NS11C-0689
A Laboratory Seismoelectric Measurement for the Permafrost Model with a Frozen-unfrozen Interface
For the Qing-Cang railway line located in the permafrost region, the freeze-thaw cycling with the seasons and spring-thaw of the permafrost are main factors to weaken the railway bed. Therefore, the determination of the frozen-unfrozen interface depth below the railway bed is important for the railway operation, and moreover, it can contribute to the evaluation of the permafrost environment effected by the railway. Since the frozen-unfrozen interface is a contact of two media with various porosity and saturation, an electric double-layer can be formed at the interface by the absorption of electrical charge to it. When a seismic wave is incident at the interface, a relative motion of the charges in the electric double-layer would induce an electromagnetic (EM) wave, or a seismoeletric conversion signal that can be measured remotely, which is potential for determining the frost depth. A simple permafrost model with a frozen-unfrozen interface was built mainly by two parts: the upper part was a frozen sand block with a 7cm thickness and the lower one with the same material was in an unfrozen state saturated with water. And the contact of the two parts simulated the frozen-unfrozen interface. The interface model was placed in a freezer, while it was heated from the bottom with a heating sheet made by the electric heating wires laid under the unfrozen part. A P-wave source transducer with 48 kHz narrow band frequency was set on the top the frozen part and driven by a square electric pulse. The six electrodes with a 1 cm even interval were fixed inside the frozen part with 1 cm vertical distance to the interface. In the experiment, all the analog signals acquired from the temperature sensors, acoustic transducers, and electrodes were sent through preamplifiers and recorded digitally by computer-based virtual instruments (VIs). At the beginning of the experiment, the first arrivals of the seismoeletric signals observed from the six electrodes with minimum offset set to be 7cm were proportional to the distances between the acoustic sources to electrodes, and thus the EM signals are originated from the stationary electromagnetic field that moves along with the acoustic waves. After the eight hours, we recognized two new events of EM waves by their exactly identical arrive times from the six electrodes. The event A with identical arrival time being close to zero is the EM interference of the high-voltage pulse exciting the acoustic source transducer. The identical arrival time 23-25 microsecond of the event B roughly equates to that of the acoustic wave travel time from the source to the interface, and it is obviously the conversion EM signal originated from the electric double-layer in the interface. With a minimum 14cm offset, the event A arrived at the same time only with greatly reduced amplitude, and the event B had not able to be detected for its weak amplitude. Another event B' with an about 50 microsecond identical arriving time could, however, be recognized, and it should be a conversion EM wave from the interface exited by the second acoustic vibration cycle from the acoustic source wave with higher amplitude, as the arrival time just equates to that of the second cycle of the narrow band acoustic wave to travel to the interface. These measurements in the laboratory show that , the electric double-layer formed at the frozen-unfrozen interface can be polarized to generate EM waves by both an EM pulse and a vibration source, which imply that the frozen-unfrozen interface of the permafrost could be surveying by both EM, and seismoelectric methods. And the results also show that the electric double-layer needs several hours to be formed in a laboratory experiment under low tempreture.
NS11C-0690
Hydrothermal system beneath Usu volcano in Japan inferred from self-potential survey
We conducted self-potential (SP) surveys on Usu volcano since July to December 2006. The compiled SP map reveals positive anomalies around Ko-Usu lava dome and at the foot of the volcano, and a negative anomaly on the top of O-Usu lava dome. The SP profile on the summit caldera shows the same pattern of 1985's. However, the peak-to-peak amplitude of the SP value is different: the 1985's of that shows 1000 mV while the 2006's shows 1400 mV. Topographic effect is clearly shown along the southwestern foot of the volcano, in which coefficient is about -2.5 mV/m. The SP profile corrected of the topographic effect reveals a large and extensive positive anomaly over +600mV and several local positive anomalies over +1000 mV on the summit caldera. The corrected SP also reveals that a positive anomaly at the south part of Ko-Usu lava dome and a negative anomaly at the O-Usu lava dome do not exist. The revealed positive SP anomalies are likely to be affected by an extensive altered layer, located beneath the summit caldera. The largest positive anomaly is shown on the ridge of Usu- Shinzan cryptodome after topographic correction. This anomaly is not regarded to be formed by hydrothermal upwelling, because any indications of fumarolic and geothermal activities have not been observed on the ridge of the volcano. The SP anomaly may be affected by intruded material into the volcano. The SP amplitude in the northwestern part of Nishiyama is very small in spite of rugged topography. An extensive low resistivity layer (< 10 ohm-m) located in the shallow part in the northwestern of Nishiyama probably shields the SP variation.
