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<![CDATA[Inference of multi-Gaussian relative permittivity fields by probabilistic inversion of crosshole ground-penetrating radar data]]>
http://geophysics.geoscienceworld.org/cgi/content/short/82/5/H25?rss=1
In contrast to deterministic inversion, probabilistic Bayesian inversion provides an ensemble of solutions that can be used to quantify model uncertainty. We have developed a probabilistic inversion approach that uses crosshole first-arrival traveltimes to estimate an underlying geostatistical model, the subsurface structure, and the standard deviation of the data error simultaneously. The subsurface is assumed to be represented by a multi-Gaussian field, which allows us to reduce the dimensionality of the problem significantly. Compared with previous applications in hydrogeology, novelties of this study include an improvement of the dimensionality reduction algorithm to avoid streaking artifacts, it is the first application to geophysics and the first application to field data. The results of a synthetic example show that the model domain enclosed by one borehole pair is generally too small to provide reliable estimates of geostatistical variables. A real-data example based on two borehole pairs confirms these findings and demonstrates that the inversion procedure also works under realistic conditions with, for example, unknown measurement errors.
]]>2017-07-06T00:30:46-07:00info:doi/10.1190/geo2016-0347.1hwp:resource-id:gsgpy;82/5/H25Society of Exploration Geophysicists2017-07-06Ground-Penetrating Radar825H25H40<![CDATA[Numerical evaluation of active source magnetics as a method for imaging high-resolution near-surface magnetic heterogeneity]]>
http://geophysics.geoscienceworld.org/cgi/content/short/82/5/J27?rss=1
We have evaluated a geophysical method that uses a low-frequency magnetic source to image subsurface magnetic heterogeneity. This active source approach can be used to image magnetic features at higher resolutions than the conventional passive geomagnetic method. Importantly, this frequency-domain active source approach is independent of the effects of remanent magnetization, which complicates the interpretation of geomagnetic data. We carried out forward modeling of frequency-domain electromagnetic (EM) data and we found that, at frequencies of a few hertz, the magnetostatic response due to the induced magnetization dominates the EM induction response. The result suggests that it is possible to make magnetic interpretation of low-frequency EM data without having to consider the conductivity structure and the corresponding EM induction effect. We compare the anomalous magnetic responses with magnetic noise components and find that the proposed active source magnetic (ASM) method has a depth of investigation of approximately 300 m. Free-space field and inductive noise are considered as the most important issues affecting the depth of investigation. We also determine the potential for linear interpretation of magnetic heterogeneity under 0.1 SI by showing that the low-frequency magnetic response can be approximated by a linear magnetic response. In our synthetic experiments, inversion of the ASM data shows a marked enhancement in resolution, with no effect of the remanent magnetization, in contrast to geomagnetic inversion. These results show that the ASM method is a useful geophysical tool, especially when high-resolution imaging of magnetic susceptibility is required or where strong remanent magnetization complicates the magnetic interpretation.
]]>2017-07-06T00:30:46-07:00info:doi/10.1190/geo2016-0435.1hwp:resource-id:gsgpy;82/5/J27Society of Exploration Geophysicists2017-07-06Magnetic Exploration Methods825J27J38<![CDATA[Well ties for seismic with severe stratigraphic filtering]]>
http://geophysics.geoscienceworld.org/cgi/content/short/82/5/IM31?rss=1
Stratigraphic filtering (SF), or short-period multiples, is prominent in cyclically stratified sedimentation with large impedance contrasts that result in normal-incident reflection magnitudes greater than 0.5. Because SF attenuates and delays the propagating wavelet, similar to the effects of Q attenuation, the integrity of well ties is often jeopardized. A method is proposed to obtain better well ties in areas with severe SF. Starting with a well-log acoustic impedance curve, two-way transmitted wavefields and their equivalent inverse filters are generated at each time sample. Because a time-varying convolution of the transmitted wavefields with the primary-only reflectivity yields the multiple reflectivity, a time-varying deconvolution of the multiple synthetic with the inverse filters yields the primary-only reflectivity. In essence, when the multiple synthetic matches the near-angle stack at a well location, the near-angle stack is deconvolved in a time-varying fashion to match the primary-only synthetic, which then constitutes a correlation with the acoustic impedance yielding a good well tie. This new well-tie technique preserves the integrity of the lithologic interpretation because stretching and squeezing the time scale of the primary-only synthetic to force a seismic match are avoided. Our well-tie method is applied to the synthetic and field data from Cooper Basin, Australia, where more than 30 coal beds are observed within a 1000 ft (304 m) interval.
