ALBERT

Description: A 1D normal moveout (NMO)-corrected and stacked pseudoprofiling method was applied to analyze the characteristic features shown on primary P - and S -wave coda and on Sp waveforms from local microearthquakes in an attempt to image prominent reflectors and to resolve shallow crustal velocity structure (~5 km) in the upper Mississippi embayment. Acoustic well log data were used to constrain the P -wave velocity in the upper 5 km. Events at close distances and with clear P and S arrivals were selected to ensure reliable NMO correction for reflections and transmissions. The observed reflections and transmissions are important controlling factors on modeling waveforms. We analyzed local earthquake data recorded at all broadband and one short-period station of the Cooperative New Madrid Seismic Network. Despite polarity differences among P , S , and Sp waveforms, consistent reflectors in the sedimentary section can be imaged across the three wave types. Correlation with a basement-penetrating well indicates that reflectors at the base of the Upper Cretaceous–Holocene Mississippi Embayment Supergroup, the base of the Cambrian–Ordovician Knox Group, and the high-velocity lower Upper Cambrian Bonneterre Formation are shown in pseudoprofiles among stations in the upper Mississippi embayment. Our study finds that a one-layer homogeneous velocity model of sediments in the ranges of 1.95–2.42 km/s for V P and 0.60–0.73 km/s for V S overlying a half-space of Paleozoic rocks with velocities in the ranges of 6.0–6.2 km/s for V P and 3.26–3.6 km/s for V S can represent shallow crustal structure in the upper Mississippi embayment. Differential times of P – PpPhp and S – SsShs appear linearly proportional to sediment thicknesses, which best fits a one-layer sediment structure with average V P 2.042±0.041 km/s and V S 0.709±0.051 km/s, in the least-squares sense predicted by the wave propagation effects. Online Material: Figures illustrating seismograms, process procedure, imaged reflectors, and resolved velocity structure for six broadband stations and one short-period station.

Description: Journal of the American Chemical Society DOI: 10.1021/jacs.6b09693

Description: We develop an automated strategy for discriminating deep microseismic events from shallow ones on the basis of the waveforms recorded on a limited number of surface receivers. Machine-learning techniques are employed to explore the relationship between event hypocentres and seismic features of the recorded signals in time, frequency and time–frequency domains. We applied the technique to 440 microearthquakes –1.7 〈  M w  〈 1.29, induced by an underground cavern collapse in the Napoleonville Salt Dome in Bayou Corne, Louisiana. Forty different seismic attributes of whole seismograms including degree of polarization and spectral attributes were measured. A selected set of features was then used to train the system to discriminate between deep and shallow events based on the knowledge gained from existing patterns. The cross-validation test showed that events with depth shallower than 250 m can be discriminated from events with hypocentral depth between 1000 and 2000 m with 88 per cent and 90.7 per cent accuracy using logistic regression and artificial neural network models, respectively. Similar results were obtained using single station seismograms. The results show that the spectral features have the highest correlation to source depth. Spectral centroids and 2-D cross-correlations in the time–frequency domain are two new seismic features used in this study that showed to be promising measures for seismic event classification. The used machine-learning techniques have application for efficient automatic classification of low energy signals recorded at one or more seismic stations.

Description: We introduce a nondiagonal seismic denoising method based on the continuous wavelet transform with hybrid block thresholding (BT). Parameters for the BT step are adaptively adjusted to the inferred signal property by minimizing the unbiased risk estimate of Stein (1980) . The efficiency of the denoising for seismic data has been improved by adapting the wavelet thresholding and adding a preprocessing step based on a higher-order statistical analysis and a postprocessing step based on Wiener filtering. Application of the proposed method on synthetic and real seismic data shows the effectiveness of the method for denoising and improving the signal-to-noise ratio of local microseismic, regional, and ocean bottom seismic data.

Description: Seismic waveforms from the Eastern Tennessee Seismic Network are corrected to the nominal Wood–Anderson (WA) torsion seismometer to obtain a total of 11,905 maximum trace amplitudes from 690 events seen on 50 different horizontal components to determine a local magnitude scale for the Eastern Tennessee Seismic Zone (ETSZ). We use the following distance-correction function –log 10 ( A 0 )=0.538( r /17)–0.0002516( r –17)+2.0, in which A 0 is the maximum amplitude measured in millimeters and r is the hypocentral distance measured in kilometers; this better agrees with reported moment magnitudes for larger events in the ETSZ. Using the normal 100 km distance for M L normalization severely overestimates M L , and we therefore chose to adopt the 17 km normalization technique. The –log 10 ( A 0 ) is very flat at distances 〉200 km, suggesting unusually low distance attenuation at local and near-regional distances from the ETSZ. The WA response reported by the International Association of Seismology and Physics of the Earth’s Interior to the nominal response shows no significant difference in the distance-dependent factor for the log A 0 term, although the revised response consistently yields M L values that are 0.1 lower than those found using the nominal response. The b -values for the currently reported duration magnitude are lower than the b -values obtained using the newly calculated M L scale. The relationship between M L and M D can be expressed as M L =0.68093 M D +0.64603. The catalog of events used in this study is complete for M L 〉1.3.

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.