Direct Arrivals Research Articles

Strategi Pengembangan Wisata Edukasi Tanaman Palawija Sebagai Atraksi Wisata di Desa Sukarame Kecamatan Pacet Kabupaten Bandung

Agritourism is an agricultural industry activity that has the expectation of direct tourist arrivals to tourist attractions organized in the hope of expanding knowledge, experience, and recreation. The purpose of this study was to determine the potential of crops as a tourist attraction and the development strategy of educational tourism of crops as a tourist attraction in Sukarame Village. The research used is qualitative research with a descriptive approach and analyzed using SWOT analysis. Data collection techniques were obtained from secondary data and primary data such as conducting interviews, observation and documentation. The data collection method used was purposive sampling with key research informants who have expertise in their respective fields. The data analysis used consists of four stages including data collection, data reduction, data presentation, and conclusion drawing. The result of this research is that Sukarame Village has great potential to develop as an agro-tourism and educational tourism destination. Active community participation and promotion through various media are needed to support this development effort. Infrastructure challenges and skills improvement need to be addressed through financial assistance and training and utilizing agritourism and agricultural education opportunities can expand the promotion and marketing of Sukarame Village.

The impact of admission modes on the treatment outcome and in-hospital mortality rate of STEMI patients undergoing PPCI

The current research on ST elevation myocardial infarction (STEMI) patients has been mostly limited to Door-to-Balloon (D-to-B) time. This study aimed to compare the effects of different hospital admission modes to on the time metrics of patients undergoing primary percutaneous coronary intervention (PPCI). It also examined the effects of these modes on in-hospital mortality and other influencing factors. The goal was to prompt healthcare facilities at all levels, including chest hospitals, the Centers for Disease Control and Prevention (CDC), and communities to take measures to enhance the treatment outcomes for patients with STEMI. A total of 1053 cases of STEMI patients admitted to Tianjin Chest Hospital from December 2016 to December 2023 and successfully underwent PPCI were selected for this study. They were divided into three groups based on the admission modes: the ambulances group (363 cases), the self-presentation group (305 cases), and the transferred group (385 cases). Multivariate logistic regression was used to explore the impact of different modes of hospital admission on the standard-reaching rate of key treatment time metrics. The results showed that the S-to-FMC time of transferred patients (OR = 0.434, 95% CI 0.316–0.596, P < 0.001) and self-presentation patients (OR = 0.489, 95% CI 0.363–0.659, P < 0.001) were more likely to exceed the standard than that of ambulance patients; The cath lab pre-activation time of self-presented patients was also less likely to meet the standard than that of ambulance patients (OR = 0.695, 95% CI 0.499–0.967, P = 0.031); D-to-W time of self-presentation patients was less likely to reach the standard than that of ambulance patients (OR = 0.323, 95% CI 0.234–0.446, P < 0.001);However, the FMC-to-ECG time of self-presentation patients was more likely to reach the standard than that of ambulance patients (OR = 2.601, 95% CI 1.326–5.100, P = 0.005). The Cox proportional hazards model analysis revealed that for ambulance patients, the time spent at each key treatment time point is shorter, leading to lower in-hospital mortality rate (HR0.512, 95% CI 0.302–0.868, P = 0.013) compared to patients admitted by other means. We found that direct arrival of STEMI patients to the PCI hospital via ambulance at the onset of the disease significantly reduces the S-to-FMC time, FMC-to-ECG time, D-to-W time, and catheterization room activation time compared to patients who self-present. This admission mode enhances the likelihood of meeting the benchmark standards for each time metric, consequently enhancing patient outcomes.