]]>2017-07-06T00:30:46-07:00info:doi/10.1190/geo2016-0695.1hwp:resource-id:gsgpy;82/5/IM31Society of Exploration Geophysicists2017-07-06Interpretation Methods825IM31IM39<![CDATA[Interferometric OBS imaging for wide-angle seismic data]]>
http://geophysics.geoscienceworld.org/cgi/content/short/82/5/Q39?rss=1
Marine wide-angle seismic data obtained using air guns and ocean-bottom seismographs (OBSs) are effective for determining large-scale subseafloor seismic velocities, but they are ineffective for imaging details of shallow seismic reflection structures because of poor illumination. Surface-related multiple reflections offer the potential to enlarge the OBS data illumination area. We have developed a new seismic imaging method for OBS surveys applying seismic interferometry, a technique that uses surface-related multiples similarly to mirror imaging. Seismic interferometry can use higher order multiple reflections than mirror imaging, which mainly uses first-order multiple reflections. A salient advantage of interferometric OBS imaging over mirror imaging is that it requires only single-component data, whereas mirror imaging requires vertical geophone and hydrophone components to separate upgoing and downgoing wavefields. We applied interferometric OBS imaging to actual 175 km long wide-angle OBS data acquired in the Nankai Trough subduction zone. We obtained clear continuous reflection images in the deep and shallow parts including the seafloor from the OBS data acquired with large spacing. Deconvolution interferometry is more suitable than correlation interferometry to improve spatial resolution because of the effects of spectral division when applied to common receiver gathers. We examined the imaging result dependence on data acquisition and processing parameters considering the data quality and target depth. An air-gun-to-OBS distance of up to 50 km and a record length of 80 s were necessary for better imaging. In addition, our decimation tests confirmed that denser OBS spacing yielded better quality and higher resolution images. Understanding crosstalk effects due to the acquisition setting will be useful to optimize methods for eliminating them. Interferometric OBS imaging merged with conventional primary reflection imaging is a powerful method for revealing crustal structures.
]]>2017-07-06T00:30:46-07:00info:doi/10.1190/geo2016-0482.1hwp:resource-id:gsgpy;82/5/Q39Society of Exploration Geophysicists2017-07-06Seismic Interferometry825Q39Q51<![CDATA[Full-waveform inversion using seislet regularization]]>
http://geophysics.geoscienceworld.org/cgi/content/short/82/5/A43?rss=1
Because of inaccurate, incomplete, and inconsistent waveform records, full-waveform inversion (FWI) in the framework of a local optimization approach may not have a unique solution, and thus it remains an ill-posed inverse problem. To improve the robustness of FWI, we have developed a new model regularization approach that enforced the sparsity of solutions in the seislet domain. The construction of seislet basis functions requires structural information that can be estimated iteratively from migration images. We implement FWI with seislet regularization using nonlinear shaping regularization and impose sparseness by applying soft thresholding on the updated model in the seislet domain at each iteration of the data-fitting process. The main extra computational cost of the method relative to standard FWI is the cost of applying forward and inverse seislet transforms at each iteration. This cost is almost negligible compared with the cost of solving wave equations. Numerical tests using the synthetic Marmousi model demonstrate that seislet regularization can greatly improve the robustness of FWI by recovering high-resolution velocity models, particularly in the presence of strong crosstalk artifacts from simultaneous sources or strong random noise in the data.
]]>2017-07-06T00:30:46-07:00info:doi/10.1190/geo2016-0699.1hwp:resource-id:gsgpy;82/5/A43Society of Exploration Geophysicists2017-07-06Geophysics Letters825A43A49<![CDATA[The seismic signature of rain]]>
http://geophysics.geoscienceworld.org/cgi/content/short/82/5/P53?rss=1
Rain has long been a problem for land seismic surveys, in terms of its effect on the condition of the surface and near surface, and also due to the seismic noise it creates when raindrops hit the ground. I measured the seismic signature of rainfall using water dripped from height using a pipette and natural rain in Winchester, England, over a three-month period. My results indicated that rain noise is concentrated at frequencies of greater than 80 Hz with a detectable range of less than 1 m. Drops of water landing directly on a geophone result in events with amplitudes nearly 30 times larger than those landing next to the geophone. Items placed on the surface of the ground, such as cables, absorb the energy of the impact and reduce the level of the resulting seismic noise. Burying geophones results in attenuation of rain noise by between 7.7 and 8.6 dB/0.1 m. But, given the effort required to bury geophones, it is likely that data processing algorithms, or the placement of vibration-absorbent matting, are likely to be the preferred strategies for dealing with the noise.
