Multi-channel Resistivity Research Articles

Use of geoelectrical resistivity method for detecting near-surface groundwater potential zones at Riyadh city, Saudi Arabia

Six profiles of the two-dimensional multi-channel electrical resistivity tomography (ERT) were conducted at the northwest part of Riyadh city using the SYSCAL Pro system with a dipole–dipole configuration to detect the near-surface water-bearing potential zones. The ERT field measurements use a series of regularly placed electrodes to measure the electrical resistivity along the profile. The results of the inverse models are displayed as cross-sections of the true resistivity distribution of the subsurface with depth along with each profile. The two-dimensional (2-D) inversion results of the electrical resistivity data indicate three geoelectric zones; 1st zone of low resistivity values (<100 Ω.m), representing two groundwater-bearing potential zones from top to bottom as follows; the 1st one extends downwards to 10- and 15-meters depth, while the 2nd zone spreads between 15- and 30-meters depth, 2nd zone with resistivities ranging from 100 to 1000 Ω.m, which represent the highly fractures limestone; the 3rd zone with resistivities greater than 1000 Ω.m reflecting compacted limestone rocks. It is noticed also some of the low resistivity pockets are isolated through the subsurface geoelectrical cross-section indicating the presence of voids and/or cavities. Based on these results, the investigated area is considered as one of the promising zones for groundwater supply in Riyadh city and its future urbanization expansion projects. This can be accessed by drilling wells down to a depth of 50 m for groundwater extraction.

Development of a Multi-Channel Array Resistivity Method Instrument System

Application scenarios and experimental methods in the field of modern electrical exploration are becoming increasingly complicated, expanding requirements on instrumentations. Currently, the commonly used Resistivity method instrument systems are mostly single-channel transmitting and receiving, which lowers the measurement efficiency. The multi-channel array Resistivity method instrument system developed in this paper adopted an improved automatic measurement method to realize array synchronous transmitting and receiving, which effectively improve the measurement efficiency. The instrument consists mainly of two parts: a transmitter and receiver subsystems. The transmitter subsystem realizes array transmitting through centralized electrode units or serial electrode management nodes. The former can support up to 256 electrodes with a maximum transmit power of 600W; the latter can support up to 65,535 electrodes with a maximum transmit power of 5kW. The receiver subsystem consists of multiple 8-channel receivers. The Root Mean Square noise of the receivers is lower than 9.38 microvolts, the dynamic range is better than 120dB, and the isolation between channels is better than 119dB. Combined with a built-in 4G and Wi-Fi module, it can achieve real-time monitoring of data quality and cloud storage functions. The results of the water tank comparison experiment between the developed instrument and GRx8-32 showed well consistent By applying the developed system to the monitoring of the perfusion process of structured cementitious materials, the relative changes in resistivity of the perfusion process can be visually monitored, which lays a foundation for the application of the instrument in two-dimensional and three-dimensional detection and time-shift monitoring in other fields.

Identification of Reclamation Area in Ancol of North Jakarta Using Resistivity Method

The coastal reclamation area is an expansion of coastal areas through technical engineering to develop new land areas. Identification of the reclamation area can be performed by detecting subsurface imaging using the resistivity method. This study used a multi-electrode (multichannel) resistivity imaging method. The resistivity imaging results show a good response of subsurface resistivity and successfully identified reclamation area with low resistivity &lt;27.8 Ωm in almost the study area. Its depth varies from 4 meters to 30 meters. The reclamation results are composed of loose rock that has not been fully compacted, so it has not been well consolidated. As a result, it will experience land subsidence if overload.