NS11C-0691
Self Potential as a method of monitoring the effectiveness of insitu remediation technologies
This project uses a 2D flow tank (dimensions 145cm x 85cm x 6.6cm) filled with sand, a gravel pack and slotted piezometer to monitor the effectiveness of commonly used groundwater sparging systems. The flow tank is designed to have a hydraulic gradient maintained by lateral flow. Two types of SP electrodes were constructed and compared - Pb-PbCl Petiau electrodes and Ag-AgCl electrodes utilizing agar as a fixing agent. The electrodes are evenly spaced along the hydraulic gradient with the reference in contact with fluid at the base of the tank. Falling head tests are conducted before and after each sparging event to monitor changes in hydraulic conductivity. Down borehole insitu remediation technologies are difficult to design, effectively monitor and validate. Often additional monitoring boreholes have to be drilled at great expense to confirm zones of influence around groundwater remediation boreholes. The results of this work contribute to real time monitoring and development of groundwater remediation technologies
NS11C-0692
Three-Dimensional SIP Imaging of Rock Core Sample: Numerical Examples
SIP (spectral IP) method is known as complex resistivity method because it measures and uses both the magnitude and the phases. SIP method had been mainly used in the field of mineral explorations, but recently SIP method extended its application to the environmental problem, because the real and imaginary components of interpreted complex resistivity are related to the hydraulic property of subsurface. In this study, we used the SIP method to monitor the physical property change during injection of CO2 gas into a rock sample in the laboratory experiments. For this purpose, we developed three-dimensional SIP modeling and inversion algorithm based on the complex resistivity. We chose the FEM (finite element method) in the modeling algorithm, and we deformed a rectangular grid to a cylinder shape to build the cylinder model, like core samples. To verify the SIP modeling algorithm, we tested our algorithm to a simple isolated block model in homogeneous half space and compare its results with those from three-dimensional integral equation method. Results from the different two methods are quite well matched. To verify the inversion algorithm developed, we applied it to the simple isolated earth model and compared its inversion result with true model. Inverted result shows smoother distribution of conductivity and phase than true model due to the smoothness constraints which are necessary for the stability of inversion. Although the values of conductivity and phase are somewhat underestimated than true value and its distribution is smoother than the given model, we can clearly see the location of conductive anomaly. We could confirm the validity of developed inversion algorithm from these results. After finishing the verification, we applied the developed algorithm to imaging of a rock core model. The core model has conductive and reactive anomalous body at the center of the model. We simulate the SIP survey using 16 electrodes on the surface of the model, and then apply developed inversion to the simulated SIP responses. Although the number of electrode is limited to 16, we can clearly see the conductive and reactive anomalous zone from the inverted results. For we have finished the development of numerical modeling and inversion algorithms, we are going to apply it to the results of laboratory test results in the near future. We hope that our developed algorithm could image the change of physical property during the CO2 injection into the rock sample.
NS11C-0693
Low Frequency Electrical Measurements of Unsaturated Filters for Wastewater Treatment
On-site wastewater treatment such as infiltration systems are used extensively for small communities all over Europe. For instance in France, the number of vertical flow constructed wetland has increased from 0 to more than 400 units between 1992 and 2005. There is a lack of relevant tools for monitoring their functionality and detecting potential failures in the systems. Electrical Resistivity Tomography (ERT) combined with Induced Polarization (IP) could potentially provide such a tool. Preliminary field studies have indicated that spatial variability in the flow pattern can be revealed by time-lapse measurements of electrical resistivity, however the interpretation of these results require better knowledge about the constitutive relationship between electrical properties and water content, water conductivity and biological clogging in typical filter media. A laboratory experiment was conducted using a four-electrode arrangement in soil sampling cylinders. Under controlled water contents the electrical resistivity and IP was measured for different levels of water conductivity and biofilm development. Finally, results were compared with empirical and theoretical relationships found in the literature, and the reliability of these methods for monitoring in wastewater treatment systems evaluated.
NS11C-0694
Geophysical monitoring of microbial activity during stimulated subsurface bioremediation
Understanding how microorganisms alter their physical and chemical environment during bioremediation is hindered by our inability to resolve subsurface microbial activity with high spatial resolution. Here we demonstrate the use of a minimally invasive geophysical technique to monitor microbe-mediated iron and sulfate reduction during acetate amendment of a uranium-contaminated aquifer near Rifle, CO. During induced polarization (IP) measurements, spatiotemporal variations in the phase response between applied and measured voltages correlated with changes in groundwater geochemistry indicative of microbial iron and sulfate reduction and sulfide mineral precipitation. The enhanced sensitivity of the high and low frequency phase responses to accumulated aqueous iron and sulfide, respectively, provide the ability to discriminate the dominant subsurface biogeochemical process. The spectral effect was verified and calibrated using a biostimulated column experiment containing Rifle sediments and groundwater. Sediments and fluids recovered from regions of the field site exhibiting an anomalous phase response were enriched in sorbed Fe(II) and cell-associated 2-4 nm diameter FeS nanoparticles. These mineral precipitates and accumulated electroactive ions altered the ability of pore fluids to conduct electrical charge, accounting for the IP response. The results reveal the usefulness of multi-frequency IP measurements for discriminating mineralogical and geochemical changes during stimulated subsurface bioremediation.
