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Abstract In recent years, the sparsity-promoting reconstruction method based on the compressed sensing theory has been rapidly developed and applied to seismic data reconstruction. Many achievements have been made toward providing high-quality reconstruction by using undersampled data. However, the problem of insufficient reconstruction in null traces still hinders us a lot in practical applications, especially for complex seismic data. Aiming to solve this problem, we made full use of the sparsity characteristics of seismic data in multiple sparse transform domains and jointly reconstructed seismic data to realize the complementary advantages of multiple sparse transforms; As such, we propose a high-precision seismic data recovery method with multi-domain sparsity constraints based on curvelet and high-resolution Radon transforms. Numerical examples by synthetic and real data showed that the new approach can achieve a better reconstruction result than the commonly used curvelet-based recovery method. Integrated with the curvelet transform to develop new recovery method, the high-resolution Radon transform has more advantages than the conventional Radon transform for overcoming the shortcomings associated with the insufficient reconstruction of high-amplitude events. At the same time, the method is also applicable for developing new reconstruction methods by combining other sparse transforms depending on the characteristics of seismic data. The reconstruction method with multi-domain sparsity constraints can easily be extended to three-dimensional situation. Highlights • A better seismic data recovery with multi-domain sparsity constraints is proposed. • Sparse representation with curvelet and Radon transforms for complex seismic data. • A joint recovery scheme is easily implemented based on sequential inversion. • New method improves the recovery effect of seismic data with high-amplitude events. • Sparse Radon transform is superior to conventional one for joint data recovery. [ABSTRACT FROM AUTHOR]

Density is an important parameter for both geological research and geophysical exploration. However, for model-driven seismic inversion methods, high-fidelity density inversion is challenging due to seismic wave traveltime insensitivity to density and crosstalk that density has with velocity. To circumvent the challenge of density inversion, some inversion methods treat density as a constant value or derive density from velocity through empirical equation. On the other hand, deep learning approaches are completely driven by data and have strong target-oriented characteristics, offering a new way to solve multiparameter coupling problems. Nevertheless, the accuracy of the inversion results of data-driven algorithms is directly related to the amount and diversity of the training data, and thus, they lack the universality of model-driven algorithms. To achieve accurate density inversion, we propose a simultaneous inversion algorithm for velocity and density that combines the advantages of data- and model- driven approaches: A neural network model (U-T), combining the U-net and Transformer architectures, is proposed to construct non-linear mappings between seismic data as inputs and the velocity and density as predictions. Next, the model-driven inversion algorithm uses the U-T prediction as the initial model to obtain the final accurate solution. In the model-driven module, envelope-based sparse constrained deconvolution is used to obtain full-band seismic data, while a variable dominant frequency full waveform inversion algorithm is used to perform multiscale inversion, ultimately leading to accurate inversion results of velocity and density. The performance of the algorithm on the Sigsbee2A and Marmousi models demonstrates its effectiveness. [ABSTRACT FROM AUTHOR]

Oceanic detachment faulting, a major mode of seafloor accretion at slow and ultraslow spreading ridges, is thought to occur during magma‐poor phases and be abandoned when magmatism increases. In this framework, detachment faulting is the result of temporal variations in magma flux, which is inconsistent with recent geophysical observations at the Longqi segment on the Southwest Indian Ridge (49°42′E). In this paper, we focus on this sequentially active detachment faulting system that includes an old, inactive detachment fault and a younger, active detachment fault. We investigate the mechanisms controlling the temporal evolution of this tectonomagmatic system by using 2D mid‐ocean ridge spreading models that simulate faulting and magma intrusion into a visco‐elasto‐plastic continuum. Our models show that temporal variations in magma flux alone are insufficient to match the inferred temporal evolution of the sequentially active detachment system. Rather we find that sequentially active detachment faulting spontaneously occurs at the Longqi segment as a function of lithospheric thickness. This finding is in agreement with an analytical model, which shows that a thicker axial lithosphere results in a smaller fault heave and that a flatter angle in lithosphere thickening away from the accretion axis stabilizes the active fault. A thicker axial lithosphere and its flatter off‐axis angle combined have the potential to modulate sequentially active detachment faulting at the Longqi segment. Our results thus suggest that temporal changes of magmatism are not necessary for the development and abandonment of detachment faults at ultraslow spreading ridges. Plain Language Summary: The oceanic lithosphere is frequently generated through slip on long‐lived faults called oceanic detachment faults, which exhume lower crustal and upper mantle materials on the seabed and form dome‐shaped features. Yet, oceanic detachment faulting is a sequentially active process and previous studies have focused on temporal variations in magma flux as the primary cause. Magma flux refers to the rate at which the continuous flow of magma moves from melted mantle at depth toward the ridge axis through volcanic vents or fissures. In the presented manuscript we show that this standard model does not work for the ultraslow spreading Southwest Indian Ridge (49°42′E). Here, we present geological and geophysical evidence that the magma flux had remained stable during the two most recent phases of detachment faulting. We investigate the mechanisms controlling the life cycle of the detachment fault by using a 2D geodynamics model. We compare model results with observations and propose that sequentially active detachment faulting at the Longqi segment is a function of lithospheric structure rather than variations in magma flux. These findings show that temporal changes of magmatism are not required for sequentially active detachment faulting to occur. Key Points: Our tectonomagmatic models provide a mechanical explanation for the sequentially active detachment fault system at the Longqi segment on the ultraslow spreading ridgesLithospheric structure at spreading centers plays a key role in the lifecycle of oceanic detachment faults [ABSTRACT FROM AUTHOR]

