C. Almagro Vidal

Retrieval of Reflections from Ambient Noise Using the Incident Fields (Point-spread Function) as a Diagnostic Tool

Seismic interferometry (SI) enables the retrieval of virtual-shot records at the location of receivers. SI with ambient noise allows the retrieval of the reflection response of the subsurface without the need of any active source. The quality of the retrieved response is dependent on the illumination characteristics of the ambient noise. For the exploration frequency band of interest, in low-seismicity regions most of the energy in the recorded noise comes from sources at or near the surface. Such sources would mainly contribute to retrieval of surface waves, which would show up as the most energetic arrivals in the retrieved results. Because of that, the generally weaker retrieved reflections would be buried by the retrieved surface waves. Aiming at improving SI retrieved reflections, we propose a diagnostic tool applied after cross-correlation that separates the correlated noise panels into surface-wave dominated and body-wave dominated. We show results of the application of the tool to modelled data for noise sources acting separately in time and for noise sources overlapping in time. Finally, we show results from the application of the diagnostic tool to ambient noise recorded in Northern Netherlands.

Turning One-sided Illumination into Two-sided Illumination by Target-enclosing Interferometric Redatuming

We present a novel method to transform seismic data with sources at the surface and receivers above and below a selected target zone in the subsurface into virtual data with sources and receivers located at the initial receiver locations. The method is based on inverting a series of multidimensional equations of the convolution- and the correlation-type. The required input data can be computed from surface seismic data with a new iterative scheme that is currently being developed. The output data contains virtual sources that illuminate the target not only from above (as in the original data), but also from below, facilitating the needs of seismic imaging and inversion in an optimal way. The method is nonlinear in the sense that all internal multiples are correctly accounted for and true amplitude in the sense that the virtual sources are forced to inherit uniform radiation patterns even though the overburden is strongly heterogeneous.

Diagnosing Virtual Source Radiation Characteristics without a Velocity Model

With seismic interferometry, controlled sources can be redatumed from the Earths surface to downhole receiver locations without a velocity model. As redatuming operators are taken directly from the data very accurate results can be achieved. The redatumed field does not only contain scattered arrivals but also a virtual source function at zero time lag. Under optimal illumination conditions, this virtual source function will be close to a bandlimited delta-function. If such conditions are not present, the virtual-source function will carry an illumination footprint, which can provide useful information on the radiation characteristics of the generated virtual source. By analyzing the contribution of each surface shot to the virtual source illumination footprint individually and selecting subsets of shots based on such analysis, virtual source radiation characteristics can be manipulated. Such manipulation can simplify interpretation and reduce costs of time-lapse virtual source data as monitor surveys can be designed economically to illuminate only particular target areas. We emphasize that all this can be done without the use of a velocity model.

Retrieval of Elastodynamic Reflections From Passive Double‐Couple Recordings

Virtual Greens functions obtained by seismic interferometry (SI) can provide valuable reflectivity data that can complement tomographic inversion schemes. However, virtual reflections are affected by illumination irregularities that are typical of earthquake-induced wavefields recorded by the receiver array. As a consequence, irregular source distributions, scattering (in case of suboptimal illumination), and complex source mechanisms can significantly disturb the retrieval of Greens function approximations by conventional SI methods. We introduce SI by full-field multidimensional deconvolution (MDD) for elastodynamic wavefields as an alternative method to obtain body wave Greens functions under those typical circumstances. The advantage of this method compared to other MDD methods is that the kernel of its governing equation is exact. This alternative formulation of the kernel pertains to several advantages: the solution is less sensitive to artifacts and utilizes the free-surface multiples in the data to estimate primary reflections. Moreover, the point spread function of the full-field MDD method corrects more affectively for irregular illumination because it also addresses irregularities caused by scattering inside the medium. In order to compare full-field MDD to existing SI methods, we model synthetic earthquake recordings in a subduction zone setting using an elastodynamic finite-difference scheme with double couples of different orientations and peak frequencies. Our results show that SI by cross correlation suffers most under these circumstances. Higher-quality reflections are obtained by the MDD methods, of which full-field MDD involves the most stable inversion, and its results are least contaminated by artifacts.

Wavefield Decomposition of Field Data, Using a Shallow Horizontal Downhole Sensor Array and a Free-surface Constraint

SUMMARY Separation of recorded wavefields into downgoing and upgoing constituents is a technique that is used in many geophysical methods. The conventional, multi-component (MC) wavefield decomposition scheme makes use of different recorded wavefield components. In recent years, land acquisition designs have emerged that make use of shallow horizontal downhole sensor arrays. Inspired by marine acquisition designs that make use of recordings at multiple depth levels for wavefield decomposition, we have recently developed a multi-depth level (MDL) wavefield decomposition scheme for land acquisition. Exploiting the underlying theory of this scheme, we now consider conventional, multi-component (MC) decomposition as an inverse problem, which we try to constrain in a better way. We have overdetermined the inverse problem by adding an MDL equation that exploits the Dirichlet free-surface boundary condition. To investigate the successfulness of this approach, we have applied both MC and combined MC-MDL decomposition to a real land dataset acquired in Annerveen, the Netherlands. Comparison of the results of overdetermined MC-MDL decomposition with the results of MC wavefield decomposition, clearly shows improvements in the obtained one-way wavefields, especially for the downgoing fields.

The Seismic Response of a Single Fracture – Synthetic Modelling and Laboratory Measurements

The response of a single fracture in an elastic medium has been derived using the linear slip theory. We have performed laboratory experiments where we have successfully isolated the transmission response of a single fracture filled with different fracture infill materials. Experimental results are compared with the synthetic complex transmission response in order to retrieve the fracture compliance. The validity and the accuracy of the linear-slip theory to model the compliance of a single fracture have been looked at. The frequency- and angle-dependence of the fracture response can be used to characterize the compliance. Our results are strongly suggestive of the possibility of fracture compliance retrieval from the elastic wave transmission (or reflection) response.