Geophysical Prospecting最新文献

The gravimetric forward method is crucial in geophysical applications for a gravimetric interpretation of the Earth's inner structure. In this study, we present the gravimetric forward modelling open-source software that incorporates a graphical user interface. This software allows data preparation, manipulation and result interpretation both spatially and spectrally. For spatial domain modelling, it uses prism and tesseroid elements, whereas in the spectral domain, it extends Parker's formulas within specified boundaries. The software's utility is demonstrated through synthetic models and real-world applications, including calculating corrections for topography, sediments and consolidated crust using ETOPO1 and CRUST1.0 models. Performance comparisons show that Parker's method delivers computation speed superior to that of the prism, tesseroid and Terrain gravity forward (TGF) software, with variances ranging within ±12 mGal for $�G_z$ and ±0.3 E for $�G_{�z�z�}$ across different geological scenarios.

Experimental investigation of the elastic behaviours of lacustrine shales remains sparse, although they play an essential role in source rock evaluation, unconventional reservoir exploration and development, and the seal integrity evaluation for geological storage of CO₂ and nuclear waste disposal. We make the ultrasonic velocity measurement of 63 organic-rich shale samples (Chang 7, Qingshankou and Lucaogou formation) from three typical lacustrine basins in China. It is found that the P- and S-wave velocity of Chang 7 and Qingshankou shale corresponding to the fresh-brackish lacustrine depositional environment is mainly impacted by the clay and organic matter content, whereas their elastic anisotropic magnitude is mostly influenced by clay content. The P- and S-wave velocities of Lucaogou shale corresponding to the saline lacustrine depositional environment are mainly affected by the total organic carbon and porosity and exhibit weak anisotropy linked to organic matter enrichment. Exponential law well captures the relationship between anisotropic magnitude and velocity perpendicular to bedding for both saline and fresh-brackish water lacustrine shales, although there exists notable discrepancy, particularly at low velocities. The disparity in elasticity between laminations has a profound impact on the magnitude of elastic anisotropy and shapes the trend of velocity variations.

Marine magnetometry very close to the targeted sources, even in very deep waters, is today a reality with the advent of autonomous underwater vehicles. We argue that a successful approach is to fully integrate the magnetometer onboard the autonomous underwater vehicle and to deal with its static magnetic noise, that is induced and permanent fields of the drone, with a 3-axis vector measurement of the Earth's magnetic field. This argument is supported by results from three magnetic surveys performed with different fluxgate magnetic sensors embedded in autonomous underwater vehicles of increasing sizes. They show that simple specifically defined figures of merit coupled to an optimized scalar calibration procedure derived from aeromagnetic and satellite-borne developments produce reliable magnetic measurements from autonomous underwater vehicles for geophysical mapping or detection applications. Results are impressive and show that even weak magnetic anomalies smaller than 10 nT can be highlighted even though the magnetic signatures of autonomous underwater vehicles can be orders of magnitude higher.

We reprocessed a 50-km long legacy 2D reflection seismic profile acquired in 1986, under a project funded by the National Geophysics Program to improve the delineation of complex geological structures that host the platinum-bearing horizons (known as the UG2 reef; a chromitite horizon) on the south-eastern edge of the Western limb of the Bushveld Complex and to investigate the continuity of the reef below the thick cover. The pre-stack seismic data quality was improved through conventional processing steps. We applied standard Kirchhoff pre-stack depth migration as well as advanced coherency migration techniques. Both imaging techniques yielded good structural images of the platinum deposits, their hanging wall and footwall rocks. In particular, the coherency migration technique has provided significantly better images in complex faulted regions, yielding a better understanding of the interrelationship between fault activity and platinum deposit distribution, and the relative chronology of tectonic events. Moreover, the regional geological structures (Crocodile River fault and Chaneng structure) that crosscut the profile are clearly defined.

To meet the demand for long-distance and highly accurate detection in tunnel construction, a vibroseis source has been introduced into tunnel forward prospecting. However, serious noise in tunnels reduces the resolution of vibroseis signals, and small-scale structure recognition in tunnels requires higher signal resolution. Thus, we develop a time-frequency domain correlation method based on the synchrosqueezed wavelet transform for tunnel vibroseis data. Time-frequency domain correlation may capture more information than a single time or frequency domain correlation method, and its effectiveness depends on the time-frequency transformation method. The synchrosqueezed wavelet transform allows us to obtain high-resolution time-frequency spectra and thus helps to extract high-resolution effective signals. The advantages and disadvantages of different correlation methods are highlighted using numerical examples, in which the high resolution and strong robustness of the time-frequency domain correlation method based on the synchrosqueezed wavelet transform are demonstrated. A detailed field case study in an iron mine tunnel further demonstrates the reliability of the time-frequency domain correlation method based on the synchrosqueezed wavelet transform for practical data.

