M. Senkaya

Joint inversion of Rayleigh‐wave dispersion data and vertical electric sounding data: synthetic tests on characteristic sub‐surface models

In the traditional inversion of the Rayleigh dispersion curve, layer thickness, which is the second most sensitive parameter of modelling the Rayleigh dispersion curve, is usually assumed as correct and is used as fixed a priori information. Because the knowledge of the layer thickness is typically not precise, the use of such a priori information may result in the traditional Rayleigh dispersion curve inversions getting trapped in some local minima and may show results that are far from the real solution. In this study, we try to avoid this issue by using a joint inversion of the Rayleigh dispersion curve data with vertical electric sounding data, where we use the common-layer thickness to couple the two methods. The key idea of the proposed joint inversion scheme is to combine methods in one joint Jacobian matrix and to invert for layer S-wave velocity, resistivity, and layer thickness as an additional parameter, in contrast with a traditional Rayleigh dispersion curve inversion. The proposed joint inversion approach is tested with noise-free and Gaussian noise data on six characteristic, synthetic sub-surface models: a model with a typical dispersion; a low-velocity, half-space model; a model with particularly stiff and soft layers, respectively; and a model reproduced from the stiff and soft layers for different layer-resistivity propagation. In the joint inversion process, the non-linear damped least squares method is used together with the singular value decomposition approach to find a proper damping value for each iteration. The proposed joint inversion scheme tests many damping values, and it chooses the one that best approximates the observed data in the current iteration. The quality of the joint inversion is checked with the relative distance measure. In addition, a sensitivity analysis is performed for the typical dispersive sub-surface model to illustrate the benefits of the proposed joint scheme. The results of synthetic models revealed that the combination of the Rayleigh dispersion curve and vertical electric sounding methods in a joint scheme allows to provide reliable sub-surface models even in complex and challenging situations and without using any a priori information.

High Resolution Deconvolution by Combining Fx Filtering and Cauchy Regularization

In this study, it is performed an inversion process that combines Cauchy regularization and FX filtering for a stable multi-channel high resolution deconvolution. Applications of the method are made on synthetic and post stack field data. From these applications, it is considered that resolution in the vertical direction and continuity in the lateral direction are more improved by successfully reduction random noise. Results show that the process provides more interpretable seismic sections.

Post-stack high-resolution deconvolution using Cauchy norm regularization with FX filter weighting

High-resolution deconvolution can mathematically be viewed as a regularized inverse problem. Besides, the result of the high-resolution deconvolution is generally accepted as reflectivity series of the layered media. On the other hand, lateral continuity is frequently poorer than vertical resolution on post-stack seismic section after application of any high-resolution deconvolution. However, because of the ill-posed inherent of the deconvolution problem, the Cauchy norm regularization term, a non-quadratic prior-information is widely used to provide the stability and uniqueness of the problem. But, it does not provide adequate quality of deconvolution if the noise in the data is strong. In this study, a stable and high-resolution deconvolution of post-stack seismic data was accomplished by an iterative inversion algorithm incorporating the Cauchy norm regularization with FX filter weighting. Cauchy norm regularization was utilized to force the solution to a spikiness structure, while the effective random noise reduction was performed by using the FX filter. Applications to synthetic and real post-stack data showed that the resolution in the vertical direction and continuity in the lateral direction are better improved. Thus, we think that this process makes seismic sections obtained especially from thin layered sedimentary basins more interpretable.

The Sensitivity Analysis as a Non-uniqueness Indicator in the Inversion of Rayleigh-wave Dispersion Curve

In the Rayleigh-wave dispersion curve (RDC) inversion, layer thicknesses are typically fixed during the inversion process and may be estimated using available a priori knowledge. But, the RDC inversion process frequently suffers from non-uniqueness due to the use of erroneous or conjectural layer thicknesses. In this work, sensitivity analysis of S-wave and layer thickness is used to reveal the reliability and accuracy of the RDC inversion results in the non-uniqueness situation. For this purpose, a synthetic data is inverted with three different initial models through the same inversion parameters and their sensitivity patterns are compared. The sensitivity analyses demonstrate that the sensitivity pattern of layer S-wave velocity and thickness may present useful information about the quality of estimated parameters. In this way, the possibility of getting trapped in non-uniqueness may be decreased.

