查看更多>>摘要：Passive surface wave methods that utilize ambient noise to investigate underground space have been widely used in densely populated urban areas. As an important step in imaging from ambient noise, dispersion measurement seriously affects the accuracy of the final inversion results. Linear stacking, the most commonly used stacking method in dispersion measurement, usually relies on the long observation time to obtain Green's functions with high signal-to-noise. However, it is sometimes difficult to ensure sufficient observation time for passive surface wave methods for exploration efficiency and environmental restrictions in urban areas, making dispersion measurement from ultra-short noise important and necessary. Considering the limitations of linear stacking, we compared the dispersion images obtained by linear stacking, phase-weighted stacking (PWS), and time-frequency phase-weighted stacking using ultra-short ambient noise to find an appropriate stacking way. Synthetic experiments show that PWS can obtain better cross-correlation functions and dispersion spectra with fewer noise segments. Two field cases further verify the reliability of applying PWS in dispersion measurement from several minutes of the continuous noise record by analyzing the phase velocities error or comparing the inversion results with the drilling data. Both synthetic experiments and field examples illustrate PWS can be used as an efficient and economical method to meet the needs of ultra-short-time imaging and real-time monitoring of urban underground structures.
查看更多>>摘要：Because of porosity and mineralogy disparity, rocks have a broad range of bulk densities, a critical property that has an important role in rock characterization. Practically, the density of rock is estimated either via wireline logging or logging while drilling tools. However, these techniques are not always accessible, making the door open for using different predicting methods such as empirical correlations. The existing empirical correlations have substantial restrictions, which limited their accuracy and reliability. This work targets developing various artificial intelligence models to predict bulk density for complex lithology during drilling (i.e., at real-time). The artificial neural network (ANN) and adaptive neuro-fuzzy inference system (ANFIS) techniques are used with the mechanical drilling parameters as inputs. This study uses a vertical well with 2912 data points from various lithologies including sand, carbonate, and shale to develop the models. Besides, a new empirical correlation for bulk density is developed based on the optimized ANN technique. A different dataset from the same tested field was used to validate the developed models. The obtained outcomes demonstrated that both ANN- and ANFISbased models estimated the rock density with high fitting accuracy. The ANFIS approach results correlation coefficient (R) values of 0.99 with an average absolute percentage error (AAPE) values of 0.83 and 0.92% in training and testing processes. Whereas the ANN approach is slightly outperformed, as described by the highest R values of 0.99 and the lowest AAPE of 0.77 and 0.93% for training and testing processes. Also, a new empirical model for bulk density estimation was derived from the developed, optimized ANN-based model, which is applicable to the extracted weights and biases. The validation process indicates the robustness and reliability of the developed models with R values of 0.99 and AAPE of 0.92 and 0.96% for ANN- and ANFIS-based models, respectively. The outcomes of this work develop models with high accuracy for real-time prediction of bulk density, which can help in rock description in an adequate method with less cost, time, and errors.
查看更多>>摘要：Velocity analysis is critical in seismic data interpretation. In sedimentary environment, 1D geometry is a reasonable assumption. Velocity analysis under such assumption has been done through the interpretation of either reflection or transmission responses. Recently, Marchenko method has also been applied to provide 1D velocity model from reflection data. However, none of the previous researchers attempted to invert the velocity model from combined physics from the reflection and transmission data with a projection constraint. The projection constraint is computed from the determinant of matrices of dot products of the velocity models. Although, the MATLAB code for the forward responses is given, the algorithm for performing the multi-physics inversion of 1D reflection and transmission data, with projection constraint, is presented. The concept is tested on two different velocity model examples with varying overburden thicknesses and layer thicknesses. Results are consistent even with up to 5% random noise. It is also worthy of note that while the near-surface velocity section remains less affected in the reflection response data, the deep-subsurface velocity values are equally resilient from the transmission response. This presents a much-desired complementary benefit for velocity analysis from a multi-physics interpretation.
查看更多>>摘要：The Horgos area of the thrust belt on the southern margin of the Junggar Basin has a good prospect for oil and gas exploration. However, because of the complex surface conditions and the coverage of the gravel layer, the seismic data on the southern margin thrust belt have some poor imaging, which makes the three-row structural pattern unclear. It is urgent to use 3D magnetotelluric (MT) exploration to investigate the distribution of gravel layer for seismic processing to build shallow model. In this paper, a 3D geoelectric model is established using MT data and logging data collected in the study area. Through the forward modeling and inversion calculation, the feasibility of using 3D MT data to explore the high resistivity gravel layer of various thickness is verified. Phase tensor analyses indicate the shallow (> 0.1 Hz) electrical structure is mainly 1D or 2D, while the deep structure is affected by 3D structure. The 3D conjugate gradients inversion method is used to process the MT data of the study area, the forward responses of the inversion result fit the observed data well and the electrical layers of the 3D inversion is consistent with the electrical characteristic revealed by the electric logging data. All these indicate the reliability of the 3D inversion results. The 3D MT results are first used to interpret the distribution and thickness of two sets of gravel layers in the study area. It is mainly used to assist the seismic exploration to solve problems involving exploration of complex surface carbonate rocks, exploration of deep complex structures and identification of shallow lithology. It proves that 3D MT is an effective technique for the gravel and shallow lithology exploration in similar regions.
