Reverse time migration of phase encoded all-order multiples

Multiples follow long paths and carry more information on the subsurface than primary reflections, making them particularly useful for imaging. However, seismic migration using multiples can generate crosstalk artifacts in the resulting images because multiples of different orders interfere with each others, and crosstalk artifacts greatly degrade the quality of an image. We propose to form a supergather by applying phase-encoding functions to image multiples and stacking several encoded controlled-order multiples. The multiples are separated into different orders using multiple decomposition strategies. The method is referred to as the phase-encoded migration of all-order multiples (PEM). The new migration can be performed by applying only two finite-difference solutions to the wave equation. The solutions include backward-extrapolating the blended virtual receiver data and forward-propagating the summed virtual source data. The proposed approach can significantly attenuate crosstalk artifacts and also significantly reduce computational costs. Numerical examples demonstrate that the PEM can remove relatively strong crosstalk artifacts generated by multiples and is a promising approach for imaging subsurface targets.

The vertical image projection method for migration stretch compensation

True amplitude inversion is often carried out without taking into account migration distortions to the wavelet. Seismic migration leaves a dip-dependent effect on the wavelet that can cause significant inaccuracies in the inverted impedances obtained from conventional inversion approaches based on 1D vertical convolutional modelling. Neglecting this effect causes misleading inversion results and leakage of dipping noise and migration artifacts from higher frequency bands to the lower frequencies. I have observed that despite dip-dependency of this effect, low-dip and flat events may also suffer if they are contaminated with cross-cutting noise, steep migration artifacts, and smiles. In this paper I propose an efficient, effective and reversible data pre-conditioning approach that accounts for dip-dependency of the wavelet and is applied to migrated images prior to inversion. My proposed method consists of integrating data with respect to the total wavenumber followed by the differentiation with respect to the vertical wavenumber. This process is equivalent to applying a deterministic dip-consistent pre-conditioning that projects the data from the total wavenumber to the vertical wavenumber axis. This preconditioning can be applied to both pre- and post-stack data as well as to amplitude variation with offset (AVO) attributes such as intercept and gradient before inversion. The vertical image projection methodology that I propose here reduces the impact of migration artifacts such as cross-cutting noise and migration smiles and improves inverted impedances in both synthetic and real data examples. In particular I show that neglecting the proposed pre-conditioning leads to anomalously higher impedance values along the steeply dipping structures.

Inter-episodes earthquake migration in the Bohai-Zhangjiakou Fault Zone, North China: Insights from numerical modeling

In this paper, we focus on why intraplate seismic initiation and migration occurs, which has widely been considered to be caused by static stress triggering caused by earthquakes, as well as post-seismic slips. To illustrate the mechanism underlying large earthquakes, in particular the migration caused by two key episodes that occurred after 1500 in the Bohai-Zhangjiakou Fault Zone (BZFZ) of North China, we developed a high-resolution three-dimensional viscoelastic finite element model that includes the active faults with vertical segmentation, their periodical locking, and the lithosphere heterogeneity. We used the birth and death of element groups to simulate stress intensity changes during the two episodes (named Episode I and II), with our results showing that the Tangshan earthquake was primarily triggered by the Sanhe-Pinggu M8.0 earthquake in 1679, whereas the Zhangbei M6.2 earthquake in 1998 was not triggered by earthquakes in Episode I. According to our work, the calculated stress changes in the different segments of the fault zone correspond to the magnitude of the triggered earthquakes. Further, the largest stress decrease was near the Sanhe-Pinggu fault and occurred the largest earthquake in Episode I, whereas the largest stress increase was near the Tangshan fault and occurred during the largest earthquake in Episode II. Given the above, we propose a model for seismic migration to describe the dynamic mechanisms of earthquake migration within the BZFZ and North China, in which the factors affecting both the seismic migration path and intensity primarily include the distance between the triggered active fault and the original fault, the coupling of the active faults, the location and scale of the low-velocity anomaly, its distance from the active fault, and the location and scale of the crustal thinning.

Dual seismic migration velocities reveal imbricated fluid diffusion and aseismic slip In a Corinth Gulf swarm (Greece)

<p>Fluid induced earthquake sequences generally appear as expanding swarms activating a particular fault. Such swarms are generally interpreted as fluid diffusion, which ignores the possibility of static, dynamic or aseismic triggering, and the existence of rapid migration. Here, we study the temporal evolution of a seismic swarm that occurred over a 10-day period in October 2015 in the extensional rift of the Corinth Gulf (Greece) using high-resolution earthquakes relocations. The seismicity radially migrates on a normal fault at a fluid diffusion velocity (~125 m/day). However, this migration occurs intermittently, with periods of fast expansion (2-to-10 km/day) during short seismic bursts alternating with quiescent periods. Moreover, the growing phases of the swarm illuminate a high number of repeaters. Therefore, we propose a new model to explain the combination of multiple driving processes for such swarms.  Fluid up flow in the fault may induce aseismic slip episodes, separated by phases of fluid pressure build-up. The stress perturbation due to aseismic slip may activate small asperities in the fault that produce bursts of seismicity during the most intense phase of the swarm. We then validated this model through hydro-mechanical modeling, where earthquakes consist in the failure of asperities on a creeping fault infiltrated by fluid. For that, we couple rate‐and‐state friction, non‐linear diffusivity and elasticity along a 1D interface. This model reproduces the dual migration speeds observed in real swarms. We show that migration speeds increase linearly with the mean pressurization, and are not dependent on the hydraulic diffusivity, as traditionally suggested.</p>

Time Evolution of Tectonic Earthquake in Ambon, Indonesia

The advancement of earthquake seismology brings a new insight into earthquake detection. Advanced signal processing by implementing sliding-window mathematical techniques of cross-correlation into data stream are able to recognize low amplitude earthquake signals even under the presence of noise. Clear detection of the onset of low amplitude seismic waves is crucial, however they often hidden by larger amplitudes of coda waves after the generation of mainshock event. By performing template matching algorithm we found a detail temporal variation of seismicity related to Mw 6.6 Ambon earthquake. In comparison, the detection level is up to eight times from conventional method. The method also reveals a seismic migration before the main event coincide with the direction of local tectonic movement derived from previous GPS analysis. Since the method able to detect the earthquake within their family, it gives a reasonably significant improvement to virtual stress-meter analysis. Highly accumulated stress preceding the main event depicted by b-value drop are clearly mapped in a high confidence level.