]]>2017-07-22T05:36:59-07:00info:doi/10.1190/geo2016-0421.1hwp:resource-id:gsgpy;82/5/P53Society of Exploration Geophysicists2017-07-22Seismic Data Acquisition825P53P60<![CDATA[Broadband seismic over/under sources and their designature-deghosting]]>
http://geophysics.geoscienceworld.org/cgi/content/short/82/5/P61?rss=1
The source depth influences the frequency band of seismic data. Due to the source ghost effect, it is advantageous to deploy sources deep to enhance the low-frequency content of seismic data. But, for a given source volume, the bubble period decreases with the source depth, thereby degrading the low-frequency content. At the same time, deep sources reduce the seismic bandwidth. Deploying sources at shallower depths has the opposite effects. A shallow source provides improved high-frequency content at the cost of degraded low-frequency content due to the ghosting effect, whereas the bubble period increases with a lesser source depth, thereby slightly improving the low-frequency content. A solution to the challenge of extending the bandwidth on the low- and high-frequency side is to deploy over/under sources, in which sources are towed at two depths. We have developed a mathematical ghost model for over/under point sources fired in sequential and simultaneous modes, and we have found an inverse model, which on common receiver gathers can jointly perform designature and deghosting of the over/under source measurements. We relate the model for simultaneous mode shooting to recent work on general multidepth level array sources, with previous known solutions. Two numerical examples related to over/under sequential shooting develop the main principles and the viability of the method.
]]>2017-07-22T05:36:59-07:00info:doi/10.1190/geo2016-0512.1hwp:resource-id:gsgpy;82/5/P61Society of Exploration Geophysicists2017-07-22Seismic Data Acquisition825P61P73<![CDATA[Image-domain velocity inversion and event location for microseismic monitoring]]>
http://geophysics.geoscienceworld.org/cgi/content/short/82/5/KS71?rss=1
Microseismic event locations obtained from seismic monitoring data sets are often a primary means of determining the success of fluid-injection programs, such as hydraulic fracturing for oil and gas extraction, geothermal projects, and wastewater injection. Event locations help the decision makers to evaluate whether operations conform to expectations or parameters need to be changed and may be used to help assess and reduce the risk of induced seismicity. However, obtaining accurate event location estimates requires an accurate velocity model, which is not available at most injection sites. Common velocity updating techniques require picking arrivals on individual seismograms. This can be problematic in microseismic monitoring, particularly for surface acquisition, due to the low signal-to-noise ratio of the arrivals. We have developed a full-wavefield adjoint-state method for locating seismic events while inverting for P- and S-wave velocity models that optimally focus multiple complementary images of recorded seismic events. This method requires neither picking nor initial estimates of event location or origin time. Because the inversion relies on (image domain) residuals that satisfy the differential semblance criterion, there is no requirement that the starting model be close to the true velocity. We determine synthetic results derived from a model with conditions similar to a field-acquisition scenario in terms of the number and spatial sampling of receivers and recorded coherent and random noise levels. The results indicate the effectiveness of the methodology by demonstrating a significantly enhanced focusing of event images and a reduction of 95% in event location error from a reasonable initial model.