The 12 October 2021, MW\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$M_W$$\\end{document}=6.4, Zakros, Crete earthquake

We study the strong 12 October 2021, MW\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$M_W$$\\end{document}=6.4, offshore Zakros, Crete earthquake, and its seismotectonic implications. We obtain a robust location (azimuthal gap equal to 17∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{\\circ }$$\\end{document}) for the mainshock by combining all freely available local, regional and teleseismic phase arrivals (direct and depth phase arrivals). Based on our location and the spatial distribution of the poor aftershock sequence we parameterise the fault area as a 30 km ×\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ imes $$\\end{document} 20 km planar surface, and using three-component strong motion data we calculate slip models for both earthquake nodal planes. Our preferred solution shows a simple, single slip episode on a NE-SW oriented, NW shallow-dipping fault plane, instead of a N-S oriented, almost vertical nodal plane. An anti-correlation of the aftershocks spatial distribution versus the maximum slip (∼\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim $$\\end{document} 27 cm) of our model further supports this, although the accuracy of the aftershock hypocentral locations could be somewhat questionable. Coulomb stress changes calculated for both kinematic models do not show substantial differences, as the aftershock seismicity within the first 3 months after the mainshock is distributed along the stress shadow zone and over the stress enhanced areas developed at the southern fault edge, induced by the mainshock. The Kasos island tide gauge record analysis shows a small signal after the earthquake, but it can hardly demonstrate the existence of tsunami waves due to the low signal-to-noise ratio. Tsunami simulations computed for the two nodal planes do not yield conclusive evidence to highlight whether the causative fault plane is NE-SW oriented, NW shallow-dipping plane, or the N-S oriented plane, nevertheless, the power spectrum analysis of the NW shallow-dipping nodal plane matches the spectral peak at 8 s period and is overall closer to the spectrum of the tide gauge record. A USGS Shakemap was also produced with all available local strong motion data and EMSC testimonies. This was also investigated in an effort to document the responsible fault. The overall analysis in this study, slightly suggests a rather westward, shallow-dipping offshore fault zone, being antithetic to the main Zakros almost vertical normal fault which shapes the coast of eastern Crete and is perpendicular to the direction of Ptolemy Trench in this area. This result agrees with seismotectonic and bathymetric evidence which support the existence of approximately N-S trending grabens, east and northeast of Crete.

A spectrum-of-spectrum filtering method to extract direct and multipath arrivals from simulations and measurements

In measurements and numerical modelling of wave propagation, undesired interference between direct and multipath arrivals can be reduced using Fourier-based signal processing methods. Existing methods, such as cepstral analysis and time-signal gating, are not applicable to all cases. Here, an alternative Fourier-based signal processing method is presented, called spectrum-of-spectrum (SoS) filtering. Its main advantage over existing methods is its ability to extract single direct or multipath arrivals for relatively short propagation distances even when subsequent arrivals do not become successively weaker. The method is based on the following steps:•Apply a lowpass filter to the real and imaginary parts of an input frequency spectrum individually, using a digital finite impulse response (FIR) filter in the frequency domain.•Recombine the filtered real and imaginary parts of the frequency spectrum to get the frequency spectrum of the direct arrival.•For extraction of the first multipath arrival, subtract the filtered frequency spectrum from the input frequency spectrum and repeat the previous steps. Repeat multiple times to extract subsequent multipath arrivals.

Sources depth estimation for a tonal source by matching the interference structure in the arrival angle domain.

A publication by McCargar and Zurk [J. Acoust. Soc. Am. 133(4), EL320-EL325 (2013)] introduced a passive source depth estimation method for a moving tonal source with a vertical line array (VLA), utilizing the depth-dependent modulation in the arrival angle domain caused by the interference between the direct and surface-reflected acoustic arrivals. Under the isovelocity approximation, this method can estimate the depth of sources at close ranges, but the depth estimation error will increase with the increase in source range, as the impact of the sound speed profile on sound propagation is ignored. This paper presents a theoretical formula for calculating the modeled interference structure in the arrival angle domain with the knowledge of the sound speed profile. By matching the measured interference structure obtained from the beamforming of the acoustic data received by the VLA with the modeled structure under different assumed source depths, the tonal source depth estimation is achieved, even for sources at the remote part of the direct arrival zone. The performance of this method is verified by simulation data, as well as experimental data radiated from a towed source and a non-cooperative passing ship.