NS11C-0695
Temporal and spectral induced polarization contribution to ore body detection and differenciation.
The aim of this paper is to show complementarity of temporal and spectral induced polarization, applied to ore body detection and differenciation. Study had been lead into well known geological background through borings, geochemical measurements, and also through some electrical resistivity tomographies. Temporal induced polarization (TIP) material and carry out: TIP had been carried out in a quite original way by employed device, and technique. Measurements had been done with SYSCAL PRO (Iris Instrument, Ltd.) transformed into ELREC PRO: this make it possible to differenciate transmitter device (with VIP generator manufactured by IRIS) from receiver one. The main interests are (i) to avoid internal coupling effects and between transmittor/receptor cables on soil, and (ii) to obtain higher electrical power (until 3000 watt) necessary to reach 30-40m depth. Voltage measurement is done through non-polarizable electrodes. Electrical chargeability and resistivity tomographies had been obtained by lateral device displacement. Pole-dipole device had been chosen because it is the best compromise between minimizing coupling effects, getting enough power to reach wanted depth, and necessary lateral resolution. Spectral induced polarization (SIP) material and carry out : SIP FUCHS II device (manufactured by Radic Research) had been used. As the device is not configurated into a multielectrode way, and as one sounding is very time consuming (about 7 hours), only 2 soundings had been done, located on major chargeability anomalies. First results: A a first analysis, a 3 lauer model can be observed: very resistivive level between 0 and 5m depth (up to 1000 ohm.m), more conductive between 5 and 20-25m depth (50 ohm.m), and finally an increasing resistivity. A finer analysis indicates some big conductive zone 50m wide (50 ohm.m) from surface until 30m depth. This could be exlplained by clay or ore body presence. Chargeability analysis indicate us values very high chargeabilities at resistivity anomaly zone (up to 80 mV/V), whereas everywhere else these values are less than 10mV/V. Such chargeability intensity may only be explained by ore body presence. At this stage we could think that there is only one kind of ore body (nevertheless geochemical analysis indicate 2 different types). Two SIP soundings had been done vertically to chargeability anomalies. There is no difference for resistivity values, but phase diagrams are very different (maximum phase is 3 Hz for one sounding and less than 0.1 Hz for the second). Some SIP modelling is now to be done to better characterize ore body nature. As a conclusion, the measurements of electrical resistivity, of chargeabilité and PPS are very complementary (fastness for the chargeability / strong capacity of differenciation for SIP).
NS11C-0696
Building Better Electrodes for Electrical Resistivity and Induced Polarization Data
In the third year of a project to understand and mitigate the systematic noise in resistivity and induced polarization measurements, we put a significant effort into understanding and developing better electrodes. The simple metal electrodes commonly used for both transmitting and receiving of electrical geophysical data are likely the Achilles" heal of the resistivity method. Even stainless steel, a commonly used electrode material because of its durability, showed only average results in laboratory tests for electrode noise. Better results have been found with non-polarizing metal-metal salt electrodes, which are widely used as surface electrodes and in IP surveys. But although they produce small measurement errors, they are not durable enough for in-situ borehole resistivity surveys, and often contain compounds that are toxic to the environment. They are also very seldom used as transmitters. In laboratory studies, we are exploring other materials and configurations for low-noise compound electrodes that will be nontoxic, inexpensive, and durable and can be used as both transmitters and receivers. Testing of the electrical noise levels of electrodes is an arduous task involving repeated measurements under varying conditions at field scales. Thus it is important to find methods of sorting out likely candidates from the mass of possible electrode configurations and construction methods. Testing of electrode impedance versus current density appears to provide simple criteria for predicting the suitability of electrodes. The best electrodes show relatively low overall contact impedance, relatively small changes in impedance with increased current density, and relatively small changes in impedance with time. Furthermore it can be shown that resistivity and induced polarization performance of electrodes is strongly correlated, so that methods of finding electrodes with low impedance and good direct current performance usually provide better quality induced polarization data and vice- versa.
NS11C-0697
Electromagnetic Coupling in Spectral Induced Polarization Data
Spectral induced polarisation (SIP) can be applied to derive hydraulic permeabilities by using the complete spectral information, i.e., by analyzing the frequency dependence (typically in the range from 1 mHz to 10 kHz) in terms of relaxation times. The relation between the Cole Cole relaxation time and the hydraulic conductivity was deduced from laboratory measurements on sedimentary rocks. Unfortunately, this relation can not be directly used to derive hydraulic conductivities from field data because they are influenced by electromagnetic coupling. This is crucial even for small arrays and particularly for conductive environments and at high frequencies. Coupling occurs between the electric wires and the ground as well as among the wires themselves. We focus on inductive coupling of the current wires to the ground. The influence of inductive coupling will be shown. Furthermore, a method for the removal of the inductive coupling is introduced and applied for a field example.