The estimation of the air gun source signature is a crucial topic in marine seismic. Typical approaches such as numerical modelling or near-field measurement-based inversion normally work in practice, but suffer from lack of knowledge on the varying sea surface reflection coefficient, the signature directional consistency and the dynamically changed source geometry. For better understanding of the above uncertainties, real far-field air gun source signatures were acquired. A customized approach combined with sparse f - k transformation and adaptive subtraction is applied to deblend the source signature from the sea-floor reflections. Characteristic of the ghost notch distribution and bubble oscillation period indicate an overall good match to the simulated results, except the latter part due to the unexpected air-leakage. Amplitude decay with incidence-angle increase is observed, indicating that the point-source spherical theory for directional signature estimation may be questionable. The sea-surface reflection coefficient appears time-variant and frequency-dependent which challenges the assumption of flat sea surface in signature estimation. An innovative notional source inversion workflow is adapted to the frequency-dependent reflection coefficient. The notional signature inversed through the adapted approach results in less ghost residuals and weaker spectral notch effect comparing with the normal inversion result, i.e., based on a constant reflection coefficient. • Wide incidence-angle (3–73°) air gun source signature is acquired at far field. • A customized deblending and denoising approach is proposed to clean the data. • Comprehensive source signature analysis is done with the assistance of simulations. • The sea-surface reflection coefficient behaves time-variant and frequency-dependent. • An innovative far-field signature based notional source inversion method is proposed. [ABSTRACT FROM AUTHOR]

[Display omitted] • A 3-D numerical model is estimated to investigate stress and strain at seafloor. • Regional stress is deviated from the spreading direction by more than 45°. • Stress rotation and shear strain controls the location of the hydrothermal field. The increased hydrothermal activity at the inside corners is interpreted to be controlled by the high-permeability detachment faults at mid-ocean ridges. Paradoxically, no hydrothermal activity is confirmed to be located at the detachment terminations, where the permeability is theoretically maximal. Here, we use a numerical model to investigate the stress state of the Longqi hydrothermal field and Trans‐Atlantic Geotraverse (TAG) hydrothermal field, which are both located on the hanging wall of a detachment fault rather than at detachment terminations. Our results show that the maximum horizontal stress S Hmax at the inside corner is deviated from the observed spreading direction by more than 45°. Meanwhile, shear strain is much higher at the insider corner and the distributions of maximal shear strain is consistent with locations of hydrothermal field. We speculate that regional stress rotation and shear strain enhance the regional permeability of the shallow crust, facilitate hydrothermal circulation on the hanging wall and therefore control the location of the hydrothermal field. Our model provides a potential mechanism for the location of the hydrothermal activity in similar geological settings and therefore provides an important constrain for the exploration of seafloor massive sulfide deposits in the future. [ABSTRACT FROM AUTHOR]

Unwanted interference noise often causes problems during seismic reflection data acquisition and processing. In this paper we present two novel data‐regrouping methods that enable improved shot‐to‐shot coherent interference noise attenuation in marine seismic data. The two approaches are based on line mixing and/or shot skipping: (1) for a 3‐D marine seismic reflection data acquisition which is typically carried out by shooting closely spaced parallel lines, we propose to mix the shot gathers from two adjacent sail lines to break up shot‐to‐shot coherent noise. This approach takes advantage of the fact that adjacent sail lines are usually acquired with a significant time interval, resulting in different types of noise; (2) for a 2‐D/3‐D acquisition, we propose an algorithm to skip and reorder the original shots to decrease the continuity of the shot‐to‐shot coherent noise. This approach could also supply more aperture data for the first or last shot from both sides, which is beneficial for the coherent noise removal at the edge area. After regrouping the original data, the coherent noise can then be further randomized or isolated by sorting and transforming to other domains, typically the common p‐shot domain where a random noise attenuation tool could be applied to identify and attenuate such noise. Field data examples confirm the effectiveness of the methods. Key Points: This paper illustrates the generation and distribution characteristics of coherent noise, especially the rig noise and seismic interferenceThis paper presents two novel approaches to break up the consistency of shot‐to‐shot coherent noise resulting in very good denoised resultStatistics show that data‐regrouping methods combined with cross‐party vessel cooperation significantly reduced the time‐sharing cost [ABSTRACT FROM AUTHOR]