]]>2017-07-06T00:30:46-07:00info:doi/10.1190/geo2016-0561.1hwp:resource-id:gsgpy;82/5/KS71Society of Exploration Geophysicists2017-07-06Passive Seismic Methods825KS71KS83<![CDATA[The impact of magnetic viscosity on time-domain electromagnetic data from iron oxide minerals embedded in rocks at Opemiska, Quebec, Canada]]>
http://geophysics.geoscienceworld.org/cgi/content/short/82/5/B165?rss=1
The magnetic viscosity (MV) effects observed at time scales between 0.01 and 10 ms at Opemiska are associated with magnetic grains of variable size in rocks. Recent observations made during a ground time-domain electromagnetic (TDEM) survey at Opemiska are consistent with four aspects of the spatial and amplitude characteristics of a MV response: (1) the Bz/t decay rate is roughly proportional to 1/t1+α, where –0.4 < α < 0.4, (2) the anomalies are mainly visible on the z-component, when the EM receiver sensor is located inside or just outside the transmitter loop, (3) there is no obvious x- or y-component response, and (4) the sites where MV effects are seen in the TDEM data are coincident with an airborne magnetic anomaly. Previous studies have demonstrated that MV could be caused by (1) fine-grained particles of maghemite or magnetite in the overburden, regolith, or soil that were formed through lateritic weathering processes, (2) volcanic glass shards from tuff containing approximately 1% by weight magnetite, which occur as grains approximately 0.002–0.01 μm in size precipitated in a spatially uniform way, or (3) from the Gallionella bacterium that precipitates ferrihydrite that oxidizes to nanocrystalline maghemite aggregates. The sites investigated at Opemiska are outcropping and well-exposed with relatively little or no overburden, and they are unfavorable for the formation of maghemite; hence, it is assumed that the source of MV seen at Opemiska cannot be the maghemite, or the other aforementioned causes. Hand samples were collected from Opemiska to identify the minerals present. Polished thin sections observed under an optical reflecting microscope identified the accessory minerals magnetite, ilmenite, and pyrrhotite, all known for their relatively high magnetic susceptibility. The use of the scanning electron microscope confirmed fine-grained magnetite grains as small as 0.667 μm. An electromagnetic induction spectrometer confirmed the viscous nature of the susceptibility of the Opemiska samples. This suggests that MV could originate not only from fine-grained magnetite and maghemite particles located in the weathered regolith but also from other iron oxides and magnetic minerals embedded in the rock itself.
]]>2017-07-22T05:36:59-07:00info:doi/10.1190/geo2017-0153.1hwp:resource-id:gsgpy;82/5/B165Society of Exploration Geophysicists2017-07-22Case Histories825B165B176<![CDATA[Temporal high-order staggered-grid finite-difference schemes for elastic wave propagation]]>
http://geophysics.geoscienceworld.org/cgi/content/short/82/5/T207?rss=1
The traditional high-order finite-difference (FD) methods approximate the spatial derivatives to arbitrary even-order accuracy, whereas the time discretization is still of second-order accuracy. Temporal high-order FD methods can improve the accuracy in time greatly. However, the present methods are designed mainly based on the acoustic wave equation instead of elastic approximation. We have developed two temporal high-order staggered-grid FD (SFD) schemes for modeling elastic wave propagation. A new stencil containing the points on the axis and a few off-axial points is introduced to approximate the spatial derivatives. We derive the dispersion relations of the elastic wave equation based on the new stencil, and we estimate FD coefficients by the Taylor series expansion (TE). The TE-based scheme can achieve (2M)th-order spatial and (2N)th-order temporal accuracy (N < 5). We further optimize the coefficients of FD operators using a combination of TE and least squares (LS). The FD coefficients at the off-axial and axial points are computed by TE and LS, respectively. To obtain accurate P-, S-, and converted waves, we extend the wavefield decomposition into the temporal high-order SFD schemes. In our modeling, P- and S-wave separation is implemented and P- and S-wavefields are propagated by P- and S-wave dispersion-relation-based FD operators, respectively. We compare our schemes with the conventional SFD method. Numerical examples demonstrate that our TE-based and TE + LS-based schemes have greater accuracy in time and better stability than the conventional method. Moreover, the TE + LS-based scheme is superior to the TE-based scheme in suppressing the spatial dispersion. Owing to the high accuracy in the time and space domains, our new SFD schemes allow for larger time steps and shorter operator lengths, which can improve the computational efficiency.
]]>2017-07-22T05:36:59-07:00info:doi/10.1190/geo2017-0005.1hwp:resource-id:gsgpy;82/5/T207Society of Exploration Geophysicists2017-07-22Seismic Modeling and Wave Propagation825T207T224<![CDATA[High-order finite-difference approximations to solve pseudoacoustic equations in 3D VTI media]]>
http://geophysics.geoscienceworld.org/cgi/content/short/82/5/T225?rss=1
Pseudoacoustic algorithms are very fast in comparison with full elastic ones for vertical transversely isotropic (VTI) modeling, so they are suitable for many applications, especially reverse time migration. Finite differences using simple grids are commonly used to solve pseudoacoustic equations. We have developed and implemented general high-order 3D pseudoacoustic transversely isotropic formulations. The focus is the development of staggered-grid finite-difference algorithms, known for their superior numerical properties. The staggered-grid schemes based on first-order velocity-stress wave equations are developed in detail as well as schemes based on direct application to second-order stress equations. This last case uses the recently presented equivalent staggered-grid theory, resulting in a staggered-grid scheme that overcomes the problem of large memory requirement. Two examples are presented: a 3D simulation and a prestack reverse time migration application, and we perform a numerical analysis regarding computational cost and precision. The errors of the new schemes are smaller than the existing nonstaggered-grid schemes. In comparison with existing staggered-grid schemes, they require 25% less memory and only have slightly greater computational cost.