Trace-wise coherent noise suppression via a self-supervised blind-trace deep-learning scheme

Seismic data denoising via supervised deep learning is effective and popular but requires noise-free labels, which are rarely available. Blind-spot networks circumvent this requirement by training directly on noisy data and have been demonstrated to be a powerful suppressor of random noise. In this work, we expand the methodology of blind-spot networks to create a blind-trace network that successfully removes trace-wise coherent noise. An extensive synthetic analysis illustrates the denoising procedure’s robustness to varying noise levels and varying numbers of noisy traces within shot gathers. It is demonstrated that the network can accurately learn to suppress the noise when up to 60% of the original traces are noisy. Furthermore, our procedure is implemented on the Stratton 3D field data set and demonstrates the restoration of the previously corrupted direct arrivals. In addition to trace-wise noise suppression, we adapt the blind-spot networks to the successful suppression of colored Gaussian noise, which exhibits varying coherent properties in time and spatial axes. Our adaptation of the blind-spot networks paves the way for its use in other applications, such as the suppression of coherent noise arising from wellsite activity, passing vessels, or nearby industrial activity.

An application of internal multiple prediction and primary reflection retrieval to salt structures

Irregular salt bodies complicate the seismic wavefield propagation and cause multipathing, leading to intricate internal multiples that cannot be easily suppressed during data processing. To investigate the performance of internal multiple removal in salt bodies, we compare two types of Marchenko-based internal multiple removal strategies. One focuses on internal multiple prediction (IMP) based on Marchenko redatuming and seismic interferometry. With the retrieved upgoing and downgoing Green’s functions, internal multiples can be reconstructed using convolutional interferometry. The second method emphasizes primary reflection retrieval (PRR) by capturing primary reflections without adaptive subtraction. It is derived from a modified method based on the Gel’fand-Levitan-Marchenko equation. Testing with the Pluto salt model, we find that for the IMP the critical point is obtaining the purely scattering components of the upgoing and downgoing Green’s functions from the complicated wavefield. Our complex salt model indicates that the predicted internal multiples’ adaptive subtraction also introduces an extra challenge for attenuation quality. We develop a layer-by-layer adaptive subtraction scheme for the IMP to obtain primary reflections. The constant-time truncation operator in the time domain makes the PRR independent of additional model information, which can exclude internal multiples from the primary reflections. However, selecting an appropriate smoothing window for truncation requires further investigation. We evaluate the weaknesses of these methods and enhance each method individually. Our numerical results indicate that these improved schemes can correctly handle internal multiples in salt structure scenarios. In the addition, we study the robustness of the IMP when an erroneous background velocity model is used to estimate the direct arrival and the effects of sampling on the virtual receiver/source lines in IMP.

Seismic prompt gravity strain signals in a layered spherical Earth

Seismic waves generated by an earthquake can produce dynamic perturbations in the Earth’s gravity field before the direct arrival of P-waves. Observations of these so-called prompt elasto-gravity signals by ground-based gravimeters and broadband seismometers have been reported for some large events, such as the 2011 MW9.1 Tohoku earthquake. Recent studies have introduced prompt gravity strain signals (PGSSs) as a new type of observable seismic gravity perturbation that can be used to measure the spatial gradient of the perturbed gravity field. Theoretically, these types of signals can be recorded by in-development instruments termed gravity strainmeters, although no successful detection has been reported as yet. Herein, we propose an efficient approach for PGSSs based on a multilayered spherical Earth model. We compared the simulated waveforms with analytical solutions obtained from a homogeneous half-space model, which has been used in earlier studies. This comparison indicates that the effect of the Earth’s structural stratification is significant. With the help of the new simulation approach, we also demonstrated how the PGSSs depend on the magnitude of the seismic source. We further conducted synthetic tests estimating earthquake magnitude using gravity strain signals to demonstrate the potential application of this type of signal in earthquake early warning systems. These results provide essential information for future studies on the synthesis and application of earthquake-induced gravity strain signals.

