Time Series Data Inversion and Monitoring Method for Cross-Hole ERT Based on an Improved Extended Kalman Filter

In recent decades, the DC resistivity method has been applied to geophysical monitoring because of its sensitivity to hydrogeological properties. However, existing inversion algorithms cannot give a reasonable image if fluid migration is sudden and unpredictable. Additionally, systematic or measurement errors can severely interfere with accurate object location. To address these issues, we propose an improved time series inversion method for cross-hole electrical resistivity tomography (cross-hole ERT) based on the Extended Kalman Filter (EKF). Traditional EKF includes two steps to obtain the current model state: prediction and correction. We improved the prediction step by introducing the grey time series prediction method to create a new regular model sequence that can infer the potential trend of underground resistivity changes and provide a prior estimation state for reference during the next moment. To include more current information in the prior estimation state and decrease the non-uniqueness, the prediction model needs to be further updated by the least-squares method. For the correction step, we used single time-step multiple filtering to better deal with the case of sudden and rapid changes. We designed three different numerical tests simulating rapid changes in a fluid to validate the proposed method. The proposed method can capture rapid changes in the groundwater transport rate and direction of the groundwater movement for real-time imaging. Model and field experiments were performed. The inversion results of the model experiment were generally consistent with the results of dye tracing, and the groundwater behavior in the field experiment was consistent with the predicted groundwater evolution process.

Time-Lapse Electrical Resistivity Tomography and Soil-Gas Measurements on Abandoned Mine Tailings Under a Highly Continental Climate, Western Siberia, Russia

Mine tailings are a very active system in which the processes of oxidation, dissolution, and the re-deposition of substances occur in real-time. Time-lapse electrical resistivity tomography and soil-gas measurements have been used on abandoned mine tailings under a highly continental climate, Western Siberia, Russia. The electrical resistivity tomography method allows the structure of the tailings to be determined, namely, its electrophysical parameters, which are related to the chemical composition and geochemical characteristics of the subsurface substance. The aim of this work is to determine the variations in the geoelectrical zoning of sulfide-bearing mine tailings depending on fluctuations in environmental conditions, i.e., ground and air temperature, in conjunction with volatile compounds of environmental concern emanating from the tailings (SO2, CS2, C2H6S). The hourly observations revealed that the configuration of the geoelectrical section varies during the day. The concentration of gases in the surface air layer varied in accordance with the ambient temperature conditions. In general, the minimum gas concentrations were determined at night, and the increase in gas concentrations began when the temperature increased. The dependence of gas formation on temperature conditions differed during the daytime and nighttime. In warmer hours, gas concentrations are highest. At night, when there was a decrease in the temperature of air and then in the ground temperature, a local increase in the concentration of all measured gases occurred at the maximum temperature difference in the air (14.1 °C), and the ground remained relatively warm (20.8 °C). There is a close relationship between ground temperature, electrical resistivity, and the rate of gas production. Local anomalies with the greatest variation in electrical resistivity are associated with the zones that have the most active gas emanations.

Multi-Scale Pseudo-Bending Raytracing for Arbitrary Complex Media

Raytracing is a fast and effective numerical simulation method of the seismic wavefield. It plays an important role in field data acquisition design, wavefield analysis, identification, and tomography. In raytracing, pseudo-bending (PB) is a fast and efficient method, but it is unsuitable for complex media with sudden velocity changes. An improved pseudo-bending raytracing method is presented in this paper, which can be applied to any complex medium. The proposed method first decomposes complex medium into multi-scale velocity components and then applies the pseudo-bending approach to the velocity components of different scales. The numerical simulation of seismic wavefield from models shows that the improved multi-scale pseudo-bending (MSPB) method can be applied to a medium with continuous velocity variation and any complex medium with abrupt velocity change.

A New Approach to Predict Hydrogeological Parameters Using Shear Waves from the Multichannel Analysis of Surface Waves Method