]]>2017-07-22T05:36:59-07:00info:doi/10.1190/geo2016-0589.1hwp:resource-id:gsgpy;82/5/T225Society of Exploration Geophysicists2017-07-22Seismic Modeling and Wave Propagation825T225T235<![CDATA[A micro ocean-bottom E-field receiver]]>
http://geophysics.geoscienceworld.org/cgi/content/short/82/5/E233?rss=1
Ocean-bottom electromagnetic (EM) receivers are used to record EM signals for marine magnetotelluric and controlled-source EM offshore data acquisition. These marine EM data are used for offshore gas hydrate and petroleum exploration. Although many conventional receivers are used for offshore data acquisition, they have deficiencies, such as a large size, high cost, and low operational efficiency. To address these limitations, we have developed a micro ocean-bottom E-field (micro-OBE) receiver. It reduces costs and deck-space use, while providing improved horizontal resolution and operational efficiency. Based on conventional receiver specifications, including low noise levels, low power consumption, and low clock-drift error, we reduced the receiver size, provided a low-cost release mechanism, integrated an acoustic telemetry module, improved the operational efficiency, and reduced the field acquisition cost. The resulting micro-OBE is comprised of a compact nylon frame, 17 inch glass sphere, logging system, batteries, recovery beacon, burn-wire release mechanism, transducer, electrode, red flag, and anchor (which eliminates the heavy and expensive acoustic release). It has no alloy aluminum press case, and it contains a minimal number of glass spheres, watertight cables, and connectors. Its size is 70x50x50 cm (not including the electrode arm, red flag, and anchor). Its weight in air is 70 kg. It has a noise level of 0.1 nV/m/rt (Hz) at 8 Hz, and it provides 33 days of battery lifetime. Offshore data acquisition was performed to evaluate the micro-OBE field performance during an offshore gas-hydrate experiment. Our results indicate that the receiver effectively functioned throughout the operation and offshore data acquisition. Micro-OBE was thus verified as providing a low cost, compact size, and high operational efficiency.
]]>2017-07-06T00:30:46-07:00info:doi/10.1190/geo2016-0242.1hwp:resource-id:gsgpy;82/5/E233Society of Exploration Geophysicists2017-07-06Electrical and Electromagnetic Methods825E233E241<![CDATA[Correlation analysis for spread-spectrum induced-polarization signal processing in electromagnetically noisy environments]]>
http://geophysics.geoscienceworld.org/cgi/content/short/82/5/E243?rss=1
In induced-polarization (IP) surveys, the raw data are usually distorted significantly by the presence of electromagnetic (EM) interferences, including cultural noise. Several methods have been proposed to improve the signal-to-noise ratio of these data. However, signal processing in an electromagnetically noisy environment is still a challenging problem. We have determined a new and simple technique based on the analysis of the correlation between the measured potential and the injected primary current signals. This processing is applied to the data acquired using a new frequency-domain IP method called the spread-spectrum induced-polarization (SSIP) approach. In this approach, we use a pseudorandom m-sequence (also called the maximum length sequence) for the injected primary current. One of the advantages of this sequence is to be essentially spectrally flat in a given frequency range. Therefore, complex resistivity can be determined simultaneously at various frequencies. A new SSIP data set is acquired in the vicinity of Baiyin mine, Gansu Province, China. The correlation between potential difference and transmitting current signals for each period can be used to assess data quality. Only when the correlation coefficient between the two signals is greater than 0.5 can the SSIP data be used for subsequent processing and tomography. We determine what threshold value should be used for the correlation coefficient to extract high-quality apparent complex resistivity data and eliminate EM-contaminated data. We then compare the pseudosections with and without using the correlation analysis. When the correlation analysis is used, the noisy data are filtered out, and the target anomaly obtained through tomography is clearly enhanced. The inversion results of the apparent complex resistivity (amplitude and phase) for the survey area are consistent with some independent geologic and drilling information regarding the position of the ore body demonstrating the effectiveness of the approach.