Inferring damage state and evolution with increasing stress using direct and coda wave velocity measurements in faulted and intact granite samples

SUMMARY A better understanding of damage accumulation before dynamic failure events in geological material is essential to improve seismic hazard assessment. Previous research has demonstrated the sensitivity of seismic velocities to variations in crack geometry, with established evidence indicating that initial crack closure induces rapid changes in velocity. Our study extends these findings by investigating velocity changes by applying coda wave interferometry (CWI). We use an array of 16 piezoceramic transducers to send and record ultrasonic pulses and to determine changes in seismic velocity on intact and faulted Westerly granite samples. Velocity changes are determined from CWI and direct phase arrivals. This study consists of three sets of experiments designed to characterize variations in seismic velocity under various initial and boundary conditions. The first set of experiments tracks velocity changes during hydrostatic compression from 2 and 191 MPa in intact Westerly granite samples. The second set of experiments focuses on saw-cut samples with different roughness and examines the effects of confining pressure increase from 2 to 120 MPa. The dynamic formation of a fracture and the preceding damage accumulation is the focus of the third type of experiment, during which we fractured an initially intact rock sample by increasing the differential stress up to 780 MPa while keeping the sample confined at 75 MPa. The tests show that: (i) The velocity change for rough saw cut samples suggests that the changes in bulk material properties have a more pronounced influence than fault surface apertures or roughness. (ii) Seismic velocities demonstrate higher sensitivity to damage accumulation under increasing differential stress than macroscopic measurements. Axial stress measured by an external load cell deviates from linearity around two-third through the experiment at a stress level of 290 MPa higher than during the initial drop in seismic velocities. (iii) Direct waves exhibit strong anisotropy with increasing differential stress and accumulating damage before rock fracture. Coda waves, on the other hand, effectively average over elastic wave propagation for both fast and slow directions, and the resulting velocity estimates show little evidence for anisotropy. The results demonstrate the sensitivity of seismic velocity to damage evolution at various boundary conditions and progressive microcrack generation with long lead times before dynamic fracture.

From strain to displacement: using deformation to enhance distributed acoustic sensing applications

SUMMARY Over a period of less than a decade, distributed acoustic sensing (DAS) has become a well-established technology in seismology. For historical and practical reasons, DAS manufacturers usually provide instruments that natively record strain (rate) as the principal measurement. While at first glance strain recordings seem related to ground motion waveforms (displacement, velocity and acceleration), not all the seismological tools developed over the past century (e.g. magnitude estimation, seismic beamforming, etc.) can be readily applied to strain data. Notably, the directional sensitivity of DAS is more limited than conventional particle motion sensors, and DAS experiences an increased sensitivity to slow waves, often highly scattered by the subsurface structure and challenging to analyse. To address these issues, several strategies have been already proposed to convert strain rate measurements to particle motion. In this study, we focus on strategies based on a quantity we refer to as ‘deformation’. Deformation is defined as the change in length of the cable and is closely related to displacement, yet both quantities differ from one another: deformation is a relative displacement measurement along a curvilinear path. We show that if the geometry of the DAS deployment is made of sufficiently long rectilinear sections, deformation can be used to recover the displacement without the need of additional instruments. We validate this theoretical result using full-waveform simulations and by comparing, on a real data set, the seismic velocity recovered from DAS with that recorded by collocated seismometers. The limitations of this approach are discussed, and two applications are shown: enhancing direct P-wave arrivals and simplifying the magnitude estimation of seismic events. Converted displacement provides better sensitivity to high velocity phases, improves broadside response and permits the direct application of conventional seismological tools that are less effective when applied to strain (rate) data.

