- Home
- A-Z Publications
- Near Surface Geophysics
- Previous Issues
- Volume 12, Issue 1, 2014
Near Surface Geophysics - Volume 12, Issue 1, 2014
Volume 12, Issue 1, 2014
-
-
Smoothness‐constrained time‐lapse inversion of data from 3D resistivity surveys
Authors M.H. Loke, T. Dahlin and D. F. RuckerABSTRACTThree‐dimensional resistivity surveys and their associated inversion models are required to accurately resolve structures exhibiting very complex geology. In the same light, 3D resistivity surveys collected at multiple times are required to resolve temporally varying conditions. In this work we present 3D data sets, both synthetic and real, collected at different times. The large spatio‐temporal data sets are then inverted simultaneously using a least‐squares methodology that incorporates roughness filters in both the space and time domains. The spatial roughness filter constrains the model resistivity to vary smoothly in the x‐, y‐ and z‐directions. A temporal roughness filter is also applied that minimizes changes in the resistivity between successive temporal inversion models and the L‐curve method is used to determine the optimum weights for both spatial and temporal roughness filters. We show that the use of the temporal roughness filter can accurately resolve changes in the resistivity even in the presence of noise. The L1‐ and L2‐norm constraints for the temporal roughness filter are first examined using a synthetic model. The synthetic data test shows that the L1‐norm temporal constraint produces significantly more accurate results when the resistivity changes abruptly with time. The model obtained with the L1‐norm temporal constraint is also less sensitive to random noise compared with independent inversions (i.e., without any temporal constraint) and the L2‐norm temporal constraint. Anomalies that are common in models using independent inversions and the L2‐norm and L1‐norm temporal constraints are likely to be real. In contrast, anomalies present in a model using independent inversions but that are significantly reduced with the L2‐norm and L1‐norm constraints are likely artefacts. For field data sets, the method successfully recovered temporal changes in the subsurface resistivity from a landfill monitoring survey due to rainwater infiltration, as well as from an experiment to map the migration of sodium cyanide solution from an injection well using surface and borehole electrodes in an area with significant topography.
-
-
-
4D time‐lapse ERT inversion: introducing combined time and space constraints
Authors M. Karaoulis, P. Tsourlos, J.‐H. Kim and A. RevilABSTRACTTime‐lapse Electrical Resistivity Tomography (ERT) can be used to characterize dynamic processes occurring in the subsurface of the Earth. It involves the installation of a permanent array of electrodes to monitor changes in resistivity associated with changes in pore‐water properties (salinity, temperature, water content) or porosity (compaction or dilation). The interpretation of time‐lapse data is complicated by both the presence of noise in the data and the influence of low sensitivity in parts of the model. A uniform space and time constraint is not able to address this problem. In this work, we propose a new approach to distinguish noise‐related artefacts to true changes in resistivity, while at the same time addressing the problem of the lack of sensitivity of electrical resistivity tomography with depth. We propose transforming the space and time constraints to be active, meaning that the regularization parameters are distributed rather than being uniform for the entire model. This way, both time‐related noise (assumed to be random) in the data and the lack of sensitivity are addressed and we can incorporate prior information in a natural way into the inversion scheme. Using this strategy, the inversion scheme is able to favour areas where the expected changes are likely to occur while filtering out areas where no changes should occur. The favoured areas can be either selected from a preliminary analysis of the data, or by incorporating other types of prior information into the system based on the process that is monitored.
-
-
-
Soil resistivity monitoring of an irrigation experiment
Authors Torleif Dahlin, Pär Aronsson and Mats ThörnelöfABSTRACTResistivity measurements were used for tracing water transport during a three‐year irrigation study. Three different rates of landfill leachate irrigation and one control treatment were applied to two cultivars of short‐rotation willow coppice. Groundwater level measurements and water sampling were carried out in pipes installed in the centre of each plot. Resistivity was measured with permanently installed electrodes along six lines running through the centre of the plots. The resistivity results were inverted to produce vertical sections of ground resistivity at different time steps and as change in resistivity relative to the start of the experiment. Changes in resistivity linked to differences in irrigation quantities and plant growth were observed. The results showed that a repeated soil resistivity measurement has the potential as a tool to monitor changes in soil water and ion contents. Furthermore, expanding zones of increasing soil resistivity immediately under and around the plants indicate that the method may be useful for imaging plant root development.