]]>2017-07-06T00:30:46-07:00info:doi/10.1190/geo2016-0109.1hwp:resource-id:gsgpy;82/5/E243Society of Exploration Geophysicists2017-07-06Electrical and Electromagnetic Methods825E243E256<![CDATA[Noise properties of Fourier deconvolution for time-domain electromagnetic soundings]]>
http://geophysics.geoscienceworld.org/cgi/content/short/82/5/E257?rss=1
The transient electromagnetic method (TEM) is widely used for mapping the earth’s subsurface resistivity structures, e.g., for groundwater or mineral exploration, using airborne and ground-based systems. Data from TEM surveys can be modified using Fourier deconvolution to correct for instrument drift, increase apparent bandwidth, or alter the apparent excitation waveform, e.g., to fuse or compare measurements from different systems, for visual presentation purposes, or to make the data compatible with a specific processing or inversion code. Although this method has been applied in several studies, little attention has been devoted to its properties with regard to noise. Using a generic analytical system model and examples featuring synthetic and field data, we perform a detailed analysis of the noise properties of Fourier deconvolution in the context of TEM. We find that although the effects from stationary noise are trivial, effects from nonstationary noise are less intuitive and more severe, e.g., causing on-time noise phenomena to degrade off-time data. In general, we observe that the method decreases the signal-to-noise ratio, and our recommendation is therefore that Fourier deconvolution should only be applied when it is desirable for presentation purposes or when strictly necessary for processing or inversion.
]]>2017-07-22T05:36:59-07:00info:doi/10.1190/geo2016-0588.1hwp:resource-id:gsgpy;82/5/E257Society of Exploration Geophysicists2017-07-22Electrical and Electromagnetic Methods825E257E266<![CDATA[Extraction of reflected events from sonic-log waveforms using the Karhunen-Loeve transform]]>
http://geophysics.geoscienceworld.org/cgi/content/short/82/5/D265?rss=1
Sonic-reflection logging, a recently developed borehole geophysical scheme, is in principle capable of providing a clear view of outside the well bore. In this type of acoustic well logging, a key technical obstacle is that the reflected wave signal is almost entirely obscured by the directly arriving P-, S-, and Stoneley wave modes. Effective extraction of these reflection signals from the full acoustic waveforms is therefore a critical data-processing step. We have examined the use of the Karhunen-Loève (KL) transform, combined with a band-limiting filter, as a technique for the extraction of reflections of interest from a mixture with directly arriving wave modes of much higher amplitude. Under the assumption that large energy (squared-amplitude) differences exist between each wave component, the direct Stoneley wave, S-wave, and the P-wave are eliminated sequentially by subtracting the most significant principal components, after which the remaining signal is seen to be dominated by reflected events. Thereafter, the extracted reflections can be used in migration to provide interpretable images of the structures outside the borehole. Synthetic data are used to develop and justify our procedure for subtraction of appropriate KL principal components. Laboratory data are used to demonstrate in detail the suppression of unwanted modes. For comparison, the multiscale slowness-time-coherence method is applied to extract reflections from the same data set. The procedure is exemplified on a field data case with attention paid in particular to the consequences to imaging of near-borehole structures.
]]>2017-07-22T05:36:59-07:00info:doi/10.1190/geo2017-0031.1hwp:resource-id:gsgpy;82/5/D265Society of Exploration Geophysicists2017-07-22Borehole Geophysics825D265D277<![CDATA[Elastic full-waveform inversion for surface topography]]>
http://geophysics.geoscienceworld.org/cgi/content/short/82/5/R269?rss=1
Elastic full-waveform inversion (EFWI) attempts to find high-resolution model parameters that are able to match observed data exactly by minimizing residuals between the observed and predicted data. However, irregular surface topography can generate problematic EFWI results. To resolve this issue, an EFWI method in the curvilinear system is presented to invert velocities for areas with surface topography. This method meshes the regions near the surface topography into body-fitted grids, and areas off surface regions into rectangular grids. We derived the gradient formulas with respect to multiple parameters in the curvilinear system and developed a step-search method to calculate the corresponding step lengths. Wiener filtering is performed on a multiscale decomposition method to invert the velocity from a low to a high wavenumber. Error analysis, using a homogeneous model with a surface topography, demonstrates the accuracy and validity of our methods. Numerical experiments with an elastic hill gully surface topography model reveal that the results obtained from our method are comparable with those of the body-fitted grid method, with the notable exception of a shortened computation time for the generation of grids and simulating wavefields. The inverted results with a modified elastic Marmousi model indicate that the inverted parameters are very well-resolved after implementing the proposed EFWI on the surface topography. The sensitivity analyzes of heterogeneous near-surface media, stochastic noise, and starting velocity errors further reveal that our method has the capability of handling complex land data.