Wave Reflection Effects on Bender Element Test of Very Stiff Clay

A common source of error in bender element testing lies in the determination of the shear wave arrival time arising from the near-field effect, which is attributed to shear wave–like motion of the compression wave obscuring the arrival of the shear waves, and can be mitigated by using a sufficiently high travel-distance-to-wavelength (L/λ) ratio. However, the L/λ ratio also is relevant for signal interference due to side reflections, and its role in this important mechanism is far less understood. Stiff materials such as cement-treated soils, which have a very short shear wave travel time, are likely to exacerbate these errors. This paper examined the causes of travel time errors in cement-treated soil using bender element tests of standard-sized triaxial specimens, large block samples, and three-dimensional finite-element analyses. The results show that the reflection of shear waves from sample boundaries generates a second pulse in the received signal, resulting in signal distortion. This can be ameliorated at sufficiently high L/λ ratios. The reflection of the compressional waves generates spurious prearrivals and distorts the profile of the direct shear wave even at high L/λ ratios. Shear and compressional wave reflections can be mitigated by increasing the specimen diameter to decouple the latter from the direct shear wave arrival.

A study on bubble suppression for deep marine reflection data acquired by a small airgun array

AbstractThe oscillatory bubble pulses generated by airguns in seawater are known to produce artefacts in seismic images. Although such artefacts are suppressible by employing larger airgun arrays in acquisition, small airgun arrays are used more often now to minimize the environmental impacts, thus raising the need for further suppressing bubble pulses at data processing stage. For a deep marine reflection dataset recently acquired by a small airgun array, we compare the effectiveness of three popular debubbling methods that estimate the far‐field source signature based on theoretical simulation and wavelets extracted from seafloor reflections and direct arrivals, respectively. In this case, due to the lack of near‐field measurements for calibration, the debubbling via simulation underperforms the two wavelet extraction methods. Overlapping events in the noisy response of seafloor sediments lead to the failure of the wavelet extraction from primary seafloor reflections. The estimated source signature based on direct arrivals achieves the best bubble suppression result, indicating the importance of signal‐to‐noise ratio and a low level of directionality of the small airgun array source.

Comparison of high-frequency components in acoustic emissions from rock fracture under Mode Ⅰ and Mode Ⅱ dominated loading

High-frequency components in acoustic emission (AE) or seismic radiation attract much attention as they are directly related to the fracture process. One of the main issues in our knowledge of high-frequency radiation is a lack of definition on the relationship between high-frequency content and source mechanisms. In this study, high-frequency emission waves due to rock fracture in tension and shear are studied experimentally and theoretically. An improved AE-signal processing method, focusing on direct-arrived waves, is proposed to pinpoint the exact high-frequency radiation in a rupture event. Current data support that for each individual AE signal the later arriving waves contain lots of low-frequency components, while the high-frequency components are mostly carried by the direct arrivals. In comparison with tensile cracks, shear cracking generates more high-frequency components and tends to radiate energy in the form of waves with larger RA values (designated as rise time/amplitude). Combining the theories from dynamic fracture and acoustic radiation, the difference in high-frequency energy release from shear and tensile cracking can be attributed to the rate of stress drop at the tips. The present findings provide a framework for a new way to identify the tensile failure in rock. It will be helpful in solving the difficulty tracking seismic or micro-seismic events from tensile sources associated with karst collapsing, hydraulic fracturing and roof falling in mines.

A workflow for processing mono‐channel Chirp and Boomer surveys

AbstractAcquiring seismic data with multichannel, multiple‐streamer and even multi‐componentsystems at sea provides excellent images of the Earth. However, the cost and complexity of operations prevent their use in busy areas such as ports or in sensitive environments such as lagoons. In the latter cases, mono‐channel Chirp or Boomer systems are the most viable instruments for marine surveys. The lack of multiple offsets prevents the use of standard tools for amplitude‐versus‐offset and velocity analysis, which are necessary for the lithological characterization of rocks, especially for the shallow sediments in offshore engineering. In this paper, we present a few recent techniques that exploit the traveltime and amplitude of multiple reflections to compensate for the offset limitation, including a new algorithm for the joint tomographic inversion of direct arrivals, primaries and multiples. We have developed a cost‐effective workflow for mono‐channel surveys based on a data‐driven, physically consistent philosophy that attempts to approximately extract lithological parameters, such as P velocity, anelastic absorption, acoustic impedance and even density. We applied the proposed workflow to a real field experiment and obtained a semi‐quantitative estimate for shallow sediments that can be used by offshore engineers.