-
-
-
Integrated geophysical survey for 3D modelling of a coastal aquifer polluted by seawater
Authors R. Martorana, L. Lombardo, N. Messina and D. LuzioABSTRACTGeophysical surveys are carried out in the coastal area of Petrosino (south‐western Sicily) to study the time evolution of seawater contamination of the coastal aquifer, probably increased due to human impact. The overexploitation of the aquifer, due to an intensive agricultural use has affected significantly the natural hydro‐geochemical state of the basin. The study is based on a processing and integrated analysis of hydrogeological, geochemical and geophysical data. In particular in the last two years seasonal time‐lapse electrical resistivity tomographies (ERT), new TDEM soundings and Multi‐Analysis Surface Wave soundings (MASW) have been carried out. The interpretation of the total set of previously existing and new geophysical data made it possible to reconstruct a three‐dimensional model of the electrical resistivity of the aquifer, aimed at defining the extent and geometry of the seawater intrusion. Furthermore, the execution of a series of high‐resolution time‐lapse electrical tomographies and a correlation analysis between geophysical measures and geo‐chemical, geological and hydrogeological data allowed to discriminate the effects of the salt concentration in the groundwater and the porosity and saturation degree of the rock on the time variations of the measured electrical resistivity. Finally, the average porosity of the rocks forming the reservoir was determined.
-
-
-
4D electrical resistivity tomography monitoring of soil moisture dynamics in an operational railway embankment
Authors J.E. Chambers, D.A. Gunn, P.B. Wilkinson, P.I. Meldrum, E. Haslam, S. Holyoake, M. Kirkham, O. Kuras, A. Merritt and J. WraggABSTRACTThe internal moisture dynamics of an aged (> 100 years old) railway earthwork embankment, which is still in use, are investigated using 2D and 3D resistivity monitoring. A methodology was employed that included automated 3D ERT data capture and telemetric transfer with on‐site power generation, the correction of resistivity models for seasonal temperature changes and the translation of subsurface resistivity distributions into moisture content based on petrophysical relationships developed for the embankment material. Visualization of the data as 2D sections, 3D tomo‐grams and time series plots for different zones of the embankment enabled the development of seasonal wetting fronts within the embankment to be monitored at a high‐spatial resolution and the respective distributions of moisture in the flanks, crest and toes of the embankment to be assessed. Although the embankment considered here is at no immediate risk of failure, the approach developed for this study is equally applicable to other more high‐risk earthworks and natural slopes.
-
-
-
3D effects on 2D resistivity monitoring in earth‐fill dams
Authors In‐Ky Cho, Ik‐Soo Ha, Ki‐Seog Kim, Hee‐Yoon Ahn, Seunghee Lee and Hye‐Jin KangABSTRACTMeasuring resistivity is a potentially powerful method of monitoring leakage zones that have developed in a dam, and their expansion over time. Generally, for embankment dams, two‐dimensional (2D) resistivity data have been measured along the dam crest for the detection of leakage zones. However, the three‐dimensional (3D) effects created by specific dam geometry and fluctuations in reservoir water levels significantly distort the 2D resistivity data measured at the dam crest. This study evaluates the 3D effects through 3D resistivity modelling software, which was developed to calculate apparent resistivity data for geometries and material distributions for embankment dams. These modelling results demonstrated that the 3D effect from the dam geometry and variations in water level is significant. Especially, in the case of monitoring, changes in 3D effects from water level fluctuations cause a spurious near‐surface layer when time‐lapse inversion is applied with a cross‐model constraint. To overcome this problem, we introduced a combined reference model constructed from the independent inversion of both time‐lapse data and original reference data. The combined reference model was able to effectively suppress the spurious near‐surface layer and to clearly image the damaged zone when the change in water level was small. However, a time‐lapse inversion using the combined reference model also failed to identify the damaged zone when the change in water level was large. Finally, by using the resistivity monitoring system devised for dam surveillance to a test dam site, resistivity monitoring data were acquired. From the time‐lapse inversion of two data sets showing a large change in water level between two measurements, it was confirmed that the variation of water levels produces the occurrence of a spurious near‐surface layer due to a strong 3D effect.
-
-
-
SP Monitoring at a Sea Dike
Authors Hye‐Jin Kang, In‐Ky Cho, Jung‐Ho Kim, Hwan‐Ho Yong, Sung‐Ho Song and Young‐Gyu ParkABSTRACTThe self‐potential (SP) method is widely used in seepage evaluation hydrological studies to monitor the integrity of infrastructure such as dams, sea dikes, and other types of flood control devices because the electric signals that are measured are directly related to seepage rate. At leaking areas along sea dikes, large SP anomalies can be generated by the rising and falling of tides. Unfortunately, SP data are often contaminated with several types of noise, such as that from drifting electrodes, telluric disturbances, and external electrical noise. Furthermore, SP signals can have high levels of spatial variability due to heterogeneity in lateral resistivity at the locations where the electrodes are installed. Because of these issues, it is very difficult to correlate the measured SP voltages with the streaming potentials associated with groundwater flows at particular points in time. To alleviate these problems, we developed a simple but effective interpretation method for SP monitoring data that involves subtracting consecutive SP voltages collected at different time points from a particular monitoring station. This subtracting procedure is able to effectively reduce spurious SP anomalies caused by electrode drift, change in resistivity, and other types of interference. Therefore, any changes observed in SP measurements over certain time frames were interpreted as resulting primarily from temporal changes in seepage flow. To demonstrate the performance of this method, we analysed SP monitoring data measured at a sea dike located on the southern coast of Korea. Our results confirmed that the SP interpretation method is able to explain changes in streaming potentials depending on the tide change over time and to detect the horizontal location of anomalous seepage zones along the sea dike.
