Diving Wave Tomography Research Articles

Applications and limitations of distributed acoustic sensing in shallow seismic surveys and monitoring

Distributed acoustic sensing (DAS) is an emerging technology gaining acceptance in a variety of seismological applications. We systematically analyze the adaptability and usefulness of horizontal DAS deployments for near-surface geophysical applications, such as surface wave inversion, diving wave tomography, and passive subsurface source location. We find that, in accordance with previous studies, DAS data are generally similar to traditional sensors and can be successfully used independently. Nonetheless, DAS data suffer from inherent limitations due to the design of the optical measurement system. Among others, we identify the gauge length, measurement directivity, and saturation at near offsets as the primary limiting factors. When operating in low-velocity environments and a standard 10 m gauge length, surface wave analysis may be constrained to the usage of low frequencies due to the wavenumber filtering effect of the gauge length. The measurement directivity generally prohibits applications that are based on upgoing P-wave energy, such as near-offset diving wave tomography. In addition, saturation at near offsets prevents reliable diving wave traveltime picking. As a consequence of these limitations, the ultrashallow resolution achievable with DAS data is poor. Source directivity also strongly limits passive source location resolution. Whereas some of the limitations can be alleviated through an optimal choice of optical parameters at the interrogator level, we mostly rely on adapted field acquisition and data processing workflows that use the undisturbed portion of the recorded signal. Eventually, the spatiotemporal resolution and relative ease of long-term deployments turn DAS into a worthwhile option for monitoring and time-lapse scenarios, or when operating in urban environments over existing infrastructure.

Diving Wave Tomography: Velocity Modelling Using First Arrival Traveltime

In hydrocarbon exploration, information carried by diving waves and post-critical reflections that are used to reconstruct the long-to-intermediate wavelength of the subsurface is an integral part of successful velocity model building. Diving wave tomography (DWT) is one of the tools for shallow velocity assessment particularly when seismic data has poor signal-to-noise ratio (SNR) with complex geologic settings where no clear reflector is present. Considering the relationship between velocity with time and space, the output from tomography plays a crucial role to align data between time and depth domain and produce a reliable image of the deeper structure where hydrocarbon reservoir is typically located. In geophysics, tomography is primarily used to correct seismic trace alignment to produce a reliable stack section. In advanced imaging it is used as an initial model for waveform inversion in an integrated workflow. In the post-processing stage, it is used to correct the misfit between well logs and seismic data and is crucial for the quantitative analysis of rock physics. In this paper, we focus on tomography and its working principle on near-surface velocity modelling. We restricted our workflow to 2D synthetic data simulating the shallow gas occurrence that is prominent in the offshore Malay Basin to demonstrate how tomography works in velocity reconstruction. Results from synthetic and real data example shows that DWT can recover local large-scale structure and improved stacked data, considering no other seismic data and constraint from well data is included in the iterative process.

Imaging complex geology through challenging surface terrain – a case study from West China

Huiwen Xie, Chao Wu, Caiming Luo, WenSheng Duan,Yingjie Zhao, Xin Ji, Sherman Yang, Xiaolong Yao, Dingxue Wang, Zhengguo Jia and Qinglin Liu describe a reprocessing project combining four 3D datasets that improved the imaging of a deep subsalt thrustfaulted target in an area of variable topography and complex near-surface conditions. Oil and gas exploration is moving into areas of increasingly complex geology, where depth imaging – along with the associated need to build accurate velocity models – is becoming an ever more important tool in efficiently finding and developing oilfields. This article presents a case study of structural imaging from an area where both the subsurface geology and surface conditions are extremely complex. The project combined several vintages of land 3D seismic data from the Dabei block, located in the Kuche foreland basin area of the northern Tarim Basin, West China. Several previous attempts had been made to reprocess these datasets, but the results were all considered inadequate to meet the imaging quality and accuracy required for reliable well placement. The goal of the new reprocessing project was to deliver accurate imaging of hydrocarbon-bearing strata beneath a large anticline in subsalt thrust-fault blocks at depths of around 7000 m. Tomographic static correction using carefully picked first breaks outperformed previous field static solutions based on upholes. A multi-domain noise attenuation sequence included a newly developed method that uses true shot and receiver coordinates, thereby handling irregularities in acquisition geometry. A tilted transverse isotropic (TTI) velocity model was built using a combination of diving wave tomography (DWT) in the shallow part; geologically constrained reflection tomography in the remainder of the overburden; and multi-azimuthal tomography in the offset vector tile (OVT) domain in the salt and target zone. In areas of thick high-velocity conglomerate deposits, a seismic joint inversion (SJI) model building technique was applied to include information from non-seismic data to better define strong lateral velocity changes.

Combining diving-wave tomography and prestack reflection tomography for complex depth imaging — A case study from mountainous western China

Reflection tomography is widely used to build velocity models for seismic depth imaging, but several factors can limit its effectiveness, particularly in resolving complexities in the near surface. A 3D data set acquired in a mountainous area of western China exhibited several such factors, including low fold of coverage in the shallow data that made it difficult to verify and adjust velocities based on measuring the flatness of common-imaging-point (CIP) gathers. The data set also presented challenges related to poor signal-to-noise ratio (S/N) and dramatic lateral velocity variation in the shallow section. The diving-wave tomography method, combined with precise first-arrival picking, provided an enhanced solution for deriving the shallow velocity field in the initial depth-migration velocity model. This was part of the innovative and enhanced velocity model-building workflow that was applied to accurately image the complex subsurface geology of this challenging data set. The workflow comprised four key steps: data conditioning, diving-wave tomography, reflection tomography, and prestack Kirchhoff depth migration. This project was the first application of diving-wave tomography combined with reflection tomography to land seismic data in China. Its demonstrated success provides a robust and effective velocity model-building solution for application in other complex depth-imaging land seismic projects in China and elsewhere.

Land Streamer Aided Geophysical Studies at Saqqara, Egypt

During December 2002 and January 2003, Montana Tech in collaboration with Ain Shams University, Cairo, collected Ground Penetrating Radar (GPR) and seismic data at Saqqara, Egypt. The purpose of this study was to see if GPR and seismic methods could detect manmade structures in the subsurface at Saqqara. In particular, land streamer aided, seismic diving-wave tomography was tested as a method to detect archaeological features. Saqqara was one of the principal necropolises of Memphis, an ancient capital of Egypt. The research site was near the 3rd Dynasty pharaoh Djoser’s Step Pyramid—the first monumental structure built entirely of stone. A preliminary GPR study of our site yielded numerous, possibly manmade features in the subsurface with a [Formula: see text] depth of penetration using [Formula: see text] antennas. A follow-up three-dimensional (3-D) GPR survey over one of the more interesting features showed a broad trench underneath the flat-lying sand that is seen at the surface. This feature is most likely manmade because the horizontally layered limestone rocks of Saqqara are inconsistent with the shape of this feature. Seismic diving-wave tomograms show that this probable manmade feature extends to a depth of [Formula: see text] into the subsurface. Moreover, we were able to complete the seismic survey faster using a land streamer consisting of gimbaled geophones than could be done using conventional planted geophones. This site has potential for further investigation and possible excavation.