Practical considerations for shallow submerged archaeological prospection with 3‐D electrical resistivity tomography

AbstractDirect current electrical resistivity method has experienced significant breakthroughs during the last decades with the development of advanced instrumentation and sophisticated inversion algorithms. These have substantially benefitted the field of archaeological prospection to extract quantitative information for the buried archaeological material in a complete three‐dimensional (3‐D) context. The obvious continuation of past human occupation and associated settlements in the ultra‐shallow part of coastal zones generated the necessity to compile methodologies for mapping submerged cultural assets. This study investigates the efficiency of dynamic floating and submerged 3‐D Electrical Resistivity Tomography (ERT) for mapping archaeological relics buried beneath the sediments in ultra‐shallow marine environments. Extensive testing is performed through numerical simulation and 3‐D inversion of synthetic apparent resistivity tomographic data. The generic resistivity model including a complex resistive structure embedded in a stratified conductive environmental regime simulated typical scenario of an archaeological feature submerged below the sea water layer. Different array configurations including Dipole–Dipole (DD), Gradient (GRD), Reciprocal Wenner (RecWEN) and Wenner‐Schlumberger (WS), which can be appropriately programmed for continuous off‐shore measurements with associated multichannel resistivity instruments, are validated in order to determine the most efficient one for such surveys. Densely spaced multiple parallel two‐dimensional (2‐D) ERT lines along a single direction composed the survey layout to extract the 3‐D apparent resistivity data set for the different electrode arrays using a 3‐D finite element program. The synthetic tomographic data were corrupted with 3% Gaussian noise and inverted with an iterative smoothness constrained inversion algorithm. The comparative results from the various tested arrays manifest the superiority of the DD in reconstructing the optimum resistivity inversion model for both the floating and submerged survey modes. Additional tests were made concerning the resolving capabilities of ERT with variable seawater thickness and target characteristics. Although accurate ‘a priori’ information regarding the water resistivity and thickness are essential for constraining the 3‐D inversion, erroneous estimation of these parameters can result to misleading results, especially for submerged survey modes. The simulation of floating and submerged 3‐D ERT surveys through synthetic modelling documented the benefits of such approach in reconstructing structured cultural material in shallow off‐shore environments. Finally, the results of a 3‐D dynamic floating ERT survey from a submerged archaeological site in Greece verified the theoretical outcomes, proposing at the same time techniques to overcome problems that can occur due to the unique conditions of the ultra‐shallow marine environment. Overall, this work enhances the conclusion that 3‐D marine ERT is a robust method to reconstruct submerged archaeological structures related to ancient built environment in ultra‐shallow marine regions.

Investigation of defective trees using electric resistivity method

The resistivity method in geophysics is used to solve various geological and engineering problems. Recently, this nondestructive method has been used on trees to investigate possible infections within the trunks by scanning resistivity variations. In this study, the electrical resistivity method has been aimed to be applied on various trees in Istanbul, Turkey to test whether the method applies to trees via regular resistivity measurement devices used in geophysics. Firstly, a multi-channel resistivity device, that is designed to automatically take measurements on the ground, is modified to carry out the measurements on trees. The measured data are processed using two different approaches. The first approach is to prepare a program in MATLAB, which is capable of adapting measurement points into a circular profile via interpolation. The data processed with this program are then gridded to prepare resistivity contour slices. The detection and handling of faulty measurements are discussed briefly in this section. The second approach is to use an open-source electrical tomography program (BERT V2) to apply inversion to the collected resistivity data. Finally, all the results and conclusions are interpreted considering the resistivity distribution within tree trunks, including sample slices from two trees that are known to have defected beforehand. As a conclusion of our studies, we have found that a regular resistivity measurement device used in earth sciences is applicable to an extent on trees to investigate possible defects within their trunks.

Application 4D Resistivity Method for Determining Effect of Water Content; Case Study ITERA Campus

