ALBERT

Description: Often in geophysical monitoring experiments time-lapse inversion models vary too smoothly with time, owing to the strong imprint of regularization. Several methods have been proposed for focusing the spatiotemporal changes of the model parameters. In this study, we present two generalizations of the minimum support norm, which favour compact time-lapse changes and can be adapted to the specific problem requirements. Inversion results from synthetic direct current resistivity models that mimic developing plumes show that the focusing scheme significantly improves size, shape and magnitude estimates of the time-lapse changes. Inversions of the synthetic data also illustrate that the focused inversion gives robust results and that the focusing settings are easily chosen. Inversions of full-decay time-domain induced polarization (IP) field data from a CO 2 monitoring injection experiment show that the focusing scheme performs well for field data and inversions for all four Cole–Cole polarization parameters. Our tests show that the generalized minimum support norms react in an intuitive and predictable way to the norm settings, implying that they can be used in time-lapse experiments for obtaining reliable and robust results.

Description: Surface nuclear magnetic resonance technique, also called magnetic resonance sounding (MRS), is an emerging geophysical method that can detect the presence and spatial variations of the subsurface water content directly. In this paper, we introduce the MRS central loop geometry, in which the receiver loop is smaller than the transmitter loop and placed in its centre. In addition, using a shielded receiver coil we show how this configuration greatly increases signal-to-noise ratio and improves the resolution of the subsurface layers compared to the typically used coincident loop configuration. We compare sensitivity kernels for different loop configurations and describe advantages of the MRS central loop geometry in terms of superior behaviour of the sensitivity function, increased sensitivity values, reduced noise level of the shielded receiver coil, improved resolution matrix and reduced instrument dead time. With no extra time and effort in the field, central-loop MRS makes it possible to reduce measurement time and to measure data in areas with high anthropogenic noise. The results of our field example agree well with the complementary data, namely airborne electromagnetics, borehole data, and the hydrologic model of the area.

Description: 〈span〉〈div〉SUMMARY〈/div〉An integral component of the surface nuclear magnetic resonance forward model involves predicting the magnitude of the transverse magnetization following excitation. To predict the transverse magnetization, the Bloch equation must be solved. Traditional surface NMR forward models solve a simplified version of the Bloch equation where the relaxation terms are neglected. A shortcoming of this approach is that it can struggle to accurately describe the impact of relaxation during pulse effects. To address this concern, an alternative forward model based on solution of the full-Bloch equation is proposed. The advantage of the proposed scheme is that it implicitly accounts for relaxation during pulse effects, increases the flexibility to implement alternative parametrizations of the inverse model, and can readily describe an arbitrary excitation protocol given that it no longer requires closed form expressions of the transverse magnetizations. To demonstrate the potential of the updated forward modelling scheme, a novel approach for the inversion of complex-valued free-induction decay (FID) data is presented. The inverse model is reparametrized in order to produce depth profiles of the water content, T〈sub〉2〈/sub〉* and T〈sub〉2〈/sub〉. This approach has great potential to enhance the ability of FID measurements to provide insights into pore size and permeability as it can provide direct sensitivity to T〈sub〉2〈/sub〉. In contrast, traditional approaches that employ a forward model based on the simplified Bloch equation and estimate only T〈sub〉2〈/sub〉* are plagued by uncertainty surrounding the link between T〈sub〉2〈/sub〉* and pore size/permeability. Synthetic and field results are presented to demonstrate the feasibility of the proposed forward model and FID inversion framework.〈/span〉

Description: 〈span〉〈div〉SUMMARY〈/div〉Aquifer properties can be obtained from envelopes of surface nuclear magnetic resonance (NMR) signals, but this demands high-quality data. To retrieve reliable envelopes using synchronous detection from the intrinsically low signal-to-noise ratio (SNR) surface NMR recordings, a variety of signal processing techniques are employed to mitigate noise. We present a different approach to retrieve complex envelopes using spectral analysis and a sliding window, which can potentially improve SNR significantly. The complex envelope is composed of the spectral values at the Larmor frequency found through the Fourier transform of surface NMR data using a sliding window. We discuss how to maximize the SNR of envelope by selecting the optimum length and shape of the sliding window. An accompanying method for determining the Larmor frequency is presented and we address how noise can deteriorate the envelope retrieval in spectral analysis. Results obtained from synthetic models and field measurements in low and high noise environments reveal that the proposed method not only improves the accuracy and efficiency of envelope retrieval, but also eliminates the transient distortion of early-time signal caused by the filtering procedure.〈/span〉

Description: 〈span〉〈div〉Summary〈/div〉An integral component of the surface nuclear magnetic resonance forward model involves predicting the magnitude of the transverse magnetization following excitation. To predict the transverse magnetization the Bloch equation must be solved. Traditional surface NMR forward models solve a simplified version of the Bloch equation where the relaxation terms are neglected. A shortcoming of this approach is that it can struggle to accurately describe the impact of relaxation during pulse effects. To address this concern, an alternative forward model based on solution of the full Bloch equation is proposed. The advantage of the proposed scheme is that it implicitly accounts for relaxation during pulse effects, increases the flexibility to implement alternative parameterizations of the inverse model, and can readily describe an arbitrary excitation protocol given that it no longer requires closed form expressions of the transverse magnetizations. To demonstrate the potential of the updated forward modelling scheme, a novel approach for the inversion of complex-valued FID data is presented. The inverse model is reparametrized in order to produce depth profiles of the water content, T〈sub〉2〈/sub〉* and T〈sub〉2〈/sub〉. This approach has great potential to enhance the ability of FID measurements to provide insights into pore size and permeability as it can provide direct sensitivity to T〈sub〉2〈/sub〉. In contrast, traditional approaches that employ a forward model based on the simplified Bloch equation and estimate only T〈sub〉2〈/sub〉* are plagued by uncertainty surrounding the link between T〈sub〉2〈/sub〉* and pore size/permeability. Synthetic and field results are presented to demonstrate the feasibility of the proposed forward model and FID inversion framework.〈/span〉