-
-
-
Geoelectrical monitoring of frozen ground and permafrost in alpine areas: field studies and considerations towards an improved measuring technology
ABSTRACTProcesses that control permafrost warming in Alpine regions are still not completely understood. Recently, geoelectrical monitoring has emerged as a useful tool to investigate thawing and freezing processes. However, high resistive environments and harsh environmental conditions pose very unfavourable conditions for automated resistivity measurements. Based on the results of several test studies, an improved data acquisition system for geoelectrical monitoring of frozen soils was developed. Furthermore, the implementation of algorithms for statistical analysis of raw data time series led to a significant improvement in the reliability of inversion results. At two Alpine sites, namely Molltaler Glacier and Magnetkopfl/Kitzsteinhorn, the adapted system was tested at soil temperature conditions between 0°C and –12°C. Data was continuously collected at both locations over nearly a full seasonal cycle. The results showed an almost linear dependency of resistivity and temperature at values above –0.5°C. At lower temperatures, the relation was non‐linear, indicating that the reduction of porosity due to the shrinking of connected brine channels was the dominating process that determined the value of resistivity. Based on the derived results, further improvements were suggested, especially for measurements at soil temperatures below –4.5°C as low injection currents make it extremely challenging to gather these.
-
-
-
Frozen ground dynamics resolved by multi‐year and year‐round electrical resistivity monitoring at three alpine sites in the Swiss Alps
Authors C. Kneisel, T. Rödder and D. SchwindtABSTRACTFrozen ground characteristics resolved by annual, seasonal/monthly and daily electrical resistivity monitoring are presented based on case studies from three alpine sites in the Swiss Alps with different surface conditions and subsurface process dynamics.
Data acquisition is achieved by different set‐ups ranging from low‐cost to automated and more expensive monitoring strategies. To ensure the reproducibility of measurement results a robust setup is required within the rough surface conditions of alpine environments, and this constitutes a fundamental precondition for time‐lapse measurements.
The selected different monitoring approaches allow for a detection and analysis of processes related to frozen ground dynamics on varying timescales. The interpretation of the geophysical data is improved by temperature measurements from various data loggers and borehole data.
All three approaches allowed detection of the interface between frozen and unfrozen ground. The variation of the frequency of measurements enabled exploration of the specific permafrost‐related problems. At one site, the multi‐annual resistivity distribution at the end of the thawing period revealed fairly stable permafrost conditions, while at the second site, year‐round measurements showed extremely divergent evolution of resistivity values in the subsurface throughout the measurement period, which could be ascribed to different site‐specific environmental parameters. Using measurements with daily resolution at the third field site, the rapid decrease in subsurface resistivity values due to the infiltration of meltwater in spring could be documented. The presented results show that the different monitoring set‐ups have their justification and are able to monitor timedependent subsurface dynamics within the scale of their temporal resolution.
The operation of an automated monitoring system allows for very efficient observation especially of short‐time processes within the active layer and the frozen ground below, the major advantage in comparison to non‐automated monitoring approaches. However, the system is cost‐intensive, requires an extensive infrastructure, and is more prone to environmental forces. For monitoring the inter‐annual and long‐term permafrost evolution, application of a fixed monitoring set‐up that is accessible throughout the year and measured manually has proven to be a robust and cost‐efficient alternative. Focusing on the long‐term permafrost evolution, set‐ups using fixed electrodes and measurements conducted as needed with a brought‐along cable is a legitimate approach. Hence, for studies in alpine permafrost environments, choice of the monitoring set‐up remains a question of the scientific problem, infrastructure facilities, and cost‐efficiency.