This is a study of the application of the time lapse resistivity method (4D resistivity) to analyze the dynamics of the movement of subsurface fluids (ground water). This study used to analyze the decline in the ground water table due to differences in the rainy and dry seasons. The 4D resistivity study was conducted at the ITERA campus in June (rainy season) and September (dry season). The resistivity method used Electrical Resistivity Tomography (ERT) with the Wenner Alpha configuration. Data acquisition uses 36 simultaneous channel with ARES Multichannel and Multielectrode resistivity meters instrument with length is 385 meters. Based on the first measurement (rainy season), it was found an unconfined aquifer layer with a moderate resistivity value (11 – 80 Ω.m) which was assumed to be tuffaceous sandstone at a depth of 11 – 35 m. Whereas in the second measurement (dry season), it was found an unconfined aquifer layer with a moderate resistivity value (11 - 80 Ω.m) which was assumed to be tuffaceous sandstone at a depth of 11-28 m. Based on the results of time lapse inversion processing, it is found that the change in resistivity for the two sets of data almost the same at all measurement points, except at certain points, which shows a greater percentage. This shows the change in resistivity values if measured at different times. This difference is due to the absorption of rainwater by the surface which then migrates into the inner layer.

Utilizing multichannel electrical resistivity methods to examine the contributions of submarine groundwater discharges

Submarine groundwater discharge is the main way to transfer land-based sources to the ocean. To verify the in situ monitoring capability of the multichannel electrical resistivity method for groundwater discharge in the coastal zone during tidal processes, in situ monitoring was carried out in different aquifers of Shilaoren Bay in September 2015 and May 2017. The electrodes were placed on the seabed, and the collection array was dipole-dipole. The monitoring results show that during tidal processes, seawater intrusion and submarine groundwater discharges in different sedimentary layers have different spatial and temporal evolution characteristics and they can clearly be reflected in time-series resistivity profiles. The percentage difference change in resistivity can be used to define the main groundwater discharge areas. Based on the theory of balance of salinity, the average discharge rate for different periods can also be estimated. Through monitoring, we find that the main discharge area is in the middle and lower parts of the intertidal zone and that the average discharge rate per unit area is 0.86 m3/h, which is the same as the actual values. From the monitoring results, we find that the multichannel electrical resistivity method has good in situ monitoring capability.

A Modified Wenner Array for Efficient Use of Eight-Channel Resistivity Meters

The Wenner array is a popular measurement strategy for acquiring geoelectrical data for one-dimensional soundings and two-dimensional profiles. An advantage of using this array is the high signal strength, as the receiving dipole expands in size proportional to the transmitting dipole. A major disadvantage of the Wenner array is the low efficiency in data acquisition, as this array can only be used in a single-channel acquisition mode. To make better use of the available open channels on multi-channel resistivity meters, we explore modifications to the Wenner array by adding additional receiver dipoles. Two of the modifications include receiver dipoles that are either larger or smaller than the Wenner dipole, but are symmetrical around the center of the transect. A third modification includes supplementing the Wenner dipole with smaller gradient dipoles that are not symmetrically arranged around the center. We found that this latter array, termed Alt_3 Wenner, performed better than other modified arrays in terms of resolution and noise. We also tested the Alt_3 Wenner array against an optimized array and found the resolution to be nearly as high, but with better noise management and channel usage efficiency. We recommend the Alt_3 Wenner array for supplementing the Wenner array if that is the array of choice.

Aplikasi metode tahanan jenis dalam studi geologi karst Gua Seropan di Gunung Kidul, Yogyakarta

In terms of geology, most areas in south of the Gunungkidul District in Central Java consist of the Wonosari formation limestone. The land is generally very dry and source of raw water is also difficult to reach. Findings on the existence of underground river in caves, however, indicate the potential amount of water within the area, especially in the eastern part of the Gunungkidul District. Although limited information available, some fishermen have discovered that Seropan cave contains fresh water source. This cave is situated at 65 m below the cliff. Initial exploration, which done using a multichannel resistivity method, confirmed the availability of freshwater in the cave and underground river. The isopach of cave depth is found in ranges of 80 200 m below the ground surface. The water of Seropan cave can be utilized by implementing pipeline or by drilling at the suggested point based on the interpretation results, i.e. 110o2223.6388 EL 8o42.874 SL. [DY1][PP2][DY1]Perbaiki grammarIn terms of geology, most areas in south of Gunungkidul District in Central Java consist of the Wonosari formation limestone. The land is generally very dry and source of raw water is also difficult to reach. Findings on the exixtence of underground river in caves, however, indicate potential amount of water within the area especially in eastern part of Gunungkidul District. Although limited information available, some fishermans has discovered that Seropan cave contain fresh water source. This cave is situated at 65 m below the cliff. Initial exploration, which done using multichannel resistivity method, confirmed the availability of freshwater in the cave and underground river. The iso pach of cave depth is found in ranges of 80 200 m below the ground surface. The water of Seropan cave can be utilized by implementing pipeline or by drilling at the suggested point based on the interpretation results i.e. 110o2223.6388 EL 8o42.874 SL.[PP2]Sudah diperbaiki

