FREQUENCY-DEPENDENT COMPLEX RESISTIVITY OF ROCK-SALT SAMPLES AND RELATED PETROPHYSICAL PARAMETERS1

The presence of water is one of the main concerns of nuclear waste disposal in rock-salt. It can be investigated using electrical properties of the rock. Laboratory measurements of frequency-dependent resistivity and other petrophysical parameters, such as porosity, water content, and specific internal surface area, have been carried out on rock-salt from the Asse mine in Germany, in order to obtain characteristic resistivity responses for the evaluation of geoelectric field methods and to develop new methods for the estimation of the water content and saturation. The laboratory method, on a.c. half-bridge for very high impedances, allows measurements of the resistivity spectrum of rock-salt in the frequency range from 15 Hz to 10 kHz. The saturation of the samples was varied artificially and was approximately 5%, 10%, 20% and 100%. The porosity varies between 0.1% and 0.5%, the water content is approximately 0.05% or less, and the initial saturation is less than 50%. The resistivity ranges from 10 MΩm at the initial saturation down to 1 kΩm for fully saturated samples. In the low-frequency range up to 100 Hz, an Archie-type relationship may be used to estimate the water content of the rock-salt from resistivity measurements. The Archie exponent m is found to be approximately 2. The resistivity is observed to be strongly dependent on frequency. The resistivity decreases with increasing frequency, with a greater decrease for small saturations and vanishing frequency dependence at complete saturation. The relative dielectric constant was found to be 6 ± 1. Saturation dependence was not observed within this error range. The measurements imply that, by measuring resistivity in rock-salt, estimations of water content and saturation, and thus the porosity, can be made in situ. This is particularly important for the safety of nuclear waste disposal in rock-salt.     

RESISTIVITY OF ROCK-SALT IN ASSE (GERMANY) AND PETROPHYSICAL ASPECTS1

In the Asse salt-mine (Germany), where extensive research is carried out on various aspects of nuclear waste disposal in rock-salt formations, the resistivity of a future construction site for a test seal at a depth of 945 m has been investigated. Measurements have been conducted, using various types of the four-point electrode configuration, on a network of 180 electrodes permanently installed in boreholes. A fully computer-controlled measurement system has been built consisting of a resistivity meter, a switchbox for electrodes and a computer with special control software. The system has interactive and programmable automatic working modes with an extensive capability for documentation and processing including the immediate evaluation of apparent resistivities and is suitable for long-term continuous observations. The average resistivity at the site which consists of almost homogeneous Stassfurt rock-salt, is approximately 0.6 × 10⁶Ωm with little variation. There is a small anomaly due to a conductive inclusion, probably a local anhydrite band. There is no indication of anisotropy so far and no significant change with time. The specific amount of water in the effective pores and its change can be estimated from the observed resistivities using the extended form of Archie's equation with pore saturation included. In this case data for the specific internal surface from samples are available and the order of permeability may be estimated using the formation factor derived from resistivities.     

DC GEOELECTRIC SOUNDING TO DETERMINE WATER CONTENT IN THE SALT MINE ASSE (FRG)*

In the course of investigations concerning disposal of radioactive waste, DC-geoelectric soundings were performed in the salt mine Asse in 1982. The survey resulted in the determination of resistivities of various salt formations. A comparison of various resistivities obtained in salt formations in the mine shows that it is possible to estimate the content of free water in salt using Archie's equation. The significant result of this survey is that salt formations in the mine can be divided into two categories according to their resistivities: 1. Salt with resistivity between 10⁷ and 10⁸Ωm (particularly Na2Sp and Na3β). The particularly pure halitic formations are to be found within this resistivity range. 2. Salt with resistivity between 10³ and 10⁵Ωm (Na2Tl, K2, transition salt). The salt formations with thin layers of anhydrite and a high content of hygroscopic salts are classified in the paper.     