-
-
-
Geoelectrical monitoring: an innovative method to supplement landslide surveillance and early warning
ABSTRACTPermanent geoelectrical monitoring, using the GEOMON4D instrumentation in combination with high resolution displacement monitoring by means of the D.M.S. system, was performed at two active landslide areas: Ampflwang/Hausruck in Austria, and Bagnaschino in Italy. These sites are part of the Austrian geoelectrical monitoring network, which currently comprises six permanently monitored landslides in Europe. Within the observation intervals, several displacement events, triggered by intense precipitation, were monitored and analysed. All of these events were preceded by a decrease of electric resistivity. The application of an innovative 4D inversion algorithm made it possible to investigate the potential processes which led to the triggering of these events. We conclude that resistivity monitoring can significantly help in the investigation of the causes of landslide reactivation. Since the results also contribute to the extrapolation of local displacement monitoring data to a larger scale, resistivity monitoring can definitely support decision‐finding in emergencies.
-
-
-
Real‐time electrical monitoring of reagent delivery during a subsurface amendment experiment
ABSTRACTAn electrical resistivity monitoring survey was conducted on a mine heap to track reagent movement during high‐pressure injections. The injections were designed to increase the dissolution of metallic gold from low‐grade ore and enhance recovery after surface leaching had ceased. The main objective of the geoelectrical monitoring was to observe the effectiveness of the injection technique and provide feedback to optimize injection parameters in real time. Real‐time assessment was achieved by monitoring the raw output current and transfer resistance on a network of borehole electrodes installed around the injection well. It was demonstrated that the output current increased significantly on particular borehole electrodes after commencement of reagent injection, when the wetting front arrived at the electrodes. When injection ceased, the electrical current returned to the initial baseline current values. The timing and distribution of the electrodes demonstrating this behaviour varied with injection depth. The internal structure of the heap was likely a controlling factor in reagent movement. Resistance, converted to apparent resistivity, was also shown to change significantly in the region near the injection. Verification of the real‐time assessment was conducted with post‐injection time‐lapse 3D tomographic inversion. While inverse modelling provides a truer 3D representation of reagent injection, the cost was shown to be a time‐lag of 3.5 days to complete the modelling. The simplicity of monitoring the raw current output and voltage can make this a powerful tool for real‐time tracking of fluid movement in the subsurface.
-
-
-
Joint interpretation of geoelectrical and soil‐gas measurements for monitoring CO2 releases at a natural analogue
Authors U. Sauer, N. Watanabe, A. Singh, P. Dietrich, O. Kolditz and C. SchützeABSTRACTThe development and validation of hierarchic monitoring concepts is essential for detecting and assessing possible leakages from storage formations, especially for carbon capture and storage (CCS) applications. Joint interpretation of various techniques (such as carbon dioxide (CO2) concentration and flux measurements, self‐potential (SP) and geoelectrical surveys) showed that the combination of geophysical methods with soil‐gas analysis for mesoscale monitoring of the shallow subsurface above geologic CO2 storages can be a valuable tool for mapping and monitoring potential CO2 spread in the subsurface. Three measurement campaigns were undertaken – May 2011, July 2011 and April 2012 – at an analogue site in the Cheb Basin, Czech Republic, with the aim of studying CO2 leakages and their temporal and spatial behaviour. Results of geoelectrical investigations give an insight into the structural features of the subsurface. CO2 discharge into the atmosphere is mostly impeded by shallow, clay‐rich, partly water‐saturated zones, which can be seen in the electrical resistivity tomography (ERT) results. Several transport processes can be identified based on SP measurements. The SP results highlight the complex behaviour of temporal variations for the flow patterns. In particular, coupled migration of gas and water plays an important influencing role in this process. Site‐specific, near surface geological features and meteorological conditions seem to exert great influence on the degassing pattern and measured CO2 values. Therefore, soil‐gas measurements represent a snapshot which illustrates both a distinct typical pattern of the soil‐gas distribution in the near subsurface and certain differences caused by soil and meteorological conditions. Observed CO2 soil‐gas anomalies and modelled results suggest that the occurrence of gas discharge is much more localized around restricted areas, often controlled by local permeability contrasts. Hence, our results show that a proposed monitoring concept should integrate SP, time‐lapse ERT, meteorological parameters and soil‐gas measurements to provide a comprehensive insight into the subsurface structures and processes.
-
Volumes & issues
-
Volume 22 (2024)
-
Volume 21 (2023)
-
Volume 20 (2022)
-
Volume 19 (2021)
-
Volume 18 (2020)
-
Volume 17 (2019)
-
Volume 16 (2018)
-
Volume 15 (2017)
-
Volume 14 (2015 - 2016)
-
Volume 13 (2015)
-
Volume 12 (2013 - 2014)
-
Volume 11 (2013)
-
Volume 10 (2012)
-
Volume 9 (2011)
-
Volume 8 (2010)
-
Volume 7 (2009)
-
Volume 6 (2008)
-
Volume 5 (2007)
-
Volume 4 (2006)
-
Volume 3 (2005)
-
Volume 2 (2004)
-
Volume 1 (2003)