Simulation of ERT surface-to-tunnel measurements.

The applicability of surface-to-tunnel electrical resistance tomography (ERT) measurements using a simulation tank for imaging subsurface targets is studied in this work. The tank was filled with water and inside of it was placed firmly a plastic construction where all targets were placed. Some of the targets that used were: a void plastic cylinder (resistive target) and metal plates (conductive target). Data were collected with a multichannel resistivity meter. The initial scope for these tests was to verify the reliability of developed inversionsoftware which was modified to cope with the specific measurements (surface-totunnel). Furthermore, during several experiments different electrode arrays (standard as well as new optimized) were validated. In our attempt to show the advantages of surface-to-tunnel measurements we analyzed the resolution for each configurationof traditional (surface) and surface-to-tunnel arrays. Studies also included testing the electrode displacement effect and also the effect of the tunnel size into the measurements.

Comparison of different sets of array configurations for multichannel 2D ERT acquisition

Traditional electrode arrays such Wenner-Schlumberger or dipole-dipole are still widely used thanks to their well-known properties but the array configurations are generally not optimized for multi-channel resistivity measures.Synthetic datasets relating to four different arrays, dipole-dipole (DD), pole-dipole (PD), Wenner-Schlumberger (WS) and a modified version of multiple gradient (MG), have been made for a systematic comparison between 2D resistivity models and their inverted images. Different sets of array configurations generated from simple combinations of geometric parameters (potential dipole lengths and dipole separation factors) were tested with synthetic and field data sets, even considering the influence of errors and the acquisition velocity. The purpose is to establish array configurations capable to provide reliable results but, at the same time, not involving excessive survey costs, even linked to the acquiring time and therefore to the number of current dipoles used.For DD, PD and WS arrays a progression of different datasets were considered increasing the number of current dipoles trying to get about the same amount of measures. A multi-coverage MG array configuration is proposed by increasing the lateral coverage and so the number of current dipoles. Noise simulating errors both on the electrode positions and on the electric potential was added.The array configurations have been tested on field data acquired in the landfill site of Bellolampo (Palermo, Italy), to detect and locate the leachate plumes and to identify the HDPE bottom of the landfill.The inversion results were compared using a quantitative analysis of data misfit, relative model resolution and model misfit. The results show that the trends of the first two parameters are linked on the array configuration and that a cumulative analysis of these parameters can help to choose the best array configuration in order to obtain a good resolution and reliability of a survey, according to generally short acquisition times.

Seasonal Electrical Resistivity Surveys of a Coastal Bluff, Barter Island, North Slope Alaska

Select coastal regions of the North Slope of Alaska are experiencing high erosion rates that can be attributed in part to recent warming trends and associated increased storm intensity and frequency. The upper sediment column of the coastal North Slope of Alaska can be described as continuous permafrost underlying a thin (typically less than 1–2 m) active layer that responds variably to seasonal thaw cycles. Assessing the temporal and spatial variability of the active layer and underlying permafrost is essential to better constrain how heightened erosion may impact material fluxes to the atmosphere and the coastal ocean, and how enhanced thaw cycles may impact the stability of the coastal bluffs. In this study, multi-channel electrical resistivity tomography (ERT) was used to image shallow subsurface features of a coastal bluff west of Kaktovik, on Barter Island, northeast Alaska. A comparison of a suite of paired resistivity surveys conducted in early and late summer 2014 provided detailed information on how the active layer and permafrost are impacted during the short Arctic summer. Such results are useful in the development of coastal resilience models that tie together fluvial, terrestrial, climatic, geologic, and oceanographic forcings on shoreline stability.

