Zhihui Zou

Assessing the reliability of low frequencies in geophone records

The reliability of low frequencies is a crucial question for geophone records. An example is the data recorded for passive seismic studies using geophones paired with TEXAN miniature recorders, where the high resonance frequency of the geophones degrades the low-frequency waveforms and increases the ambiguity in phase picking. We have evaluated the reliability of such data from a mobile array of geophoneTEXAN pairs in central China. The frequency response of the geophones can be easily established using the Power Spectral Density Ratio (PSDR) between records of the geophoneTEXAN pairs and a broadband seismometer occupying the same sites. This method allows a quantification of the retrievable frequencies of short-period seismic data and a reliable spectral extension. For instance, the retrievable frequency band of the data recorded by 4.5-Hz geophones can be extended down to 0.3 Hz for regional M2.0 earthquakes in our study area, and down to 0.04 Hz for surface wave of a M6.2 teleseismic event in Indonesia. After applying an inverse filter within the retrievable frequency band, the quality of the data is improved significantly and matched well with the records of a nearby broadband permanent station. The new method is useful for assessing and extracting low-frequency information from geophone data.

Evaluation of multi-scale full waveform inversion with marine vertical cable data

Seismic illumination plays an important role in subsurface imaging. A better image can be expected either through optimizing acquisition geometry or introducing more advanced seismic migration and/or tomographic inversion methods involving illumination compensation. Vertical cable survey is a potential replacement of traditional marine seismic survey for its flexibility and data quality. Conventional vertical cable data processing requires separation of primaries and multiples before migration. We proposed to use multi-scale full waveform inversion (FWI) to improve illumination coverage of vertical cable survey. A deep water velocity model is built to test the capability of multi-scale FWI in detecting low velocity anomalies below seabed. Synthetic results show that multi-scale FWI is an effective model building tool in deep-water exploration. Geometry optimization through target oriented illumination analysis and multi-scale FWI may help to mitigate the risks of vertical cable survey. The combination of multi-scale FWI, low-frequency data and multi-vertical-cable acquisition system may provide both high resolution and high fidelity subsurface models.

Tomographic velocity model building of the near surface with velocity-inversion interfaces: A test using the Yilmaz model

First-arrival traveltime tomography is a popular approach to building the near-surface velocity models for oil and gas exploration, mining, geoengineering, and environmental studies. However, the presence of velocity-inversion interfaces (VIIs), across which the overlying velocity is higher than the underlying velocity, might corrupt the tomographic solutions. This is because most first-arrival raypaths will not traverse along any VII, such as the top of a low-velocity zone. We have examined the impact of VIIs on first-arrival tomographic velocity model building of the near surface using a synthetic near-surface velocity model. This examination confirms the severe impact of VIIs on first-arrival tomography. When the source-to-receiver offset is greater than the lateral extent of the VIIs, good near-surface velocity models can still be established using a multiscale deformable-layer tomography (DLT), which uses a layer-based model parameterization and a multiscale scheme as regularization. Compared with the results from a commercial grid-based tomography, the DLT delivers much better near-surface statics solutions and less error in the images of deep reflectors.

The role of acquisition geometry and components for imaging microseismicity

Summary Radiation pattern of microseismicity is an important indicator of fracture orientation and distribution during reservoir fluid injection events. The quality of mapping the source radiation pattern by reverse time modeling is evaluated here. Among many factors influencing the imaging quality, the acquisition geometry and the component of input data are the most important. For instance, using a straight line of receivers will resulted in two imaged sources: the true source and a mirror source. The amplitudes of the two imaged sources are the same for single-component data, but the true imaged source has much higher amplitude than that of the mirror source for twocomponent data. One way to improve the image quality is to increase the array aperture, such as using both surface and wellbore receivers rather than just the surface receivers. Our results show that a larger aperture will result in better imaged source that matches well with the true source in both amplitude distribution and angle distribution of displacement. Those results will be useful for reservoir monitoring acquisition design and the study of induced microseismicity.

Velocity modeling and inversion techniques for locating microseismic events in unconventional reservoirs

A velocity model is an important factor influencing microseismic event locations. We review the velocity modeling and inversion techniques for locating microseismic events in exploration for unconventional oil and gas reservoirs. We first describe the geological and geophysical characteristics of reservoir formations related to hydraulic fracturing in heterogeneity, anisotropy, and variability, then discuss the influences of velocity estimation, anisotropy model, and their time-lapse changes on the accuracy in determining microseismic event locations, and then survey some typical methods for building velocity models in locating event locations. We conclude that the three tangled physical attributes of reservoirs make microseismic monitoring very challenging. The uncertainties in velocity model and ignoring its anisotropies and its variations in hydraulic fracturing can cause systematic mislocations of microseismic events which are unacceptable in microseismic monitoring. So, we propose some potential ways for building accurate velocity models.