]]>2017-07-06T00:30:46-07:00info:doi/10.1190/geo2016-0349.1hwp:resource-id:gsgpy;82/5/R269Society of Exploration Geophysicists2017-07-06Seismic Inversion825R269R285<![CDATA[A low-frequency deghosting method: Analysis and numerical tests]]>
http://geophysics.geoscienceworld.org/cgi/content/short/82/5/V285?rss=1
The low-frequency component of seismic data can be beneficial for several reasons: improved signal penetration into the earth, enhanced resolution, and better constrained inversion results. We have developed a detailed analysis of a deghosting solution for the low-frequency spectrum of marine seismic pressure data. The advantages of this low-frequency deghosting method are: (1) it can be applied in the spatial domain, (2) it is applicable for horizontal streamers and for streamers with a mild depth variation, and (3) it is a fast-track solution that can be used flexibly as a preprocessing, or premigration step. The disadvantages of this method are: (1) it is an approximation to the full-deghosting operator and cannot infill the ghost notches of the spectrum, except near 0 Hz, and (2) it has decreasing effectiveness with a larger source/receiver depth. Numerical tests on the synthetic and field data sets indicate that this method is promising in deghosting data, up to at least half the frequency of the first nonzero ghost notch.
]]>2017-07-06T00:30:46-07:00info:doi/10.1190/geo2016-0116.1hwp:resource-id:gsgpy;82/5/V285Society of Exploration Geophysicists2017-07-06Signal Processing825V285V296<![CDATA[A discrepancy-based penalty method for extended waveform inversion]]>
http://geophysics.geoscienceworld.org/cgi/content/short/82/5/R287?rss=1
Extended waveform inversion globalizes the convergence of seismic waveform inversion by adding nonphysical degrees of freedom to the model, thus permitting it to fit the data well throughout the inversion process. These extra degrees of freedom must be curtailed at the solution, for example, by penalizing them as part of an optimization formulation. For separable (partly linear) models, a natural objective function combines a mean square data residual and a quadratic regularization term penalizing the nonphysical (linear) degrees of freedom. The linear variables are eliminated in an inner optimization step, leaving a function of the outer (nonlinear) variables to be optimized. This variable projection method is convenient for computation, but it requires that the penalty weight be increased as the estimated model tends to the (physical) solution. We describe an algorithm based on discrepancy, that is, maintaining the data residual at the inner optimum within a prescribed range, to control the penalty weight during the outer optimization. We evaluate this algorithm in the context of constant density acoustic waveform inversion, by recovering background model and perturbation fitting bandlimited waveform data in the Born approximation.
]]>2017-07-22T05:36:59-07:00info:doi/10.1190/geo2016-0326.1hwp:resource-id:gsgpy;82/5/R287Society of Exploration Geophysicists2017-07-22Seismic Inversion825R287R298<![CDATA[Amplitude variation with offset-friendly bootstrapped differential semblance]]>
http://geophysics.geoscienceworld.org/cgi/content/short/82/5/V297?rss=1
Spectral noise, low resolution, and attenuation of semblance peaks due to amplitude variation with offset (AVO) anomalies hamper the reliability of velocity analysis in the semblance spectrum for seismic data processing. Increasing resolution and reducing noise while accounting for AVO has posed a challenge in various semblance schemes due to a trade-off in resolution and AVO detectability. A new semblance scheme is introduced that aims to remove this trade-off. The new scheme uses the concepts of bootstrapped differential semblance with trend-based AB semblance. Results indicate that the new scheme indeed increases spectral resolution, reduces noise, and accounts for AVO anomalies. These improvements facilitate velocity control for automatic and manual picking methods and, hence, provide a means for more reliable apparent velocity models.
]]>2017-07-22T05:36:59-07:00info:doi/10.1190/geo2016-0395.1hwp:resource-id:gsgpy;82/5/V297Society of Exploration Geophysicists2017-07-22Signal Processing825V297V309