still missing. This paper reports a signal processing method for data acquired on flexible pavements using GPR. To demonstrate the viability of the method, a dataset collected on a real-life flexible pavement was used for processing purposes. An overview of the use of non-destructive testing (NDT) methods in the field, including GPR, is first given. A multi-stage method is then presented including: (i) raw signal correction; (ii) removal of lower frequency harmonics; (iii) removal of antenna ringing; (iv) signal gain; and (v) band-pass filtering. Use of special processing steps such as vertical resolution enhancement, migration and time-to-depth conversion are finally discussed. Key considerations about the effects of each step are given by way of comparison between processed and unprocessed radargrams. Results have proven the viability of the proposed method and provided recommendations on use of specific processing stages depending on survey requirements and quality of the raw dataset