Adaptive time-lapse optimized survey design for electrical resistivity tomography monitoring

Adaptive optimal experimental design methods use previous data and results to guide the choice and design of future experiments. This paper describes the formulation of an adaptive survey design technique to produce optimal resistivity imaging surveys for time-lapse geoelectrical monitoring experiments. These survey designs are time-dependent and, compared to dipole–dipole or static optimized surveys that do not change over time, focus a greater degree of the image resolution on regions of the subsurface that are actively changing. The adaptive optimization method is validated using a controlled laboratory monitoring experiment comprising a well-defined cylindrical target moving along a trajectory that changes its depth and lateral position. The algorithm is implemented on a standard PC in conjunction with a modified automated multichannel resistivity imaging system. Data acquisition using the adaptive survey designs requires no more time or power than with comparable standard surveys, and the algorithm processing takes place while the system batteries recharge. The results show that adaptively designed optimal surveys yield a quantitative increase in image quality over and above that produced by using standard dipole–dipole or static (time–independent) optimized surveys.

Ground-truthing Electrical Resistivity Methods in Support of Submarine Groundwater Discharge Studies: Examples from Hawaii, Washington, and California

Submarine groundwater discharge (SGD) is an important conduit that links terrestrial and marine environments. SGD conveys both water and water-borne constituents into coastal waters, where these inflows may impact near-shore ecosystem health and sustainability. Multichannel electrical resistivity techniques have proven to be a powerful tool to examine scales and dynamics of SGD and SGD forcings. However, there are uncertainties both in data aquisition and data processing that must be addressed to maximize the effectiveness of this tool in estuarine or marine environments. These issues most often relate to discerning subtle nuances in the flow of electricity through variably saturated media that can also be highly conductive (i.e., seawater). Three contrasting field sites were examined for this study to assess the effectiveness of electrical resistivity techniques in varying coastal settings by comparing resistivity data to direct salinity and resistivity observations, quantifying changes in lithology and beach geomorphology, and fine-tuning inversion protocols. The three study sites all have substantial (up to 85 cm day−1) submarine groundwater discharge rates, but the hydrologic, oceanographic, and geologic characteristics of the sites are all very different. At a site in Pelekane Bay on the Big Island of Hawaii, seasonal flooding introduces very high concentrations of fine to coarse sediment into the bay. Near-shore circulation is limited in Pelekane Bay, so this newly introduced sediment can become deposited in the bay where it accumulates over time. At a site in Hood Canal, a fjord within Puget Sound, Washington, SGD rates can be high because of the large tidal range, abundant recharge, and steep hydrologic gradients. At Younger Lagoon in northern California, the flow of groundwater towards the coast is much more parsimonious, but here marine processes, including recirculated seawater, are important in controlling the flow of material towards the coast. Rigorous ground-truthing at each field site showed that multi-channel electrcial resistivity techniques can reproduce the scales and dynamics of a seepage field when such data are correctly collected, and when the model inversions are tuned to field site characteristics. Such information can provide a unique perspective on the scales and dynamics of exchange processes within a coastal aquifer—information essential to scientists and resource managers alike.

Integrated geophysical approach in assessing karst presence and sinkhole susceptibility along flood-protection dykes of the Loire River, Orléans, France

Non-invasive geophysical methods are often used for detecting near-surface defects and monitoring seepage in river dykes or dams. Between 2008 and 2009, a series of geophysical experiments was conducted to detect karst features below the dykes of the Loire River, Orléans, France. Multi-channel analysis of seismic surface waves (MASW) was used to obtain the shear wave velocity (Vs) profile of the subsurface below the dykes. As an effective approach to investigating the structure of the dykes, multi-channel resistivity surveys were also used to evaluate the electrical properties of material inside and under the dykes. This study discusses the exploration strategy and results for several sections of the dyke system. Based on the experimental data, Vs contours are used to geometrically and qualitatively describe the subsurface under the dykes and identify areas of mechanical weakness corresponding to karst features, while resistivity contours allow distinguishing interactions between the karst and the dykes through the identification of areas of downward material movement. Known and unknown anomalies are identified. A practical approach that combines seismic and electric resistivity results is proposed to assess karst presence and sinkhole hazards along the investigated dykes using a susceptibility index referring to the likelihood level of the events. As a validation method, areas of strong karst susceptibility index along the dykes are compared to known collapse event (sinkholes, dyke breaches, etc.) locations, available in databases.