Description: The near juxtaposition of the Makgadikgadi Basin (Botswana), the world’s largest saltpan complex, with the Okavango Delta, one of the planet’s largest inland deltas (technically an alluvial megafan), has intrigued explorers and scientists since the middle of the 19 th century. It was clear from early observations that the Makgadikgadi Basin once contained a huge lake, paleo–Lake Makgadikgadi. Several authors have since speculated that this lake also covered wide regions to the north and west of the Makgadikgadi Basin. Our interpretation of unusually high-quality helicopter time-domain electromagnetic (HTEM) data indicates that paleo–Lake Makgadikgadi extended northwestward at least into the region presently occupied by the Okavango Delta. The total area of paleo–Lake Makgadikgadi exceeded 90,000 km 2 , larger than Earth’s most extensive freshwater body today, Lake Superior (North America). Our HTEM data, constrained by ground-based geophysical and borehole information, also provide evidence for a paleo-megafan underlying paleo–Lake Makgadikgadi sediments.

Description: 〈span〉〈div〉Summary〈/div〉Aquifer properties can be obtained from envelopes of surface nuclear magnetic resonance (NMR) signals, but this demands high quality data. To retrieve reliable envelopes using synchronous detection from the intrinsically low signal-to-noise ratio (SNR) surface NMR recordings, a variety of signal processing techniques are employed to mitigate noise. We present a different approach to retrieve complex envelopes using spectral analysis and a sliding window, which can potentially improve SNR significantly. The complex envelope is composed of the spectral values at the Larmor frequency found through the Fourier transform of surface NMR data using a sliding window. We discuss how to maximize the SNR of envelope by selecting the optimum length and shape of the sliding window. An accompanying method for determining the Larmor frequency is presented and we address how noise can deteriorate the envelope retrieval in spectral analysis. Results obtained from synthetic models and field measurements in low and high noise environments reveal that the proposed method not only improves the accuracy and efficiency of envelope retrieval, but also eliminates the transient distortion of early-time signal caused by the filtering procedure.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉There is a growing need for detailed investigation of the top 30–50 m of the subsurface, which is critical for infrastructure, water supply, aquifer storage and recovery, farming, waste deposits, and construction. Existing geophysical methods are capable of imaging this zone; however, they have limited efficiency when it comes to creating full 3D images with high resolution over dozens to hundreds of hectares. We have developed a new and highly efficient towed transient electromagnetic (tTEM) system, which is capable of imaging the subsurface up to depth of 70 m at a high resolution, horizontally and vertically. Towed by an all-terrain vehicle, the system uses a 2×4  m transmitter coil and has a z-component receiver placed at 9 m offset from the transmitter. The tTEM uses dual transmitter moment (low and high moment) measurement sequence to obtain the early and late time gates corresponding to shallow and deep information about the subsurface layers. The first bias-free gate is as early as 4  μs from beginning of the ramp (1.4  μs after end of ramp). Data are processed and inverted using methods directly adopted from airborne electromagnetics. The system has been successfully used in Denmark for various purposes, e.g., mapping raw materials, investigating contaminated sites, and assessing aquifer vulnerability. We have also used the tTEM system in the Central Valley of California (United States) for locating artificial recharge sites and in the Mississippi Delta region, to map complex subsurface geology in great detail for building hydrogeologic models.〈/span〉

Description: Helicopter time-domain electromagnetic (HTEM) surveying has historically been used for mineral exploration, but over the past decade it has started to be used in environmental assessments and geologic and hydrologic mapping. Such surveying is a cost-effective means of rapidly acquiring densely spaced data over large regions. At the same time, the quality of HTEM data can suffer from various inaccuracies. We developed an effective strategy for processing and inverting a commercial HTEM data set affected by uncertainties and systematic errors. The delivered data included early time gates contaminated by transmitter currents, noise in late time gates, and amplitude shifts between adjacent flights that appeared as artificial lineations in maps of the data and horizontal slices extracted from inversion models. Multiple processing steps were required to address these issues. Contaminated early time gates and noisy late time gates were semiautomatically identified and eliminated on a record-by-record basis. Timing errors between the transmitter and receiver electronics and inaccuracies in absolute amplitudes were corrected after calibrating selected HTEM data against data simulated from accurate ground-based TEM measurements. After editing and calibration, application of a quasi-3D spatially constrained inversion scheme significantly reduced the artificial lineations. Residual lineations were effectively eliminated after incorporating the transmitter and receiver altitudes and line-to-line amplitude factors in the inversion process. The final inverted model was very different from that generated from the original data provided by the contractor. For example, the average resistivity of the thick surface layer decreased from $$\sim 1800$$ to $$\sim 30\hbox{ \hspace{0.17em} }\hbox{ \hspace{0.17em} }\mathrm{\Omega m}$$ , the depths to the layer boundaries were reduced by 15%–23%, and the artificial lineations were practically eliminated. Our processing and inversion strategy is entirely general, such that with minor system-specific modifications it could be applied to any HTEM data set, including those recorded many years ago.