M. van der Baan

Automation of Shear-Wave Splitting Measurements using Cluster Analysis

The propagation of two orthogonally polarized shear waves, or shear- wave splitting, is arguably the most robust indication of seismic anisotropy in the Earth. This splitting can be parameterized in terms of the polarization of the fast shear-wave and the lag time between fast and slow components dt. These two parameters provide constraints on the mechanism causing the anisotropy. All meth- ods of calculating splitting require a shear-wave analysis window to be selected. Then the and dt that best account for the splitting in that window are calculated. Conventionally the shear-wave analysis window is picked manually. However, man- ual window selection is laborious and also very subjective; in many cases different windows give very different results. We present a method for automating the selec- tion of the window. First, the splitting analysis is performed for a range of window lengths. Then a cluster analysis is applied in order to find those measurements that are stable over many different windows. Once clusters of stable results have been found, the final choice of shear-wave analysis window corresponds to the measure- ment with the lowest error in the cluster with the lowest variance. Resulting estimates of and dt are objective, and very large datasets can be analyzed easily. The success of the technique is illustrated with application to a microseismic dataset of 324 events, which confirms previously published results using manually selected analysis windows.

The 1998 Valhall microseismic data set: An integrated study of relocated sources, seismic multiplets, and S-wave splitting

Werelocate303microseismiceventsrecordedin1998bysensorsinasingleboreholeintheNorthSeaValhalloilfield.Asemiautomated array analysis method repicks the P- and S-wave arrival times and P-wave polarizations, which are needed to locate these events. The relocated sources are confined predominantly toa50-m-thickzonejustabovethereservoir,andlocationuncertaintiesarehalfthoseofpreviousefforts.Multipletanalysisidentifies 40 multiplet groups, which include 208 of the 303 events. The largest group contains 24 events, and five groups contain 10 or more events. Within each multiplet group, we further improve arrival-time picking through crosscorrelation, which enhances the relative accuracy of the relocated events and reveals that more than 99% of the seismic activity lies spatially in three distinct clusters. The spatial distribution of events and waveform similarities reveal two faultlike structures that match well with north-northwest‐south-southeast-trending fault planes interpreted from 3D surface seismic data. Most waveform differencesbetweenmultipletgroupslocatedonthesefaultscanbeattributed to S-wave phase content and polarity or P-to-S amplitude ratio. The range in P-to-S amplitude ratios observed on the faults is explained best in terms of varying source mechanisms. Wealsofindacorrelationbetweenmultipletgroupsandtemporal variations in seismic anisotropy, as revealed by S-wave splitting analysis. We explain these findings in the context of a cyclic rechargeanddissipationofcap-rockstressesinresponsetoproduction-driven compaction of the underlying oil reservoir. The cyclic nature of this mechanism drives the short-term variations in seismic anisotropy and the reactivation of microseismic source mechanismsovertime.

Signal Extraction and Automated Polarization Analysis of Multicomponent Array Data

We present a method to extract seismic signals from three-component array data and estimate their polarization properties at each station. The technique is based on a singular value decomposition (SVD) of the complex three-component analytic signal and applies to linearly as well as elliptically polarized seismic phases. To increase accuracy we simultaneously analyze data from different stations and apply a noise weighting based on prearrival data. For polarization analysis, an au- tomated routine is also included. The automated routine selects the data window with the best signal-to-noise ratio from which to obtain a polarization. A linearity measure and a confidence interval accompany the polarization estimate at each station in the array. We test our technique for automated polarization analysis on synthetic P-wave data and compare results with those from other methods. A microseismic dataset from the North Sea provides a unique opportunity to statistically compare previous and independently obtained P-wave polarizations with those provided by the auto- mated technique presented here. We conclude that, for P-wave polarization analysis, our method is robust and significantly more accurate than conventional, mainly man- ual methods. This is especially so on data with polarized and correlating background noise. It is also faster and provides meaningful quality estimates.

Interpretation of resonance frequencies recorded during hydraulic fracturing treatments

Hydraulic fracturing treatments are often monitored by strings of geophones deployed in boreholes. Instead of picking discrete events only, we here use time-frequency representations of continuous recordings to identify resonances in two case studies. This paper outlines an interpretational procedure to identify their cause using a subdivision into source, path, and receiver-side effects. For the first case study, two main resonances are observed both at depth by the downhole geophones and on the surface by two broadband arrays. The two acquisition networks have different receiver and path effects, yet recorded the same resonances; these resonances are therefore likely generated by source effects. The amplitude pattern at the surface arrays indicates that these resonances are probably due to pumping operations. In the second case study, selective resonances are detected by the downhole geophones. Resonances coming from receiver effects are either lower or higher frequency, and wave propagation modeling shows that path effects are not significant. We identify two possible causes within the source area, namely, eigenvibrations of fractures or non-Darcian flow within the hydraulic fractures. In the first situation, 15–10 m long fluid-filled cracks could generate the observed resonances. An interconnected fracture network would then be required, corresponding to mesoscale deformation of the reservoir. Alternatively, systematic patterns in non-Darcian fluid flow within the hydraulic fracture could also be their leading cause. Resonances can be used to gain a better understanding of reservoir deformations or dynamic fluid flow perturbations during fluid injection into hydrocarbon and geothermal reservoirs, CO2 sequestration, or volcanic eruptions.