Utilizing multichannel electrical resistivity methods to examine the dynamics of the fresh water–seawater interface in two Hawaiian groundwater systems

Multichannel electrical resistivity (ER) measurements were conducted at two contrasting coastal sites in Hawaii to obtain new information on the spatial scales and dynamics of the fresh water–seawater interface and rates of coastal groundwater exchange. At Kiholo Bay (located on the dry, Kona side of the Big Island) and at a site in Maunalua Bay (Oahu), there is an evidence for abundant submarine groundwater discharge (SGD). However, the hydrologic and geologic controls on coastal groundwater discharge are likely to be different at these two sites. While at Kiholo Bay SGD is predominantly through lava tubes, at the Maunalua Bay site exchange occurs mostly through nearshore submarine springs. In order to calculate SGD fluxes, it is important to understand the spatial and temporal scales of coastal groundwater exchange. From ER time series data, subsurface salinity distributions were calculated using site‐specific formation factors. A salinity mass balance box model was then used to calculate rates of point source (i.e., spatially discreet) and total fresh water discharge. From these data, mean SGD rates were calculated for Kiholo Bay (∼9,200 m3/d) and for the Maunalua Bay site (∼5,900 m3/d). While such results are on the same order of magnitude to geochemical tracer‐derived SGD rates, the ER SGD rates provide enhanced details of coastal groundwater exchange that can enable a more cohesive whole watershed perspective.

3D crosshole ERT for aquifer characterization and monitoring of infiltrating river water

The hydrogeological properties and responses of a productive aquifer in northeastern Switzerland are investigated. For this purpose, 3D crosshole electrical resistivity tomography (ERT) is used to define the main lithological structures within the aquifer (through static inversion) and to monitor the water infiltration from an adjacent river. During precipitation events and subsequent river flooding, the river water resistivity increases. As a consequence, the electrical characteristics of the infiltrating water can be used as a natural tracer to delineate preferential flow paths and flow velocities. The focus is primarily on the experiment installation, data collection strategy, and the structural characterization of the site and a brief overview of the ERT monitoring results. The monitoring system comprises 18 boreholes each equipped with 10 electrodes straddling the entire thickness of the gravel aquifer. A multichannel resistivity system programmed to cycle through various four-point electrode configurations of the 180 electrodes in a rolling sequence allows for the measurement of approximately 15,500 apparent resistivity values every 7 h on a continuous basis. The 3D static ERT inversion of data acquired under stable hydrological conditions provides a base model for future time-lapse inversion studies and the means to investigate the resolving capability of our acquisition scheme. In particular, it enables definition of the main lithological structures within the aquifer. The final ERT static model delineates a relatively high-resistivity, low-porosity, intermediate-depth layer throughout the investigated aquifer volume that is consistent with results from well logging and seismic and radar tomography models. The next step will be to define and implement an appropriate time-lapse ERT inversion scheme using the river water as a natural tracer. The main challenge will be to separate the superposed time-varying effects of water table height, temperature, and salinity variations associated with the infiltrating water.