Electrical resistivity characterization and defect detection on a geosynthetic clay liner (GCL) on an experimental site

In this paper we analyze the onsite characterization of a geosynthetic clay liner (GCL) that serves to ensure the impermeability of a landfill cap by DC electrical methods. The imaging of the GCL geoelectrical properties is a challenging problem because it is a very thin (between 4 and 7 mm thick) and resistive layer (from 100,000 to 2,000,000 Ω·m) depending on meteorological conditions and aging. We compare results obtained using electrical resistivity tomography (ERT) using two different kinds of arrays (dipole–dipole DD and Wenner–Schlumberger) on an experimental site with engineered defects. To confirm these results and to find the real onsite GCL resistivity we have performed sampling of the posterior distribution of this parameter using vertical electrical sounding (VES) inversions. Different VES methods were extracted from ERT with DD array and converted into a Schlumberger array.As a main conclusion the dipole–dipole array provides a better resistivity resolution of the defects than the Wenner–Schlumberger array. On ERT images, the defect detection seems to be impossible if the GCL has very high resistivity, as it happened when it was put in place. Taking into account the equivalence rules, the inversions are in both cases (ERT and VES) compatible. The GCL resistivity estimated from PSO (particle swarm optimization) varies from 3.0 10⁵ to 1.10⁶ Ω·m depending on saturation conditions during the twenty first months of its placing. Then, the resistivity dropped to 4.10⁴–9.10⁴ Ω·m, indicating a probable chemical damage of the GCL due to aging. Finally the fact that the VES inversions are solved via PSO sampling allows for the detection of a very thin and resistive layer and opens the possibility of performing micro VES surveys along the landfill to detect possible GCL defects.     

Characterization of aquifers in the Vientiane Basin,Laos, using Magnetic Resonance Sounding and Vertical Electrical Sounding

The aim of this study is to define and characterize water bearing geological formation and to test the possibility of using geophysical techniques to determine the hydrogeological parameters in three areas in the Vientiane basin, Laos. The investigated areas are part of the Khorat Plateau where halite is naturally occurring at depths as shallow as 50 m in the Thangon Formation. Magnetic Resonance Sounding (MRS) has been used in combination with Vertical Electrical Sounding (VES) in different geological environments. In total, 46 sites have been investigated and the MRS and VES recognized the stratigraphic unit N₂Q_1–3, consisting of alluvial unconsolidated sediments, as the main water bearing unit. The aquifer thickness varies usually between 10 and 40 m and the depth to the main aquifer range from 5 to 15 m. The free water content is here up to 30%, and the decay times vary between 100 and 400 ms, suggesting a mean pore size equivalent to fine sand to gravel. The resistivity is highly variable, but usually around 10–1500 Ω-m, except for some sites in areas 1 and 2, where the aquifer is of low resistivity, probably related to salt water. Hydraulic and storage-related parameters such as transmissivity, hydraulic column, have been estimated from the MRS. The MRS together with VES has been shown to be a useful and important tool for identifying and distinguishing freshwater from possible salt-affected water as well as the salt-related clay layer of the Thangon Formation. This clay layer is characterized by very low free water content and a resistivity lower than 5 Ω-m and can be found in all 3 areas at depths from 15 to 50 m.     

Time‐lapse electrical resistivity imaging of solute transport in a karst conduit

The use of electrical resistivity surveys to locate karst conduits has shown mixed success. However, time‐lapse electrical resistivity imaging combined with salt injection improves conduit detection and can yield valuable insight into solute transport behaviour. We present a proof of concept above a known karst conduit in the Kentucky Horse Park (Lexington, Kentucky). A salt tracer solution was injected into a karst window over a 45‐min interval, and repeat resistivity surveys were collected every 20 min along a 125‐m transect near a monitoring well approximately 750 m downgradient from the injection site. In situ fluid conductivity measurements in the well peaked at approximately 25% of the initial value about 3 h after salt injection. Time‐lapse electrical resistivity inversions show two broad zones at the approximate conduit depth where resistivity decreased and then recovered in general agreement with in situ measurements. Combined salt injection and electrical resistivity imaging are a promising tool for locating karst conduits. The method is also useful for gaining insight into conduit geometry and could be expanded to include multiple electrical resistivity transects. Copyright © 2015 John Wiley & Sons, Ltd.     