Focusing functions correction in Marchenko imaging with deep learning and transfer learning

The Marchenko method is a subsurface approach to calculate Green's functions from the subsurface to the surface. It requires only reflection response from the surface and direct arrivals from the selected subsurface focal points to the surface. Marchenko imaging can use these obtained Green's functions to create images for all the focal points (image points). Focusing functions are defined to relate Green's functions with the surface reflection response. Estimating focusing functions is the first step in the calculation of Marchenko imaging. However, the estimation of incorrect direct arrivals with the initial model can lead to errors in the focusing functions, thus affecting the effect of the image. In addition, Marchenko imaging is not an efficient imaging method, especially when the imaging area is particularly large, since the number of image points corresponds to the times of solving Marchenko equations. Therefore, we introduce a deep learning and transfer learning approach for the focusing functions correction. We construct a neural network and train the model in a small imaging region by deep learning. Then update this model for extended area by transfer learning which enables the network to work on new data dissimilar to the training data. Hence, our work can quickly obtain accurate focusing functions in a larger region instead of training an extra model for correcting new focusing functions. Numerical examples shows that our network not only helps to reduce errors in the focusing functions, but also can be applied to cope with other limitations of the Marchenko method like travel time error and imperfectly sampled reflection response.

Time of arrival estimation for underwater acoustic signal using multi-feature fusion

Underwater target detection (UTD) is widely applied in ocean exploration. A key issue for the time-of-arrival (ToA)-based UTD algorithm is to localize the first direct path in an underwater multipath channel. However, the conventional Matched Filter (MF) suffers from a deterioration of the estimation accuracy when the direct arrival of the received signal is non-strongest. Therefore, this paper proposes a multi-feature fusion ToA estimation algorithm. First, a multi-decision strategy including the coarse ToA range estimation and the fine ToA estimation is proposed to realize a balance between computational complexity and estimation accuracy. Second, the coarse ToA range estimation is realized by multi-level spectrum analysis. Then, the coarse ToA range is used to localize the fine ToA utilizing the multi-feature fusion ToA estimation. Finally, the simulation and experimental results are provided to validate the proposed ToA estimation algorithm. It clearly shows that without training data, the proposed algorithm can not only deal with the multipath acoustic channel problem but also improve the accuracy of the ToA estimation compared to other algorithms.

Field Evaluation of Two Impulsive Downhole Seismic Sources in Crosswell and Reverse VSP Geometries and High-Resolution Characterization of Near-Surface Texas Gulf Coast Sediments

Borehole seismic methods have been widely used for characterizing the shallow subsurface. Accurate analysis of their data is aided by a solid understanding of the borehole sources' characteristics. This study presents a field evaluation of two impulsive borehole seismic sources (Trident's Scorpion sparker and RT Clark's Ballard weight drop) in crosswell and reverse vertical seismic profile (RVSP) geometries at a Gulf of Mexico coastal site with two shallow vertical wells. The acquired data is then utilized to characterize the near-surface coastal sediments. The Scorpion source generated P-wave dominant frequencies that were recorded as 650 Hz and 250 Hz in the crosswell and RVSP geometries, respectively. For the Ballard source in the two geometries, the P-wave dominant frequencies were 1100 Hz and 250 Hz. We were also able to pick direct S-wave arrivals with the Ballard source, and their dominant frequencies were 100 Hz and 40 Hz for in situ and surface recordings, respectively. The average signal-to-noise ratio (SNR) recorded with the Scorpion data for the crosswell geometry and RVSP, respectively, is 13 and 6, and for the Ballard source, 62 and 30. We also investigated the source radiation patterns and signature wavelets. Seismic tomography was performed for the area between the two wells. Low P-wave and S-wave velocities correspond to three fresh water-saturated sand zones identified from drilling cuttings and previous well log data. Also, the plot of velocity of P-wave (Vp) versus S-wave (Vs) fits reasonably to the Mudrock Line. Both sources can excite repeatable energetic seismic signals up to 150 m away and could be useful in many geotechnical settings and even single-well imaging.