查看更多>>摘要：The development of a three-dimensional (3D) hydrogeologic site model requires detailed knowledge of an area's hydrostratigraphy, information typically obtained from core logs, historical well records, outcrop mapping, and/ or surface geophysics. Due to the high cost of drilling, a 3D model may rely on various statistical techniques to interpolate lithological boundaries between points located tens of kilometers or more apart. This situation can result in poorly constrained or simplified lithostratigraphic models, with under-sampled features such as karst and buried bedrock valleys. The non-invasive nature of airborne geophysical surveys combined with its dense sampling capability can fill critical data gaps. This study evaluates the capacity of a helicopter borne frequency-domain electromagnetic (FDEM) and residual magnetic survey using the ResolveTM system over a buried bedrock valley incised in a multi-layered dolostone aquifer within a complex glaciated environment in an urban/agri-cultural region of southern Ontario, Canada. The resulting airborne data was evaluated against two high-resolution ground-based electrical resistivity transects oriented orthogonal to the valley, alongside a published Quaternary geological model. A comparison between our local geophysically-based model and the regional geological model shows that the airborne electromagnetic surveys can markedly improve characterization of bedrock valley morphology and Quaternary sedimentary unit architecture that is comparable to that obtained using high-resolution surface-based electrical resistivity measurements with 10 m electrode spacing. The scale of heterogeneity that can be resolved by the airborne survey reveals the level of petrophysical variability that can be seen within the valley infill. Our study demonstrates the potential of airborne FDEM surveys in semi-urban agricultural environments where detailed characterization of Quaternary sediment architecture and bedrock topography is needed to inform 3D groundwater flow system variability.
查看更多>>摘要：This study aims to employ one of the most significant geophysical methods, known as the ground penetrating radar (GPR) method, in the determination of reasons for the underground cavities, the size of deformations and fractures in the ground and structure, the geometric behavior patterns, and the soil-structure interaction and changes in a historic structure. In addition to the instrumental examination, an observational investigation of the current situation of the mosque was also carried out. Iskender Pasha Mosque, which is a rare mosque having a tabhane (guest house), is one of the important historical mosques regarded as built-in Mimar Sinan style in Diyarbakir, Turkey in 1552. The instrumental examination part of the study was conducted totally in 13 different regions of the mosque and its courtyard. Within this scope, 8 GPR measurements were taken on the ground of the mosque and its immediate environment and 5 GPR measurements were taken on the walls of the mosque. The GPR measurements indicated that there are serious subsidence anomalies under the mosque and its courtyard. Besides, there is a water table under the portico of the mosque which may lead to big problems in the future. In addition, observationally detected structural deteriorations were supported by geophysical GPR measurements. It was revealed that problems dealing with the leveling irregularities on the floor of the harim, portico, and courtyard are directly related to the anomalies under the ground. Moreover, it was achieved from both instrumental and observational investigations that the anomalies under the ground are directly the reasons for the anomalies in the walls of the mosque. GPR results demonstrated that this technique can be used effectively to determine anomalies in both the carrier system of the historic structures and the ground under the historic structures without any damage.
Kumar, ThineshSeelam, Naresh KumarRao, G. Srinivasa
15页
查看更多>>摘要：Coal exploration in the Indian scenario is challenging due to plenty of carbon contents and dirt bands within a coal seam. Manual interpretation of geophysical logging data in such conditions is time-consuming and tedious due to the non-linear behaviors of well log signals. This paper intends to apply supervised machine learning techniques such as Support Vector Machine (SVM), Decision Tree (DT), Random Forest (RF), Multi-Layer Perceptron (MLP), and Extreme Gradient Boosting technique (XGBoost) for meticulously interpreting such banded coal seams from geophysical logs. To investigate the efficacy of the above-mentioned five ML techniques in the Indian scenario, we have considered Gamma-ray, Density, and Resistivity logs from four boreholes drilled in Talcher coalfield, Eastern India. ML model training results indicate that all the prediction models have scored more than 88% accuracy scores in classifying carbonaceous and non-coal lithofacies. Receiver Operating Characteristics (ROC) curves of different ML models suggest that the obtained area under the curve (AUC) is positive and above the main diagonal for all lithotypes. Finalized ML models with appropriate hyper-parameters are also applied on the nearby drilled wells, and the outputs are validated against geological core data. It is found that all ML models have acquired an overall accuracy score greater than 80% in all three test wells, conveying that ML techniques are a potential solution for dealing with banded coal seam problems in the Indian scenario.
