ALBERT

Description: A classical split perfectly matched layer (PML) method has recently been applied to the scalar arbitrarily wide-angle wave equation (AWWE) in terms of displacement. However, the classical split PML obviously increases computational cost and cannot efficiently absorb waves propagating into the absorbing layer at grazing incidence. Our goal was to improve the computational efficiency of AWWE and to enhance the suppression of edge reflections by applying a convolutional PML (CPML). We reformulated the original AWWE as a first-order formulation and incorporated the CPML with a general complex frequency shifted stretching operator into the renewed formulation. A staggered-grid finite-difference (FD) method was adopted to discretize the first-order equation system. For wavefield depth continuation, the first-order AWWE with the CPML saved memory compared with the original second-order AWWE with the conventional split PML. With the help of numerical examples, we verified the correctness of the staggered-grid FD method and concluded that the CPML can efficiently absorb evanescent and propagating waves.

Description: 〈span〉〈div〉Abstract〈/div〉China ranks second and third in global oil and natural gas consumption, and fifth and sixth in global oil and natural gas production, respectively (〈a href="https://pubs.geoscienceworld.org/tle#R1"〉U.S. EIA, 2018〈/a〉). In the past 25 years, China's oil consumption has increased 3.5 times, and natural gas consumption is rising rapidly as well. China is increasing its investment in the petroleum industry, with a goal of significantly expanding domestic oil and gas production. Complex geology, rough surface conditions, and the need to explore deep targets, unconventional resources, and offshore reservoirs pose great challenges to geophysical exploration. Geophysical technologies in China thus have advanced significantly in data acquisition, processing, and interpretation. To demonstrate the development and applications of geophysical technologies in the exploration, development, and production of oil and gas resources, we invited academic and industry experts to present recent studies on exploration geophysics in China.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Geophysics aims to image subsurface geologic structure and identify different geologic units. While the former has dominated the interpretation of applied geophysical data, the latter has received much less attention. This appears to have persisted despite applications such as those in mineral exploration that inherently rely on the inference of geologic units from geophysical and geologic observations. In practice, such activities are routinely carried out in a qualitative manner. Thus, it is meaningful to examine this aspect and to develop a system of quantitative approaches to identify different geologic units. The development of geophysical inversions in the last three decades makes such interpretation tools possible. We refer to this newly emerging direction as 〈span〉geology differentiation〈/span〉 and the resultant representation of geology model as a quasi-geology model. In this article, we will provide an overview of the historical background of geology differentiation and the current developments based on physical property inversions of geophysical data sets. We argue that integrating multiple physical property models to differentiate and characterize geologic units and work with the derived quasi-geology model may lead to a step change in maximizing the value of geophysical inversions.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Between July 2018 and January 2019, Shell acquired two vertical seismic profiling (VSP) surveys in two deepwater wells in the Gulf of Mexico by using a seismic while drilling (SWD) tool. Each survey included a rig source zero-offset VSP and a boat source offset VSP. The main objective of the surveys is to delineate the salt-sediment boundary at the salt base and flank. We design and execute the complex VSP surveys with emphasis on optimization, efficiency, and integration. We develop a comprehensive analysis and processing method to integrate P-wave sediment and salt proximities with converted PS salt proximity. We use SWD-VSP surveys to demonstrate how we define the salt boundary with the integrated results. Our results show that we can delineate the salt boundary with better accuracy and with a high degree of confidence. These successful VSP surveys provide significant business and technical value.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉We have evaluated a field test in the city of Singapore to assess the feasibility of the passive seismic survey for bedrock depth determination and to further investigate the optimal acquisition parameters. The ambient noise field, dominated by urban traffic noise, is recorded passively for the application of seismic interferometry. Spectral analysis indicates that the traffic-induced noise by local roads is concentrated between 3 and 25 Hz. We use multiple signal classification beamforming for wavefield direction of propagation analysis. We apply seismic interferometry to retrieve the surface-wave part of the Green’s functions, based on which surface-wave dispersion relations are extracted and further inverted for 1D S-wave velocity profiles. Subsequently, we compare the inversion results from seismic interferometry with borehole logs at multiple sites in Singapore and establish that the bedrock depths are well-determined using passive seismic methods within a maximum error of 3 m. By investigating the convergence of the crosscorrelograms, we ascertain that the best compromise of cost, efficiency, and accuracy for a passive site investigation in Singapore can be achieved in 15 min in the morning of a working day using an array as short as 30 m with six vertical geophones, although these parameters should be reinvestigated at other sites and other times. The success of this case study demonstrates that accurate near-surface site investigation can be achieved with faster acquisition, fewer receivers, and a smaller acquisition footprint compared with conventional methods, all of which improve the efficiency particularly in a highly developed urban environment.〈/span〉

