ALBERT

Description: The traditional approach to both earthquake and Global Positioning System (GPS) location problems in a homogeneous half-space produces a nonlinear relationship between a set of known positions, seismic stations or GPS satellites, and an unknown point, an earthquake hypocenter or GPS receiver. Linearization, followed by an iterative inversion, is typically used to solve both problems. Although sources and receivers are swapped in the earthquake and GPS location problems, the observation equation is the same for both, due to the principle of reciprocity. Consequently, the mechanical part of the solution of the equations is the same and single-step closed-form solutions for the GPS location problem, such as the Bancroft algorithm, can also be used to solve for earthquake hypocenters in a homogeneous half-space. This article applies the Bancroft algorithm to synthetic and real data for the Charlevoix seismic zone and compares the location of ~1200 events estimated with both the Bancroft algorithm and HYPOINVERSE. The Bancroft algorithm shows quantifiable improvements in accuracy compared with traditional methods. We also show how tools commonly used by the GPS community, such as the geometric dilution of precision, can be used to better estimate the precision of the results obtained by a seismic network.

Description: Three small-scale refraction and gradiometry experiments were performed to investigate whether off-the-shelf exploration geophones and seismographs can be used to perform meaningful gradiometry measurements. Relative calibration of the geophones was attempted through huddle tests and spectral measurements of ambient ground motions. The results show that relative gains cannot be completely characterized because of geophone/ground interaction. Numerical tests show that typically observed 4.5% gain errors introduce a standard deviation of 0.03 s/km and 1.97° about the correct input slowness and azimuth, respectively, for 100,000 realizations of synthetic array data. A standard linear refraction experiment was performed to investigate the slowness of P and Rayleigh waves from hammer sources to compare with measurements taken from two gradiometer designs. One design consists of four 6-instrument gradiometers in a linear array to investigate the spatial and temporal variation of horizontal slowness and propagation azimuth for sources close to the array as well as to test the location abilities of the entire gradiometer array. Its location estimate for a shot 10 m from the center of the array using the two closest cells was close to the actual source position with a 1.38-m error. The gradiometers were able to correctly determine the slowness values of the P and surface waves identified in the refraction profile. A second gradiometer experiment involves superimposed cells to explore precision in calculation of spatial gradients. Significant increases in the precision of the wave attributes occur when a larger number of averaged time-shifted waveforms are used as the reference-station displacement in place of a single-reference-station displacement waveform. The concept of center-station correlation is introduced to avoid spurious amplitude errors from drastically affecting the wave parameter estimates. We conclude that the off-the-shelf equipment can be used to construct small dense gradiometer arrays that can be used to infer wave attributes that are important for the interpretation of wavefields in exploration seismology experiments.

Description: The Mississippi Embayment overlies the New Madrid seismic zone in the central United States and consists of a thick succession of unconsolidated sediments that covers the Paleozoic basement rock. The high-velocity contrast at the basement–sediment interface and the near-vertical ray paths of teleseismic P -waves generate very large P to S conversions on the radial component and large P reverberations on the vertical component. This characteristic of P -wave propagation is used to study P - and S -wave resonance in the sedimentary layer. Horizontal-to-vertical (H/V) and vertical-to-horizontal (V/H) power spectral ratios are calculated for a time window around the teleseismic P -wave arrivals at broadband stations of the USArray Transportable Array, Northern Embayment Lithosphere Experiment, and the New Madrid Cooperative Seismic Network in the embayment. Using a map of sediment thickness, we developed models for average P - and S -wave velocity-versus-sediment thickness. The resulting shear-wave velocity model matches very well with the model obtained from an H/V ambient noise study in the region. The shear-wave velocity model shows low velocity near the edges of the embayment with average velocities increasing with increasing sediment thickness, consistent with increased sediment compaction. Fundamental resonance frequency for S waves ranges from 0.2 to 0.4 Hz for the embayment sediments. Electronic Supplement: Two figures showing the synthetic test results and a table that includes peak resonance frequency and average V P and V S .

Description: Detailed shear and compressional gradient velocity structure for the unconsolidated sediments of the Mississippi Embayment (ME) have been obtained through simultaneous inversion of radial and vertical teleseismic transfer functions for 60 broadband stations inside the ME. Transfer functions were calculated by deconvolving a vertical array beam from the radial and vertical displacement at each station inside the embayment. The vertical array beam was constructed by stacking all the vertical components of motions for stations on the bedrock and outside the embayment to reduce the effect of incoherent scattering, improving estimates of the source time function and stabilizing the deconvolution. A bounded variable least-squares inversion technique was applied to invert for velocity nodes on the top and bottom of the sediment layer. P -wave velocity changes from 1.0 km/s near the surface and increases with depth from 3.5 to 4 km/s in deeper parts. S -wave velocity varies from 0.3 km/s and increases to 1.6 km/s in deeper sections to the south. Electronic Supplement: Gradient velocity structure of the northern Mississippi Embayment sediments for 60 broadband stations.