查看更多>>摘要：Investigation of three-dimensional forward modeling for the ground-airborne frequency-domain electromagnetic (GAFEM) method and the characterization of its responses are presented. The electric field Helmholtz equation in the frequency domain is discretized by the Galerkin weighted residual method. The secondary electric field is the unknown to be solved for, discretized using edge-element basis functions. The edge-element basis functions are also used as the weight functions in the Galerkin approach. The solution of the discretized system is then obtained by a direct matrix-equation solver. To verify the effectiveness of the forward-modeling method and the accuracy of the corresponding code for computing the EM fields and reproducing their physical behavior, the numerical solutions computed using the presented method are compared against the equivalent solutions calculated from other techniques for a 3-D conductivity model. The good agreement between the various numerical methods and codes for this model demonstrates the accuracy of the responses computed by the method presented here. The method is then used to analyze the characteristics of GAFEM responses by comparing the magnetic fields and transfer functions (i.e., the magnetic field components Hx, Hy and Hz, and the transfer functions Tzx = Hz / Hx and Tzy = Hz / Hy) for different source-receiver configurations, altitudes of the observations and frequencies for several 3-D earth models. The aim is to improve the understanding of GAFEM responses and to determine the preferred measurement components and survey parameters. The responses at different frequencies of a complex model that includes topography are also investigated to study the characteristics of GAFEM responses of realistic earth models.
查看更多>>摘要：The growing dependency on groundwater resources in semi-arid hard rock terrains has steadily emerged into desaturation of shallow aquifers limiting them to the sporadically distributed bedrock fractures. Additionally, the heterogeneity in weathering and fracturing distribution elevates the ambiguity for geophysical mapping. This study demonstrates at three distinct lithological sites using integrated gradient resistivity profiling (GRP), vertical electrical sounding (VES), and electrical resistivity tomography (ERT) followed by exploratory borehole drilling and electrical logging to pin point the bedrock fractures in granitic hard rocks of Southern India. We observe, the VES and ERT efficacious in characterizing the major hydro-stratigraphy that consists top soil, followed by underneath unsaturated weathered zone, and bedrock with almost no noticeable signatures of the saturated bedrock fractures. But, the anomalies detected in GRP profiles signify the occurrence of deep bedrock fractures which corroborates with the first derivatives of resistivity contours derived from the ERT data. The qualitative delineation of fractures with low resistivity peaks in GRP complements the borehole lithologs where the bedrock fractures encountered at 70-200 m depth with 1.49-3.84 litres per second (lps) of groundwater yield. The study further points forward that greater the resistivity contrast in GRP anomaly, higher is the possibility of saturated fractures in bedrock. It concludes, the GRP as precursor and key approach for pin pointing the deep fractures which can be further characterized using VES and ERT to appraise the groundwater potentiality, though, at local scale in granitic hard rock terrain.
查看更多>>摘要：Full-waveform inversion (FWI) serves as a useful tool to quantitatively investigate the properties of the media. One of the greatest challenges is its enormous computational cost, especially for the three-dimensional (3D) case. The multisource encoding strategy is a crucial method to speed up FWI. However, current 3D elastic FWI algorithms with the simultaneous sources, such as random time series and random polarity methods, may be subjected to the crosstalk noise, which severely degrades inversion quality. We present a novel 3D elastic crosstalk-free multisource full-waveform inversion method (MFWI). The encoded sources with a series of single frequency components are excited to perform the seismic wavefield simulation. Extracting the wavefield corresponding to each single-frequency harmonic source by phase-sensitive detection (PSD) algorithm, the multi source wavefield can be decomposed. Therefore, summing the gradients with respect to the model parameters of all decomposed single frequencies can effectively avoid the crosstalk in our MFWI algorithm. Furthermore, to increase the practicability and universality of the algorithm, we adopt the graphic processing unit (GPU) based on the domain decomposition to improve the MFWI algorithm. The multi-scale strategy with a flexible encoding number in each scale helps to enhance the robustness of our algorithm. The numerical examples demonstrate that our method can achieve domain decomposition simulation and obtain 3D crosstalk-free multisource elastic wave inversion results.
NSTL
Elsevier