The imaging condition is a crucial component of the reverse time migration. In its conventional form, it involves cross-correlating the extrapolated source- and receiver-side wavefields. Effective imaging conditions are usually developed to suppress imaging artefacts (e.g. low-wavenumber artefacts) and enhance the image quality. For acoustic reverse time migration, not only the scalar pressure but also their spatial and/or time derivatives are used in the imaging condition, similar to the gradient terms of adjoint tomography. These operations implicitly introduce additional angle-domain weighting factors to the image results. In this study, based on an analysis of angle-dependent properties of the existing imaging conditions, we propose a new imaging condition tailored for acoustic reverse time migration. It can be implemented efficiently using the variables within the finite-difference solver. Without explicitly measuring wave propagation directions, the proposed imaging condition can naturally suppress the low-wavenumber artefacts while maintaining a relatively wider imaging aperture, thereby corresponding to a broader wavenumber sampling range. Additionally, the evolved imaging conditions for imaging elastic P–P and S–S scattering and reflections are also formulated. In the angle domain, we conduct a comparative analysis between existing imaging conditions and the newly proposed ones. Various numerical examples are provided to demonstrate the advantages of the new imaging conditions. A comprehensive understanding of their angle-domain properties may be further beneficial to constructing reasonable inversion strategies for full waveform inversion.

The construction of an accurate and high-resolution reservoir parameter model is crucial for reservoir characterization. However, due to the band-limited characteristics of seismic data, the inversion results heavily rely on the accuracy of the initial model. Most existing techniques for constructing an initial model interpolate well logging data within the stratigraphic framework, neglecting the effect of the stratigraphic sequence, which compromises the reliability of the initial model. The stratigraphic sequence is essential for dividing stratigraphic evolution stages and defining a geological relationship between reservoirs within the stratigraphic framework. Therefore, an initial model construction method constrained by stratigraphic sequence representation is proposed for pre-stack seismic inversion. The process begins with establishing the stratigraphic framework using horizon and fault data. Subsequently, the collaborative sparse representation algorithm is used to learn a joint dictionary that captures the relationship of structural features between seismic data and stratigraphic sequence from the well logging data. In the process of seismic data representation, the stratigraphic sequence is accurately represented in three-dimensional space by sharing sparse coefficients in the joint dictionary. Finally, the elastic parameter model is constructed by integrating the stratigraphic framework, stratigraphic sequence and well logging data, providing a reliable initial model for pre-stack seismic inversion. The main innovation of the proposed method is the three-dimensional representation of the stratigraphic sequence. A synthetic example demonstrates that the proposed method produces a more accurate initial model than conventional interpolation methods. Additionally, when applied to field data, it yields satisfactory results even without complete S-wave velocity well logging data.

A new method for estimating petrophysical properties from elastic well log or seismic data is evaluated on data from the Carnarvon Basin, Northwest Australia. The study has utilized well and seismic inversion data covering part of the Triassic-aged fluvio-deltaic Mungaroo Formation on the Exmouth Plateau. The method applied is based on several recently published papers to use acoustic impedance, velocity ratio V_p/V_s and estimated constants to calculate clay volume (V_CL), effective porosity and water saturation (S_W). The case study showed exceptional results on well data. A strong match is observed between petrophysically derived V_CL, effective porosity and S_W and the estimates derived from elastic logs. When applied to seismic inversion volumes, pay in the wells is predicted from seismic, and porosity of the sands can be estimated with confidence. Petrophysical properties for nearby direct hydrocarbon indicator–supported prospects could also be evaluated, although an imprint of the direct hydrocarbon indicator was observed on the V_CL prediction, and overall predictions are less than expected based on regional well results. Using these results, a minor modification is proposed to the equations used, and a workflow is derived to enable easy application to other projects. The modified approach was validated on the well data from the original publication. The results also indicate that the approach can be used to help identify erroneous synthetic V_s estimates in limited data settings.

Although the early-Precambrian crystalline basement is now only sporadically exposed in the northern and south-western parts of the Yangtze Block, it is supposed to have a widespread distribution beneath its Neoproterozoic and Phanerozoic covers. Here we present results of regional aeromagnetic data processing in consideration of remanent magnetization to investigate the spatial distribution of the early-Precambrian basement buried deep under the Upper Yangtze Block and surrounding areas. The direct analytic signal amplitude of the aeromagnetic anomalies, which is less affected by the magnetization direction, reveals a broader basement below the Sichuan Basin, extending far north to the Micang Mountain. A comparison between the direct analytic signal amplitude with the reduction to the pole aeromagnetic anomalies indicates that possible remanent magnetization exists beneath the Micang Mountain near the boundary between the Qinling Orogen and Sichuan Basin. The automatic depth from extreme points transform is then performed on the direct analytic signal amplitude to estimate the depth to the early-Precambrian crystalline basement. A synthetic model of a magnetic interface with remanent magnetization and random noise shows that the depth from extreme points method is able to resolve variable basement depths. Application of the depth from extreme points method to the direct analytic signal amplitude of the Upper Yangtze Block presents meaningful results about the early-Precambrian crystalline basement undulations. It is shallow and uplifted beneath the Sichuan Basin, extending north to the Qinling-Dabie Orogen, probably corresponding to the ancient Chuanzhong palaeo-uplift. Although it gradually deepens to the east, the deepest basement is buried under the Jiangnan Orogen, which is likely associated with the collision-induced crustal thickening between the Yangtze and Cathaysia blocks during the assembly of the Columbia supercontinent. Large gas fields around the Sichuan Basin are found at the slopes or depressions between basement uplifts, indicating that the deep marine carbonate rocks in the south and east of Sichuan Basin, particularly those located at the slopes or depressions between ancient basement uplifts, are favourable targets for further petroleum exploration.