For near-surface contaminant characterization, the accurate prediction of hydrogeological parameters in anisotropic and heterogeneous environments has been a challenge since the last decades. However, recent advances in near-surface geophysics have facilitated the use of geophysical data for hydrogeological characterization in the last few years. A pseudo 3-D high resolution P-wave shallow seismic reflection survey was performed at the P Reactor Area, Savannah River Site, South Carolina in order to delineate and predict migration pathways of a large contaminant plume including trichloroethylene. This contaminant plume originates from the northwest section of the reactor facility that is located within the Upper Atlantic Coastal Plain. The data were collected with 40 Hz geophones, an accelerated weight-drop as seismic source and 1 m receiver spacing with near- and far-offsets of 0.5 and 119.5 m, respectively. In such areas with near-surface contaminants, a detailed subsurface characterization of the vadose zone hydraulic parameters is very important. Indeed, an inexpensive method of deriving such parameters by the use of seismic reflection surveys is beneficial, and our approach uses the relationship between seismic velocity and hydrogeological parameters together with empirical observations relating porosity to permeability and hydraulic conductivity. Shear wave velocity ( Vs) profiles were estimated from surface wave dispersion analysis of the seismic reflection data and were subsequently used to derive hydraulic parameters such as porosity, permeability, and hydraulic conductivity. Additional geophysical data including core samples, vertical seismic profiling, surface electrical resistivity tomography, natural gamma and electrical resistivity logs allowed for a robust assessment of the validity and geological significance of the estimated Vs and hydrogeological models. The results demonstrate the usefulness of this approach for the upper 15 m of shallow unconsolidated sediments even though the survey design parameters were not optimal for surface wave analysis due to the higher than desired frequency geophones.

Seismic Shear-Wave Characterization of Sand and Gravel Groundwater Aquifers in Northern Illinois

Groundwater is a nearly exclusive water resource, specifically for the communities which are part of the Chicago metropolitan area. However, water shortage is predicted for many communities in this region, and demand for locating and delineating groundwater is increasing to fulfill the water supply. Shallow sand and gravel aquifers within the glacial deposits of the area specifically are high volume aquifer and less stressed compare to deeper bedrock aquifer. Yet, these aquifers are poorly understood in terms of their extent and lateral variability. This study applied the shear-wave seismic reflection method to delineate the thickness, lateral extent, and internal variability of these aquifers. We acquired horizontally polarized shear-wave (SH-waves) reflection data along five profiles of a total length of 11 km using the land streamer technology in McHenry County in northern Illinois to delineate sand and gravel aquifers. As shear waves propagate through the rock matrix and less sensitive to the presence of water, information from nearby borings and water wells aided the interpretation of the acquired SH-wave seismic profiles. We delineated multiple sand and gravel units of potential aquifers of different thicknesses and lateral extent along with the seismic profiles. The relatively higher vertical and lateral resolution of the shear-waves reflection method and its insensitivity to water saturation or chemistry made it an ideal method to resolve sand and gravel units of potential aquifers within the complex geological environment if aided by water-well information.

Rayleigh Wave Dispersion Curve Inversion with the Artificial Bee Colony Algorithm

Rayleigh wave dispersion curve inversion is a non-linear iterative optimization process with multi-parameter and multi-extrema. It is difficult to carry out inversion and reconstruction of stratigraphic parameters quickly and accurately with a single linear or non-linear inversion for the data processing of Rayleigh waves with complex seismic geological conditions. We proposed a new method that combines artificial bee colony algorithm (ABC) and damped least squares algorithm (DLS) to invert Rayleigh wave dispersion curve. First, food sources are initialized in a large scale of the model based on the prior geological information. Then, after three kinds of bee operators (employed bees, onlooker bees and scout bees) transform each other and perform search optimization with several iterations, the targets are converged near the optimal solution to obtain an initial S-wave velocity model. Finally, the final S-wave velocity model is obtained by local optimization of DLS inversion with fast convergence and strong stability. The correctness of the method has been verified by one high-velocity interlayer model, and it was further applied to a real Rayleigh wave dataset. The results show that our method not only absorbs the advantages of ABC global search optimization and strong adaptability, but also makes full use of the advantages of DLS inversion, such as high accuracy and fast convergence speed. The inversion strategy can effectively suppress the inversion falling into local extrema, get rid of the dependence on an initial model, enhance the inversion stability, further improve the convergence speed and inversion accuracy, while has good anti-noise ability.

Optimal Transceiver Distance of Controlled-source Audio-frequency Magnetotelluric Sounding Method

Currently, the use of Cagniard apparent resistivity and phase is still the main method and means to process and interpret controlled-source audio-frequency magnetotelluric (CSAMT) sounding data. The CSAMT data must meet the conditions in the far zone to use the magnetotelluric (MT) sounding inversion interpretation method. The conditions in the far zone are directly related to transceiver distance; Hence, the distance measured by the CSAMT method is directly related to the reliability and credibility of the data. In this study, based on the formation wave and ground wave of the analytical expression for the electric field in a uniform half-space condition, the response laws of the formation wave and the ground wave are analyzed. After rigorous mathematical derivation, the formula for determining the optimal transceiver distance of the CSAMT is obtained, and the steps for using this formula in practical application are given. The actual field application verifies the rationality of the derived formula. This study provides a reference for the design of a CSAMT field source, and effectively guides the determination of CSAMT transceiver distance in the field.