洱海南部近岸微囊藻原位增殖的季节变化及影响因子

2017年7月—2018年4月在洱海南部近岸开展了4个季度的野外调查,利用细胞分裂频率法研究微囊藻原位增殖的季节变化及影响因子,结果显示:微囊藻细胞分裂频率的变化范围为6.13%～29.23%,白天显著大于夜间,但两个点位间和表中底3层间的差异均不显著;经计算,微囊藻原位生长速率的均值为(0.36±0.06)d^-1,总体高于其它有相关研究的湖泊。同时,微囊藻细胞分裂频率和原位生长速率具有显著的季节变化,表现为冬季最大(1月,19.65%±4.10%、(0.39±0.01)d^-1),秋季(10月,14.48%±4.73%、(0.36±0.02)d^-1)和夏季(7月,12.77%±3.81%、(0.37±0.07)d^-1)次之,春季(4月,10.37%±2.64%、(0.30±0.06) d^-1)最小,其中营养盐浓度的作用不明显,而地表太阳辐射和水温等影响较大。进一步分析发现,除地表太阳辐射外,影响细胞分裂频率昼夜变化的其它因子具有一定的季节异质性。研究结果可为深入研究洱海微囊藻水...     

Coupling two radar backscattering models to assess soil roughness and surface water content at farm scale

Abstract

Remote sensing techniques are useful for agro-hydrological monitoring at the farm scale because the availability of spatially and temporally distributed data improves agricultural models for irrigation and crop yield optimization under water scarcity conditions. This research focuses on the surface water content retrieval using active microwave data. Two semi-empirical models were chosen as these showed the best performances in simulating cross and co-polarized backscatter. Thus, these models were coupled to obtain reliable assessments of both soil water content and soil roughness. The use of the coupled model enables one to avoid using roughness measured in situ. Remote sensing images and in situ data were collected between April and July 2006 within the European Space Agency-funded project AgriSAR 2006. The images data set includes L-band in HH, VV and VH polarizations acquired from the airborne E-SAR sensor, operated by the German Aerospace Centre. Results were validated using in situ soil water content and roughness measurements. The results show that reliable assessment of both soil roughness (r ² up to ?0.8) and soil water content (r ² ? 0.9) can be retrieved in fields characterized by low fractional coverage.

Editor D. Koutsoyiannis; Associate editor C. Onof

Citation Capodici, F., Maltese, A., Ciraolo, G., La Loggia, G., and D’Urso, G., 2013. Coupling two radar backscattering models to assess soil roughness and surface water content at the farm scale. Hydrological Sciences Journal, 58 (8), 1677–1689.     

Using geophysical surveys to test tracer‐based storage estimates in headwater catchments

Hydrogeophysical surveys were carried out in a 3.2 km² Scottish catchment where previous isotope studies inferred significant groundwater storage that makes important contributions to streamflow. We used electrical resistivity tomography (ERT) to characterize the architecture of glacial drifts and make an approximation of catchment‐scale storage. Four ERT lines (360–535 m in length) revealed extensive 5–10 m deep drift cover on steeper slopes, which extends up to 20–40 m in valley bottom areas. Assuming low clay fractions, we interpret variable resistivity as correlating with variations in porosity and water content. Using Archie's Law as a first approximation, we compute likely bounds for storage along the ERT transects. Areas of highest groundwater storage occur in valley bottom peat soils (up to 4 m deep) and underlying drift where up to 10 000 mm of precipitation equivalent may be stored. This is consistent with groundwater levels which indicate saturation to within 0.2 m of the surface. However, significant slow groundwater flow paths occur in the shallower drifts on steeper hillslopes, where point storage varies between ~1000 mm–5000 mm. These fluxes maintain saturated conditions in the valley bottom and are recharged from drift‐free areas on the catchment interfluves. The surveys indicate that catchment scale storage is >2000 mm which is consistent with tracer‐based estimates. Copyright © 2016 John Wiley & Sons, Ltd.     