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Description: Distributed acoustic sensing (DAS) is a novel technology that uses an optical fiber cable as a sensor for acoustic signals and can take almost any downhole fiber-optic installation or deployment and turn the fiber-optic cable into a large downhole seismic array. This array can provide enhanced vertical seismic profile (VSP) imaging and monitor fluids and pressure changes in the hydrocarbon-production reservoir. Walkaway VSP data acquired over a formerly producing well in northeastern China provided a rich set of high-quality DAS walkaway VSP data. A standard VSP data preprocessing workflow was applied, followed by prestack Kirchhoff time migration. In the DAS preprocessing step, we were faced with additional challenges: strong coherent noise due to cable slapping and ringing along the borehole casing. Compared with an earlier offset VSP data set using 327 levels acquired with conventional 3C downhole geophones in the same well, the final preprocessed DAS walkaway VSP has a larger vertical aperture, resulting in a wider lateral image. The single-well DAS walkaway VSP images provide a good result with higher vertical and lateral resolution than the surface seismic in the objective area. The vertical-well environment, which lacks the ability to effectively "clamp" the sensor to the borehole-casing wall by touching, creates a unique set of challenges. Although earth signal was recorded with almost all the shots, there was also a considerable amount of noise. Much of the noise was due to the physical placement of the wireline in the well and was expressed by slapping and ringing. Reported here are lessons learned in handling the wireline cable and subsequent special DAS data processing steps developed to remediate some of the practical wireline deployment issues. Optical wireline cable as a conveyance of fiber-optic cables for VSP in vertical wells will open the use of the DAS system to wider applications.

Description: Borehole gravity is an effective method for directly imaging anomalous mass distributions at large depths, and it has braod applications in mineral and petroleum exploration and production. The method was presently limited to vertical gravity data only. The anticipated development of vector gravimeters may change this scenario dramatically. There was a need to understand the potential capability of vector gravimetry and to develop corresponding modeling and interpretation techniques. We introduced a set of numerical modeling tools for vector gravity and examined the relative information content of vertical and vector gravity data in borehole environments. Through simulations, we illustrated that vector gravity was superior to vertical gravity in applications such as locating and characterizing anomalous mass and in tracking the movement of fluid injection in reservoirs.

Description: We present a reformulation of reduction to the pole (RTP) of magnetic data at low latitudes and the equator using equivalent sources. The proposed method addresses both the theoretical difficulty of low-latitude instability and the practical issue of computational cost. We prove that a positive equivalent source exists when the magnetic data are produced by normal induced magnetization, and we show that the positivity is sufficient to overcome the low-latitude instability in the space domain. We further apply a regularization term directly to the recovered RTP field to improve the solution. The use of equivalent source also naturally enables the processing of data acquired on uneven surface. The result is a practical algorithm that is effective at the equatorial region and can process large-scale data sets with uneven observation heights.

Description: We present a case study of applying 3D inversion of gravity gradiometry data to iron ore exploration in Minas Gerais, Brazil. The ore bodies have a distinctly high-density contrast and produce well-defined anomalies in airborne gravity gradiometry data. We have carried out a study to apply 3D inversion to a $$20\hbox{ \hspace{0.17em} }\hbox{ \hspace{0.17em} }{\mathrm{km}}^{2}$$ subarea of data from a larger survey to demonstrate the utility of such data and associated inversion algorithm in characterizing the deposit. We examine multiple density contrast models obtained by first inverting $${T}_{zz}$$ ; then $${T}_{xz}$$ , $${T}_{yz}$$ , and $${T}_{zz}$$ jointly; and finally all five independent components to understand the information content in different data components. The commonly discussed $${T}_{zz}$$ component is sufficient to produce geologically reasonable and interpretable results, while including additional components involving horizontal derivatives increases the resolution of the recovered density model and improves the ore delineation. We show that gravity gradiometry data can be used to delineate the iron ore formation within this study area.