Geophysical investigation of tumuli by means of surface 3D Electrical Resistivity Tomography

Abstract Tumuli are artificially erected small hills that cover monumental tombs or graves. In this work, the surface three-dimensional (3D) Electrical Resistivity Tomography (ERT) method, composed of dense parallel two-dimensional (2D) tomographies, was used to investigate the properties of the tumuli filling material and to resolve buried archaeological structures inside the tumuli. The effectiveness of the method was investigated by numerical modeling and through 3D inversion of synthetic apparent resistivity data. A resistivity model that simulates the inhomogeneous tumulus material and the tombs that are buried inside the tumulus was assumed. The Dipole–Dipole (DD), Pole–Dipole (PD), Pole–Pole (PP), Gradient (GRAD), Midpoint-Potential-Referred (MPR) and Schlumberger Reciprocal (SCR) arrays, which are suitable for multichannel resistivity instruments, were tested. The tumulus topography (pyramid or capsized cup) was incorporated into the inversion procedure through a distorted finite element mesh. The inversion procedure was based on a smoothness constrained Gauss–Newton algorithm in which the Active Constraint Balancing (ACB) method was also applied in order to enhance the least-squares resolving power and stability. Synthetic modeling showed that the different tumulus layers and the horizontal contact of the artificial tumulus material with the natural background soil were reconstructed by all of the tested electrode arrays. Generally, PD and the GRAD arrays comprise the optimum choices to investigate the subsurface properties of a tumulus and locate buried tombs. The MPR model was inferior to the GRAD model, while the DD, PP and SCR models had the poorest resolution. It was also shown that the inversion models are practically independent from the survey direction and the topography shape of the tumulus. The real field data collected employing the PD array along a small tumulus from the archaeological site of Vergina in northern Greece enhanced the synthetic modeling results. The inversion model outlined a number of archaeological structures that exhibit a high possibility to correlate with graves. Overall, this work signifies that the surface 3D ERT method can provide a valuable tool in the non-destructive archaeological exploration of tumuli.

Monitoring of Leachate Recirculation in a Bioreactor Landfill by 2-D Electrical Resistivity Imaging

At bioreactor landfills, biodegradation parameters are carefully controlled to achieve faster stabilization of the waste mass. For example, liquid (leachate) injection into the waste mass is used to increase waste moisture content and thereby enhance waste degradation. The objective of this study is to validate the use of non-intrusive two-dimensional electrical resistivity imaging (ERI) for monitoring a liquid injection system, and to determine the relationship between measured resistivity and water content variations in waste. Temporal variations of waste electrical resistivity, during leachate recirculation, are mainly linked to water and ionic content variations. Two-dimensional ERI monitoring was carried out at a municipal solid waste landfill using a multi-channel resistivity meter to follow transient resistivity variations during leachate recirculation in the waste mass. Resistivity variations mainly correspond to variations in water content, and to temperature variations resulting from the injection of cold leachate into the warm waste mass. Laboratory experiments can be used to calibrate water content variations in the waste as a function of variations in electrical resistivity. Although such an approach is realistic for short measurement periods (hours), it is not reliable over longer time periods (months) since other parameters, such as biodegradation of the waste, may also influence its electrical resistivity.

Integrated geophysical survey in defining subsidence features on a golf course

Subsidence was observed at several places on the Salina Municipal Golf Course in areas known to be built over a landfill in Salina, Kansas. High-resolution magnetic survey (~5400 m2), multi-channel electrical resistivity profiling (three 154 m lines) and microgravity profiling (23 gravity-station values) were performed on a subsidence site (Green 16) to aid in determining boundaries and density deficiency of the landfill in the vicinity of the subsidence. Horizontal boundaries of the landfill were confidently defined by both magnetic anomalies and the pseudo-vertical gradient of total field magnetic anomalies. Furthermore, the pseudo-vertical gradient of magnetic anomalies presented a unique anomaly at Green 16, which provided a criterion for predicting other spots with subsidence potential using the same gradient property. Results of multi-channel electrical resistivity profiling (ERP) suggested the bottom limit of the landfill at Green 16 was around 21 m below the ground surface based on the vertical gradient of electric resistivity and a priori information on the depth of the landfill. ERP results also outlined several possible landfill bodies based on their low resistivity values. Microgravity results suggested a –0.14 g cm−3 density deficiency at Green 16 that could equate to future surface subsidence of as much as 1.5 m due to gradual compaction.