TEM study of the geoelectrical structure and groundwater salinity of the Nahal Hever sinkhole site, Dead Sea shore, Israel

Since the 1990s a large number of sinkholes have appeared in the Dead Sea (DS) coastal area. Sinkhole development was triggered by the lowering of the DS level. In the literature the relationship between the sinkholes and the DS level is explained by intrusion of relatively fresh water into the aquifer thereby dramatically accelerating the salt dissolution with creation of subsurface caverns, which in turn cause sinkholes. The main goal of our project was detection and localization of relatively fresh groundwater. During our study we used the transient electromagnetic method (TEM) to measure the electrical resistivity of the subsurface. As a test site we selected Nahal Hever South which is typical for the DS coast. Our results show that resistivity of the shallow subsurface reflects its vertical and lateral structure, e.g., its main hydrogeological elements explain the inter-relations between geology, hydrogeology, and sinkholes. The TEM method has allowed detailed differentiation of layers (clay, salt, etc.) in the subsurface based on their bulk resistivity. The 10 m-thick salt layer composed of idiomorphic crystals of halite deposited during the earlier Holocene period was extrapolated from borehole HS-2 through the study area. It was found that in Nahal Hever the typical value of the bulk resistivity of clay saturated with the DS brine varies between 0.2 and 0.3 Ωm, whereas saturated gravel and sandy sediments are characterized by resistivity between 0.4 and 0.6 Ωm. The high water salinity of the aquifer (enveloping the salt layer) expressed in terms of resistivity is also an important characterization of the sinkhole development mechanism. The electrical resistivity of the aquifer in the vicinity of the salt unit and its western border did not exceed 1 Ωm (in most cases aquifer resistivity was 0.2-0.6 Ωm) proving that, in accordance with existing criteria, the pores of the alluvial sediments are filled with highly mineralized DS brine. However, we suggest that the criterion of the aquifer resistivity responsible for the salt dissolution should be decreased from 1 Ωm to 0.6 Ωm corresponding to the chloride concentration of approximately 100 g/l (the chloride saturation condition reaches 224 g/l in the northern DS basin and 280 g/l in the southern one).Based on TEM results we can reliably conclude that in 2005, when most of sinkholes had appeared at the surface, salt was located within a very low resistivity environment inside sediments saturated with DS brine. Intrusion of relatively fresh groundwater into the aquifer through the 600 × 600 m²area affected by sinkholes has not been observed.     

Relative performance of focused and unfocused devices in a two-dimensional borehole environment

Borehole direct current (dc) resistivity problems are solved using finite difference method (FDM) primarily to extend the log interpretation in the two-dimensional (2D) domain. Transitional invaded zone, flushed zone, uncontaminated zone, shoulder beds and borehole filled with mud are simulated simultaneously in the model. Linear variation of resistivity is assumed for transitional invaded zone. Normal, lateral and modified laterolog7 (LL7⁴) electrode configuration (discussed in the text) responses were computed using expanding rectangular grid system. LL7⁴ responses with variable geometric factor did show some superiority over the conventional normal and lateral for conductive target beds sandwiched between the resistive beds. For relatively small borehole diameter, all three probes can detect resistive target bed satisfactorily. However, for highly conductive mud both LL7⁴ and normal electrode configuration fail to detect any signal from the target bed. Bypassing effect of the current for LL7⁴ is more than that in normal log. Lateral log shows a signal of conductive invaded zone generated by the conductive mud. Bucking current ratio peaks and the geometric factor peaks in LL7⁴ can detect the bed boundaries.     