RefPick: A GUI Application to Pick First Arrival Times and Data Processing on Seismic Refraction Data

The quantified interpretation of the seismic refraction record depends on correct and confident picking first arrival times. However, traditional first arrival picking techniques are not enough if the refraction data are poor. This study presents the implementation of a GUI interface, RefPick, which has been established to detecting first arrivals on seismic refraction data, also data processing and inversion. While the interface allows picking the first arrival times by semi-automatically based correlation technique and manually, in addition, the seismic data can be filtered by f-k and Butterworth filters. The interface provides to export the first arrivals in the format of common evaluation software. Also, the inversion will be useful to invert the first data set within RefPick interface. By means of the RefPick, a seismic refraction data can be evaluated within one user-friend, open source GUI application.

Investigation of Bedrock Topography and Soil Dynamic Parameters of Uzungöl Settlement with Shallow Seismic Methods

Trabzon-Uzungol settlement and construction areas take place on landslide deposits which occurred due to an historical landslide. Shallow seismic reflection, seismic refraction first arrival tomography and MASW methods were used to reveal the soil structure and characterization of this deposit. The obtained P and S-wave velocity sections indicate the topographic change of the depth of the basic rock in ~ 13.5m in lateral direction. On the other hand, dynamic elastic parameters of that soil on the bedrock were calculated for geotechnical engineering studies. Despite the fact that the soil classification indicates variability from place to place, it was determined to be medium-stiff. However, all the obtained data are coherent to each other when the previous drilling studies in the study area and a geologic cross section including two wells with P and S-wave velocity sections are compared. Consequently, it is thought that all this information will contribute to the engineering studies planned for Uzungol to be done safely without damaging the environment and will be a scientific base for the future studies. Also, the contribution of the use of refraction first arrival tomography and MASW methods together are indicated to determine the soil structure and characters safely in this study.

1D Joint Inversion of Rayleigh Fundamental Mode and Electric Sounding Curves: Synthetic Tests

Layer thickness is important model parameter as far as S-wave velocity that contributes to Rayleigh phase velocity. But, S-wave velocity is only inverted parameters in the traditional Rayleigh-wave inversion process. Thus, using erroneous thickness information in inversion process will cause unreliable or non-unique S-wave velocity profile. However, layer thickness can be incorporated into process by the joint inversion approach to increase accuracy of sub-surface model. This study presents 1D joint inversion of Rayleigh fundamental mode curve with electric sounding data, to use layer thickness as variable in the inversion process, contrary from separate inversion of Rayleigh dispersion curve. The proposed method was tested on noise-free and Gaussian noisy synthetic data. Results demonstrate that reliable vertical shear velocity profile can be obtained by the joint inversion approach, even there is no a priori.

Contribution of Simplified Surface Waveform Calculation to Separate Dispersion Curves in MASW

Accurate picking of dispersion curves is the most critical stage in Multichannel Analysis of Surface Waves (MASW) method. Although there is several acquisition and numerical techniques for the optimal extraction, sometime, this requirement cannot be valid due to interference, which includes different modes, noises, and also lateral inhomogeneity in the near-surface earth. Hence, the interference between the different modes of surface waves can be so serious that neither fundamental, nor higher mode dispersion curve can be extracted with an accurate and a reliable. For this, we calculate surface waveforms, including geometrical and intrinsic dispersion, based on the summation of the particular seismic impulses corresponding to particular frequency-phase velocity curve points for geophone positions from the seismic source. This methodology is demonstrated by using both synthetic and real shot gathers. Synthetic calculations provide us a simple way to check accuracy of dispersion curve picking on a shot gather, which it is the fundamental or higher mode.