In this paper, we describe the results of a first pilot experiment of passive seismic monitoring near the active Longqi hydrothermal vent at the Southwest Indian Ridge during Chinese cruise DY115-30. During 102-day experiment, we recorded over 2,000 seismic events on a single ocean bottom seismometer with hydrophone close to the hydrothermal field. We classified these events into the following three classes based on their time–frequency characteristics: (1) volcano tectonic microearthquakes (VTMs); (2) regional earthquakes; (3) short-duration events (SDEs). We preliminarily located the 1,277 VTMs using a single-station location method. Our results revealed hypocenters primarily beneath the axial valley ridge which is about 10 km away from hydrothermal vent and extending to the upper mantle at a depth of 15 km. Two discrete swarms of VTMs occurred during our survey period. The SDEs show a complex frequency characteristics ascribed to clusters of monochromatic waves around different frequencies. This suggests that SDEs result from combination of harmonic waves resonating in fluid-filled pipes and cracks. [ABSTRACT FROM AUTHOR]

Hydrothermal processes in detachment settings at slow and ultraslow spreading ridges differ greatly from those at melt-rich faster spreading ridges. Active detachment faulting provides the possibility for off-axis high-temperature hydrothermal vents located far away from the heat source beneath the axial volcanic ridge. Seismic data from the slow spreading Mid-Atlantic ridge revealed that hydrothermal fluids may exploit detachment faults to extract heat from a melt zone near the crust-mantle interface. However, knowledge of the subsurface structure and the kinematic processes of detachment faults, and their interaction with hydrothermal fields at the slowest spreading ridges is still insufficient. Here, we report new insights on the seismicity beneath the Longqi hydrothermal field at the ultraslow spreading southwest Indian ridge from three ocean bottom seismometer monitoring experiments. The seismicity outlines the subsurface geometry of a detachment faulting system (DF1 and DF2). The strongly flexed DF1 (45°) is a mature detachment fault with a domed-shaped OCC where ultramafic rocks are exposed on the seafloor. An active young DF2 with intense earthquake activity along the steep subsurface (67°) suggests the initiation phase of rotation. The diversity of hydrothermal activities in the Longqi detachment-hydrothermal systems is closely related to the evolution of detachment faults. Additionally, both the non-transform discontinuity on the western margin of the Longqi-1 field and local faults facilitate hydrothermal circulation. Our study provides baseline observations for a Longqi-type of hydrothermal circulation at the inside corner associated with detachment faulting and non-transform offsets. • Micro-earthquakes outline the subsurface geometry of a detachment system beneath the Longqi hydrothermal field at the SWIR. • The diversity of hydrothermalism at Longqi depends on the evolution of the detachment faults. • Seismicity and hydrothermal activity fade out with distance to the non-transform discontinuity. [ABSTRACT FROM AUTHOR]

The limited bandwidth of a single source normally restricts the imaging resolution for the subsurface target. Multi-frequency acquisition is a promising way to enhance the resolution with reasonable data fusion. An Ormsby wavelet based theoretical study confirms the significance of both the low and high frequencies for improving the resolution by means of peak duration and side-lobe oscillation level. To avoid over-boosting the noise during the data fusion, we propose a joint deconvolution approach to associate the single-frequency data with the target function directly. A weighted matched filter is accordingly derived based on the signal-to-noise ratio of the multi-frequency data. Error analysis shows that the joint weighted deconvolution based data fusion achieves better noise control compared to the direct-summation based deconvolution. Both wedge model based synthetic result and ground penetrating radar acquired field data example confirm the effectiveness of the proposed data fusion approach on resolution enhancement. • Importance of bandwidth extension on improving the overall resolution was verified. • Presented a novel joint weighted deconvolution (JWD) method for data fusion. • JWD could better handle noise-boosting issue compared to the direct-summation way. • Both synthetic and field GPR data confirmed the effectiveness of this approach. [ABSTRACT FROM AUTHOR]