Correlation of vertical electrical sounding and electrical borehole log data for groundwater exploration

We have correlated the longitudinal unit conductance CL obtained from interpreted vertical electrical sounding data with the formation resistivity R_t and the formation resistivity factor F, obtained by carrying out electrical borehole logging. Interpreted geophysical data of eleven soundings and two electrical borehole log records are used for the analysis. The geophysical data used were acquired in a sedimentary basin. The study area is called Lower Maner Basin located in the province of Andhra Pradesh, India. Vertical electrical soundings were carried out using a Schlumberger configuration with half current electrode separation varying from 600–1000 m. For logging the two boreholes, a Widco logger‐model 3200 PLS was used. True formation resistivity R_t was calculated from a resistivity log. Formation resistivity factor F was also calculated at various depths using R_t values. An appreciable inverse relation exists between the correlated parameters. The borehole resistivity R_t and the formation resistivity factor F decrease with the increase in the longitudinal unit conductance CL. We have shown the use of such a relation in computing borehole resistivity R_t and formation resistivity factor F at sites that posses only vertical electrical sounding data, with a fair degree of accuracy. Validation of the correlation is satisfactory. Scope for updating the correlation is discussed. Significance and applications of the relation for exploration of groundwater, namely to update the vertical electrical sounding data interpretation by translating the vertical electrical sounding data into electrical borehole log parameters, to facilitate correlations studies and to estimate the porosity (φ), permeability (K) and water saturation S_w of water bearing zones are discussed.     

MODEL STUDIES ON SOME ASPECTS OF RESISTIVITY AND MEMBRANE POLARIZATION BEHAVIOUR OVER A LAYERED EARTH*

Electrolytic model tank experiments to study resistivity and time domain induced polarization (IP) response over layered earth models were initiated primarily to facilitate the understanding of field results. Alternate layers of clay and sand (or clay-coated sand) with, in some cases, a surficial layer of water were assembled in the tank and resistivity and IP measurements made for a range of electrode spacings using the Wenner configuration. Graphite and silver-silver chloride electrodes were used as current and potential electrodes respectively. Clay-coated (3% by weight) sand was found to generate stronger polarization than either clay or sand alone. Apparent chargeability m_a was observed to be positive for a nonpolarizable surface layer. For a polarizable surface layer, the sign of IP was controlled by the polarizability, the thickness of the second layer, and the spacing of the electrode spreads. The apparent chargeability m_a can theoretically change sign from positive to negative and vice versa with a gradual increase in electrode spacing, and such negative IP effects were obtained in a few observations. A simultaneous decrease in IP and an increase in resistivity, which is a qualitative diagnostic feature for the occurrence of clean freshwater sand aquifers, could also be generated in the model tank experiment. Combined resistivity and IP soundings were carried out near Fredericton Junction and Tracy, New Brunswick, Canada. Field curves are presented along with the model curves for qualitative comparison and understanding of IP behaviour over a layered earth. Twenty-five out of twenty-seven soundings show only positive apparent chargeabilities, whereas two show chargeability sign changes (positive/negative/positive). The model study gives reason to believe that surface soils and Quaternary gravel boulder deposits near Fredericton Junction are relatively non-polarizable. As an auxiliary experiment, sand and clay were taken in different proportions by weight and mixed thoroughly with water in a cement mixer. The mixtures were then compressed with a suitable die and plunger under 3.6 Pa pressure to prepare cylindrical samples of height 18 cm and diameter 15.5 cm. IP measurements were done on the flat faces using the Wenner configuration with a= 2 cm. Chargeability was found to be negative for 100 and 90% clay mixtures. It reached a positive maximum for an 80% clay-20% sand mixture and then decreased gradually with increasing sand and decreasing clay content.     

高密度电阻率法在海底金矿含水构造探测中的应用

海底金矿上覆岩层中含水直接对矿床的开采构成威胁,选择山东三山岛金矿新立矿区海底-135m水平沿脉巷道,采用地球物理高密度电阻率法对该巷道635m测线以下200m深度范围内岩体中的含水构造进行了坑道探测,利用三种装置(温纳装置(Wenner)、偶极装置(Dipole-Dipole)、Schlumberger装置)进行实施,相互验证,取得了很好的探测结果.温纳、偶极和Schlumberger三种装置的视电阻率反演结果反映在测线(-135m沿脉巷道)以下至30m深度为一高阻层,表明自-135~-165m的岩体中已不含水或含少量的水;测线以下30~60m段为低阻层,反映-165~-200m的岩体中含基岩裂隙水;测线以下大于60m的地段为特高阻层,反映-200m以下岩体含水性逐渐变差.该探测结果与矿区水文地质结构调查分析结论具有很好的一致性,表明了坑道高密度电阻率法探测含水构造是可行的.     