Retrieving drill bit seismic signals using surface seismometers

Seismic while drilling (SWD) is an emerging borehole seismic imaging technique that uses the downhole drill-bit vibrations as seismic source. Without interrupting drilling, SWD technique can make near-real-time images of the rock formations ahead of the bit and optimize drilling operation, with reduction of costs and the risk of drilling. However, the signal to noise ratio (SNR) of surface SWD-data is severely low for the surface acquisition of SWD data. Here, we propose a new method to retrieve the drill-bit signal from the surface data recorded by an array of broadband seismometers. Taking advantages of wavefield analysis, different types of noises are identified and removed from the surface SWD-data, resulting in the significant improvement of SNR. We also optimally synthesize seismic response of the bit source, using a statistical cross-coherence analysis to further improve the SNR and retrieve both the drill-bit direct arrivals and reflections which are then used to establish a reverse vertical seismic profile (RVSP) data set for the continuous drilling depth. The subsurface images derived from these data compare well with the corresponding images of the three-dimension surface seismic survey cross the well.

Impact and solutions of seawater heterogeneity on wide-angle tomographic inversion of crustal velocities in deep marine environments—Numerical studies

The seawater column is typically taken as a homogeneous velocity layer in wide-angle crustal seismic surveys in marine environments. However, heterogeneities in salinity and temperature throughout the seawater layer result insignificant lateral variations in its seismic velocity, especially in deep marine environments. Failure to compensate for these velocity inhomogeneities will introduce significant artifacts in constructing crustal velocity models using seismic tomography. In this study, we conduct numerical experiments to investigate the impact of heterogeneous seismic velocities in seawater on tomographic inversion for crustal velocity models. Experiments that include lateral variation in seawater velocity demonstrated that the modeled crustal velocities were contaminated by artifacts from tomographic inversions when assuming a homogeneous water layer. To suppress such artifacts, we propose two strategies: 1) simultaneous inversion of water velocities and the crustal velocities; 2) layer-stripping inversion during which to first invert for seawater velocity and then correct the travel times before inverting for crustal velocities. The layer-stripping inversion significantly improves the modeling of variation in seawater velocity when preformed with seismic sensors deployed on the ocean bottom and in the water column. Such strategies improve crustal modeling via wide-angle seismic surveys in deep-marine environment.

An Evaluation of Reverse-Time Imaging of Clustering Earthquakes

Clustering earthquakes refer to the seismic events that occur closely with each other in time and space. Because their overlapping waveform records make it difficult to pick the first arrivals, the hypocenters of clustering earthquakes cannot be determined accurately by traveltime location methods. Here we apply a reverse-time imaging (RTI) method to map clustering earthquakes. Taking the advantage of directly using waveforms, the RTI method is capable to map either a single small earthquake or some densely distributed clustering earthquakes beneath a 2-D seismic array. In 3-D case the RTI method is successfully applied to locate the long-offset doublet earthquakes using the data from a set of sparsely distributed surface stations. However, for the same acquisition geometry, the RTI encounters challenges in mapping densely distributed clustering earthquakes. While it is obvious that improving the mapping of clustering earthquakes requires a denser receiver network with wider range of illumination angles, it is necessary to verify the actual resolution of the RTI method with synthetic data. In our study area in the Three Gorges region, our tests in 3-D case suggest that some events beneath the linear aligned sub-arrays have reasonable resolution.

Phase characteristic analysis of continuous depth air-gun source wavelet

Air guns are important sources for marine seismic exploration. Far-field wavelet of air gun arrays, as a necessary parameter for pre-stack processing and source models, plays an important role during marine seismic data processing and interpretation. When an air gun fires, it generates a series of air bubbles. Similar to onshore seismic exploration, the water forms a plastic fluid near the bubble; the farther the air gun is located from the measurement, the more steady and more accurately represented the wavelet will be. In practice, hydrophones should be placed more than 100 m from the air gun; however, traditional seismic cables cannot meet this requirement. On the other hand, vertical cables provide a viable solution to this problem. This study uses a vertical cable to receive wavelets from 38 air guns and data are collected offshore Southeast Qiong, where the water depth is over 1000 m. In this study, the wavelets measured using this technique coincide very well with the simulated wavelets and can therefore represent the real shape of the wavelets. This experiment fills a technology gap in China.

Blind Testing of Deformable Layer Tomography using Near-Surface First Arrivals

It is beneficial to both the developers and users of seismic imaging algorithms to learn their applicability and limitations. using the first-arrival dataset generated by the session chairs for an unknown synthetic near-surface velocity model, we have started to conduct a series of tests using deformable layer Tomography (DLT). in this method we parameterize the target model with a number of thickness-varying layers and the velocity function of each layer can be constant, gradient, or laterally varying. We use traveltimes of first arrives and reflections to invert for either the layer geometry, or the layer velocity function, or both simultaneously. We use a multi-scale scheme to regularize the Inversion for layer geometry and velocity functions. Details of the DLT method and field data examples are available in our publications. in this blind test, the objective is to estimate a 300-meter-wide near-surface P-wave velocity model using 10,100 first arrivals. We designed the velocity model building work in several steps. Firstly, we estimated the long-wavelength model by restricting the DLT to invert for the layer geometry of some constant-velocity layers. This step has resulted in a multi-layer model whose traveltime residues have a standard deviation of less than 1.2 ms. Secondly, we fixed the layer geometry, and invert for the laterally varying velocity functions. the solution from this step has reduced the standard deviation of the traveltime residuals to about 1.05 ms, which is close to the 1-ms standard deviation of the added noise. We plan to quantify the resolution and to characterize the multiple models of similar levels of data fitness. together with other researchers in this session, we expect to learn a great deal about the behaviors of different methods and characteristics of all nonunique solutions.