Drill‐bit position monitoring using seismic‐while‐drilling data; numerical and field examples from Sweden

AbstractThe undesired drill‐bit deviation is a source of drilling risks and requires monitoring. Seismic‐while‐drilling is one potential method to achieve this and has been tested in a number of previous studies. In August 2020 in Örebro, Sweden, we conducted an experiment to test the feasibility of seismic‐while‐drilling drill‐bit positioning and other applications of the method. We used the hammer drill‐bit signal generated while drilling a 200 m deep well in hardrock conditions and implemented vertical stacking of the subsequent impulsive signals from the hammer source, generating enhanced direct arrivals from the drill‐bit. Then, we used the relative arrival times to estimate the drill‐bit position for selected bit depths, confirming our methodology with two numerical studies. We successfully estimated the position of the drill‐bit for the numerical examples and for several of the real data examples, with the accuracy dependent on the receiver array geometry and the quality of the data. We conclude that this drill‐bit positioning method shows potential for near‐real‐time monitoring in drilling operations that could be applicable for both impulsive and noise‐retrieved drill‐bit signals.

Sensitivities of diffracted PKKPab waves to the velocity structures in the lowermost mantle

SUMMARY Diffractions of PKKPab (${\rm{PKKP}}_{{\rm{ab}}}^{{\rm{diff}}}$) along the core mantle boundary (CMB) have been observed well beyond its cutoff distance in recent studies, making it useful for improving the spatial sampling to constrain the lowermost mantle velocity structures. However, the diffractions of PKKPab waves may occur at one of the three CMB interaction points (core entry, underside reflection and exit), giving rise to uncertainties of the sampling region. Moreover, the sensitivity kernels of the non-geometrical ${\rm{PKKP}}_{{\rm{ab}}}^{{\rm{diff}}}$ in the lowermost mantle are difficult to obtain with classical ray theory and can be expected to be more complicated than the typical banana-doughnut shape for direct arrivals. In this study, we address these two issues by analysing the finite-frequency delay-time sensitivity kernels of the ${\rm{PKKP}}_{{\rm{ab}}}^{{\rm{diff}}}$ waves computed by numerical algorithms based on full-wave theory. We find that the diffraction effects for the ${\rm{PKKP}}_{{\rm{ab}}}^{{\rm{diff}}}$ waves are most significant near their core entry and exit regions. For a dominant period of 1 s, the estimated widths of the first Fresnel zones on the mantle side of these two areas are about 60 km. To further investigate the sensitivities of ${\rm{PKKP}}_{{\rm{ab}}}^{{\rm{diff}}}$ to different structures in the lowermost mantle, we conduct a series of 1D and 2D high-frequency (∼1 Hz) modelling experiments. Our results show that the travel times and amplitudes of the ${\rm{PKKP}}_{{\rm{ab}}}^{{\rm{diff}}}$ waves are sensitive to large-scale P-wave anomalies (with Vp perturbations of ± 2 per cent and thicknesses of more than 100 km) and small-scale ultra-low velocity zones (ULVZs; with Vp reduction of 5 per cent or 10 per cent and thickness of tens of kilometers). However, the slownesses of the ${\rm{PKKP}}_{{\rm{ab}}}^{{\rm{diff}}}\ $ waves remain nearly unchanged in the perturbed models. We explain this unexpected result by the differential delay-time sensitivity kernels for stations at similar epicentral distances. Our results demonstrate both the advantages and limitations of the ${\rm{PKKP}}_{{\rm{ab}}}^{{\rm{diff}}}$ waves in studying the structures at the base of the mantle.

Impact and Potential Risk of Acute Myocardial Infarction on Consultation Type During the COVID-19 Pandemic: A Single-Center Experience

Backgraound: Studies considering consultation types, such as walk-in, direct arrival by emergency medical service, or referral are rare in the coronavirus disease 2019 (COVID-19) era. The aim of this study was to compare the time course and outcome of acute myocardial infarction (AMI) and to examine the relation of the consultation types (walk-in and referral) with the time course during COVID-19 era.