Combined electrical and electromagnetic imaging of hot fluids within fractured rock in rugged Himalayan terrain

Integrated electrical resistance tomography (ERT) and short-offset transient electromagnetic (TEM) measurements were carried out to investigate a geothermal area in the Main Central Thrust (MCT) zone of Garhwal Himalayan region, India. The study area is located around Helang on either side of Alaknanda River and it is dotted with hot water springs with water temperature of 45°–55 °C emerging at the surface.To assess the geothermal potential and its lateral and vertical extension in and around the hot water springs in the study area, 7 ERT profiles and 21 TEM stations on 7 profiles were established around the hot water spring and at far distant locations. The 2D inversion of ERT data indicates a low resistivity (< 50 Ωm) zone in the vicinity of hot springs, which appears to be associated with an underground water channel through the fractured rock. The bedrock resistivity is very high (> 1000 Ωm) whereas the resistivity of the weathered near surface soil at a far distant location from the hot spring is low (< 100 Ωm) again. A common feature of all TEM data is the sign reversal observed at roughly 10 μs. The consistent sign reversal in all TEM data indicates the existence of the multi-dimensionality of the geoelectrical structure. Therefore, the TEM data were treated by using the SLDM (Spectral Lanczos Decomposition Method) 2D/3D forward modeling code based on the finite difference algorithm. The resistivity structure obtained from ERT data was used as an input for the modeling of TEM data. Based on the joint analysis of the ERT and TEM data it can be inferred that geothermal anomalies associated with the hot spring in the MCT zone are a local feature appearing as a low resistivity zone (< 50 Ωm) at shallow depth (< 100 m) in the vicinity of the hot spring region.     

Geological controls on submarine groundwater discharge in Long Bay,South Carolina (USA)

A combination of geophysical methods including continuous electrical resistivity and high-resolution Chirp sub-bottom profiling were utilized to characterize geologic controls on pore fluid salinity in the nearshore of Long Bay, SC. Resistivity values ranged from less than 1 Ω m to greater than 40 Ω m throughout the bay. Areas of elevated electrical resistivity suggest the influence of relatively fresher water on pore water composition. Geophysical evidence alone does not eliminate all ambiguity associated with lithological and porosity variations that may also contribute to electrical structure of shallow marine sediments. The anomalous field is of sufficient magnitude that lithological variation alone does not control the spatial distribution of elevated electrical resistivity zones. Geographical distribution of electrical anomalies and structures interpreted from nearby sub-bottom profiles indicates abrupt changes in shallow geologic units control preferential pathways for discharge of fresh water into the marine environment. Shore parallel resistivity profiles show dramatic decreases in magnitude with increasing distance from shore, suggesting a significant portion of the terrestrially driven fresh SGD in Long Bay is occurring via the surficial aquifer within a few hundred meters of shore.     

Determination of electrical resistivity,its anisotropy and heterogeneity on drill cores: a new method1