A New Parametric Method for Time Delay Estimation

In this paper, a new parametric method for time delay estimation is proposed. The method, classified under the generalized cross-correlation (GCC) approach, uses a couple of identical FIR filters to process the data from each sensor. A set of filters is designed to maximize the output cross-correlation at each lag. The lag associated with the maximum of all filtered cross-correlation is the estimate of the time delay,and its associated FIR filter is the optimum processor. The proposed method is implemented in the time domain and does not need spectral information as for the classical GCC methods. Its implementation uses eigen-decomposition algorithms and requires one input parameter which is the order of the FIR filter. It is equivalent to applying a data-driven bandpass filter to the cross-correlogram to emphasise the source signal. The proposed method is compared with standard methods in a simulation study. Simulation results show very good performance for the case of short data records and at low to moderate SNR levels.

Wavelet estimation and blind deconvolution of realistic synthetic seismic data by log spectral averaging

Homomorphic wavelet estimation was a popular tool in the late 70s. It consists of taking the logarithm of traces transformed to the frequency domain. This maps a convolution in the time domain to an addition in the log(frequency domain). Wavelet estimation was however done on single traces. We improve on this concept by introducing a multi-trace approach. In current marine acquisition the source wavelet is very uniform across an entire survey. We assume that we have selected many traces, randomly sampled from a large area, that are all characterized by different reflectivity series. Averaging the log(spectra) then recovers the original source wavelet. The advantage of this approach is that it does not make any assumptions on the phase of the wavelet. After wavelet estimation, a standard deconvolution algorithm can be used to improve the resolution of the seismic data.

Semblance-Based Anisotropy Parameter-Estimation in Layered VTI Media Using Rational Interpolation

Summary The τ-p domain is the natural domain for anisotropy-parameter estimation in horizontally layered media. However, the need to transform the data to the τ-p domain or to pick traveltimes in the t-x domain, is a practical disadvantage. To overcome this, we combine a τ-p domain inversion technique with rational interpolation of traveltimes in the t-x domain. This combination results in a highly accurate and efcient semblance-based method for anisotropy parameter-estimation from the moveout of P-waves in layered transversely isotropic (TI) media with a vertical symmetry axis (VTI).

Widening of normal fault zones due to the inhibition of vertical propagation

Abstract In this paper, we document the early stage of fault-zone development based on detailed observations of mesocale faults in layered rocks. The vertical propagation of the studied faults is stopped by layer-parallel faults contained in a weak layer. This restriction involves a flat-topped throw profile along the fault plane and modifications of the fault structures near the restricted tips, with geometries ranging from planar structures to fault zones characterized by abundant parallel fault segments. The ‘far-field’ displacement (i.e. the sum of the displacement accumulated by all the fault segments and the folding) measured along the restricted faults exhibiting this segmentation may have flat-topped shapes or triangular shapes when fault-related folding is observed above the layer-parallel faults. We develop a model from the observations. In this model, during the course of restriction, a fault forms as a simple isolated planar structure, then parallel fault segments successively initiate to accommodate the increasing displacement. We assume that, eventually, the fault propagates beyond the layer-parallel fault. This model implies first that fault widening is controlled by the fault capacity to propagate vertically in the layered section. Likewise, owing to restriction, fault growth occurs with non-linear increases in maximum displacement, length and thickness.

Low-Frequency Tremor Signals from a Hydraulic Fracture Treatment in Northeast British Columbia, Canada

The Rolla Microseismic Experiment (RME) was undertaken by the Microseismic Industry Consortium August 7-28, 2011 to record a multistage hydraulic fracture stimulation of a Triassic unconventional gas reservoir in northeastern British Columbia, Canada. The microseismic deployment included a 6-level downhole toolstring with low-frequency (4.5 Hz) geophones, a set of 21 portable broadband seismograph systems, and a 12-channel surface array comprised of 10-Hz geophones. Although we did not observe LPLD events based on how they have been previously described, our data exhibit high-amplitude signals in the 8-15 Hz band. These signals are monotonic and have been interpreted as resonance of fluid-filled cracks or successions of small repetitive events. We have also detected several instances of discrete microseismic events with unusually low frequency. An apparent tendency for low-frequency tremors to precede high-frequency microseismicity in our data provides a tantalizing suggestion that these processes may be genetically linked.

Interpretation of Resonance Frequencies Recorded during Hydraulic Fracturing

The frequency content of continuous passive seismic recordings of hydraulic fracturing are sometimes used to gain information on the numerous microseismic events occurring during the fluid injection. In addition, resonance frequencies are frequently recorded as well. Resonances can be used in many ways due to their multiple origins possible. In every circumstance, all possible sources have to be reviewed to define their respective influence. We present two different experiments showing resonances with presumably different origins. For the first experiment, the low-frequency resonances (5-50 Hz) are only recorded by downhole geophones and broadband stations on the surface that are close to the injection well. For the second experiment, four resonances at 17, 35, 51 and 60 Hz are detected. The fluid injection being at approximately the same depth as the receivers, the path effect influence will be limited and the resonances coming from receiver effects are anticipated to be outside the frequency range of the observed resonances. A possible source would be the resonance of fluid-filled cracks. The size of a crack corresponding to a resonance of 17 Hz is calculated to be 17 m. These resonances would then correspond to mesoscale deformation of the reservoir.