Description: Standard P -wave receiver function analyses in polar environments can be difficult because reverberations in thick ice coverage often mask important P -to- S conversions from deeper subsurface structure and increase ambient noise levels, thereby significantly decreasing the signal-to-noise ratio of the data. In this study, we present an alternative approach to image the subsurface structure beneath ice sheets. We utilize downward continuation and wavefield decomposition of the P -wave response to obtain the up- and downgoing P and S wavefield potentials, which removes the effects of the ice sheet. The upgoing P wavefield, computed from decomposition of the waveform at a reference depth, is capable of indicating ice layer thickness. This simple step removes the necessity of modeling ice layer effects during iterative inversions and hastens the overall velocity analysis needed for downward continuation. The upgoing S wave is employed and modeled using standard inversion techniques as is done with receiver functions at the free surface using a least-squares approximation. To illustrate our proof of concept, data from several Antarctic networks are examined, and our results are compared with those from previous investigations using P - and S -wave receiver functions as well as body- and surface-wave tomographic analyses. We demonstrate how our approach satisfactorily removes the ice layer, thus creating a dataset that can be modeled for crustal and upper-mantle structure. Solution models indicate crustal thicknesses as well as average crustal and upper-mantle shear-wave velocities. Electronic Supplement: Figure of measured data, the vertical-component stack used in deconvolution, and the resultant vertical, radial, and tangential transfer functions.

Description: Recorded seismic signals are often corrupted by noise. We have developed an automatic noise-attenuation method for single-channel seismic data, based upon high-resolution time-frequency analysis. Synchrosqueezing is a time-frequency reassignment method aimed at sharpening a time-frequency picture. Noise can be distinguished from the signal and attenuated more easily in this reassigned domain. The threshold level is estimated using a general cross-validation approach that does not rely on any prior knowledge about the noise level. The efficiency of the thresholding has been improved by adding a preprocessing step based on kurtosis measurement and a postprocessing step based on adaptive hard thresholding. The proposed algorithm can either attenuate the noise (either white or colored) and keep the signal or remove the signal and keep the noise. Hence, it can be used in either normal denoising applications or preprocessing in ambient noise studies. We tested the performance of the proposed method on synthetic, microseismic, and earthquake seismograms.

Description: 〈span〉〈div〉Summary〈/div〉A dense seismic array, composed of over 5000 stations with an average spacing close to 120 m was deployed in Long Beach, CA, by NodalSeismic and Signal Hill Petroleum as part of a survey associated with the Long Beach oilfield. Among many interesting wave propagation effects that have been reported by others, we observe that the coda of teleseismic P waves display waves caused by obvious local scattering from the Signal Hill popup structure between strands of the Newport-Inglewood fault. The density of the seismic array allows space-based methods, such as the Curvelet transform, to be investigated to separate the teleseismic and local scattered wavefields. We decompose a synthetic wavefield composed of a teleseismic plane wave and local scattered spherical wave in the curvelet domain to test the plausibility of our curvelet analysis and then apply the technique to the Long Beach array dataset. Background noise is removed by a soft thresholding method and a declustering technique is applied to separate the teleseismic and local scattered wavefield in the curvelet domain. Decomposed results illustrate that the signal-to-noise ratio of the teleseismic P wave can be significantly improved by curvelet analysis. The scattered wavefield is composed of locally propagating Rayleigh waves from the pop-up structure and from the Newport Inglewood fault itself. Observing the wavefield both in space and time clearly improves understanding of wave propagation complexities due to structural heterogeneity.〈/span〉

Description: The coastal plains of the central and eastern United States contain deep sections of unconsolidated to poorly consolidated sediments. These sediments mask deeper crustal and upper-mantle converted phases in teleseismic receiver functions through large amplitude, near-surface reverberations. Thick sediments also amplify ambient noise levels to generally reduce data signal-to-noise ratios. Removing shallow-sediment wave-propagation effects is critical for imaging deep lithospheric structures. A propagator matrix formalism is used to downward-continue the wave field for teleseismic P waves into the midcrust and then to separate the upgoing S-wave field from the total teleseismic response of the P wave to expose deep Sp conversions. This method requires that the Earth model from the surface to the reference depth be known. Teleseismic P-wave data for the Memphis, Tennessee, station (MPH) are analyzed using a reference-station deconvolution technique to produce vertical and radial P-wave transfer functions. These transfer functions are modeled using a simple model parameterization for sediment structure through grid inversion. The inverted Earth model is incorporated into the wave-field continuation and decomposition technique to estimate the upgoing S-wave field at 10 km depth in the crust. Moho and possible deeper Ps conversions are identified with this process.