This paper presents a new method for the measurement on core samples of their electrical resistivity, its anisotropy and heterogeneity. The equipment used has been developed in the field laboratory of the German Continental Deep Drilling program KTB in the north-east of Bavaria on the western rim of the Bohemian Massif. The apparatus measures the resistivity at a fixed frequency as a function of the drill core azimuth and along the core by moving point electrode configurations. From these azimuth and depth dependences, mean values of resistivity and additional information about its anisotropy and heterogeneity are determined. Geometrical averaging is used, because the resistivity data follows a log normal distribution. The quantitative parameters ‘azimuth factor’, corresponding to horizontal anisotropy, and ‘heterogeneity factor’ are introduced. The depth logs of resistivity, azimuth factor and heterogeneity factor, measured on cores obtained from the KTB main drill hole (gneisses and amphibolites) at depths between 4150 m and 8080 m are presented. The geometrically averaged mean values of resistivity of gneisses and amphibolites are in the same range (? 10³Ωm). The resistivities tend to decrease with depth. The stress release of the drill cores during recovery produces microcracks which may partially account for this effect. Reduced resistivities (down to 150 Ωm) within an amphibolite core correlate with an alteration zone. One sample of this core displays alteration from fresh to completely altered. This sample is also electrically heterogeneous (heterogeneity factor ? 2). Other samples with uniform low alteration are more homogeneous heterogeneity factor ? 1.4). In general, higher anisotropies are observed in gneisses (mean azimuth factor 2.8), lower anisotropies in amphibolites (mean azimuth factor 1.3). Examples of isotropic and homogeneous samples, as well as anisotropic and heterogeneous samples are also presented.     

Resistivity monitoring of the tephra barrier at Crater Lake,Mount Ruapehu,New Zealand

The eruptions of Mt Ruapehu in the North Island of New Zealand in 1995 and 1996 caused a tephra barrier to be formed across the outlet of Crater Lake. By 2005 seepage from the refilled lake into the barrier raised the possibility of an eventual collapse of the barrier, releasing a catastrophic lahar down the mountain.As part of an extensive monitoring programme of the tephra barrier, direct current (dc) resistivity surveys were carried out on a number of lines along and across it in order to test whether the extent of the seepage could be measured (and monitored) by geophysical means. Two dimensional inversion of measured apparent resistivity data showed that between the initial measurements, made in January 2005, and February 2006, there was a gradual decrease in resistivity above the old outlet from ~ 50–60 Ωm to ~ 30 Ωm. This gave the first indication that lake water was seeping into the barrier. Between October and December 2006 there was a rapid rise in lake level to only 2 m below the top of the barrier, and a further resistivity survey in January 2007 showed that there had been a further decrease in resistivity throughout the entire barrier with values dropping to < 10 Ωm. The extent of this low resistivity indicated that the barrier was now saturated. At this stage lake water was penetrating the barrier and starting to cause erosion on its downstream side. Catastrophic collapse occurred on 18 March 2007, accompanied by a lahar in the Whangaehu river valley.Subsequent forward 3D numerical modelling of the resistivity structure of the barrier has confirmed that the observed changes in measured resistivity were directly related to the progress of seepage of lake water into the barrier.     

A NEW PARAMETER FOR THE INTERPRETATION OF INDUCED POLARIZATION FIELD PROSPECTING (time-domain) *

In a previous paper it has been shown that we can relate the transient IP electric field E_p , existing in a rock after a step wave of polarizing current, with the steady-state current density J_ss during the current step wave as follows: E_p =ρ' J_ss . This relation may be interpreted as a generalized Ohm's law, valid in linear cases, in which ρ’(fictitious resistivity) is defined as the product of the true resistivity ρ with the chargeability m. Supposing E _p=— grad U_p and applying the divergence condition div J_ss = o, one can, for a layered earth, obtain a general expression for the depolarization potential U_p as a solution of Laplace's equation ?²U_p= o. Since the mathematical procedure for the solution of this last equation is identical to that used in resistivity problems, we propose now the introduction of an apparent fictitious resistivity ρ'_a (defined as the product of the apparent resistivity ρ_a with the apparent chargeability m_a) as a new parameter for the interpretations of IP soundings carried out over layered structures with a common electrode array. The most general expression of ρ'_a as a function of the electrode distance turns out to be mathematically identical to the general expression of ρ'_a. Therefore it is possible to interpret a ρ'_a field curve using the same standard graphs for resistivity prospecting with the usual method of complete curve matching. In this manner a great deal of work is saved since there is no need to construct proper m_a graphs for the interpretation of IP soundings, as it has been done up to now. Finally some field examples are reported.