Yervant Vichabian

Induced-polarization detection and mapping of contaminant plumes

Several laboratory and scaled model investigations suggest that organic contaminants affect the surface electrical properties of exposed soils/rocks and therefore produce measurable induced polarization IP signatures. However, there is little field evidence of an IP methodology for contaminant mapping. A2D time-domain IP method is developed for mapping the FS12 contaminant plume at the Massachusetts Military Reservation MMR located in Cape Cod, Massachusetts. The FS-12 plume consists of approximately 265 m 3 of fuel that erupted from a broken underground pipeline in the early 1970s. Benzene and ethylene dibromide EDB are the primary contaminants at FS-12, with concentrations exceeding the allowed maximum concentration levels MCL, while other constituents of the plume did not exceed their MCL. Therefore, the contaminants of interest are benzene and EDB, partly because of their health risk and partly because they present the highest concentrations 2400 and 1000 g/L, respectively among the plume constituents and are therefore more likely to be related to the polarization source. IP data were acquired along a survey line that partially transects the plume extending over contaminated and uncontaminated zones and were inverted to give 2D resistivity and chargeability plots to 100 m depth and a horizontal extent of 400 m. By separately inverting IP data derived from time windows located at short and long decay times, a timedomain gross spectral chargeability difference is produced. Both the chargeability and gross spectral chargeability difference show good agreement with the known location of the plume from monitoring wells, with the IP chargeability section suggesting contaminant distribution detail that cannot otherwise be inferred from the sparse borehole distribution.

Self potentials in cave detection

The major application of the self potential (SP) method has been in mineral exploration and in recent years increasingly in environmental and engineering investigations. The SP method simply measures a naturally occurring potential between electrodes on the surface or in boreholes. There are three mechanisms that generate self potentials: streaming potentials due to fluid flow, electrochemical potentials generated by concentration differences of electrolytes, and thermoelectric potentials from temperature gradients.

Electromagnetic cave-to-surface mapping system

This paper presents the principle, design, construction, and methodology for an electromagnetic (EM) system to be used in the detection/location and mapping of underground cavities using surface measurements. The EM instrument consists of a loop-loop transmitter/receiver system with the transmitter placed inside the cavity. The transmitters position and depth are determined by analyzing the shape and distribution of the transmitted field on the surface. From the perspective of a cylindrical coordinate system, the vertical component of the through rock transmitted magnetic field peaks at the point where the transmitter and receiver are vertically collinear. On the other hand, the horizontal component reaches a minimum at this point. Based on these observations, a procedure is presented and tested that efficiently locates the position as well as the depth of the transmitter. A physical model for the system was developed and compared to the results of calibration experiments, with very good agreement. The model allows the study of different responses for EM waves/fields propagating through a homogenous Earth of different electrical characteristics and therefore enables several type-curves to be generated that aid in the development of an optimal system.

Minimization of self-potential survey mis-ties acquired with multiple reference locations

Self-potential (SP) surveys often involve many interconnected lines of data along available roads or trails, with the ultimate goal of producing a unique map of electric potentials at each station relative to a single reference point. Multiple survey lines can be tied together by collecting data along intersecting transects and enforcing Kirchhoff’s voltage law, which requires that the total potential drop around any closed loop equals zero. In practice, however, there is often a nonzero loop-closure error caused by noisy data; traditional SP processing methods redistribute this error evenly over the measurements that form each loop. The task of distributing errors and tying lines together becomes nontrivial when many lines of data form multiple interconnected loops because the loop-closure errors are not independent, and a unique potential field cannot be determined by processing lines sequentially. We present a survey-consistent processing method that produces a unique potential field by minimizing the ...

Investigations of andesitic volcanic debris terrains: Part 2 — Geotechnical

Here, results of geotechnical investigations for the proposed Dominica international airport are presented. The main geotechnical investigative method is comprised of boreholes to 30 m depth with standard penetrating testing (SPT) and undisturbed Shelby tube and disturbed split-spoon soil sampling. The geotechnical investigation confirms the findings of a companion geophysical investigation, which concludes that bedrock is not likely to be encountered within the proposed depths of excavation for the airport. Geophysical and geotechnical data correlations are developed. Seismic velocity and the number N of blow counts with SPT (SPT N) appear to be well correlated by a linear model. A model relationship between SPT and seismic dynamic elastic modulus is developed using seismic velocity. SPT N is better correlated with the dynamic elastic modulus than with seismic velocity. The study shows that the seismic velocity and the derived dynamic elastic moduli can accurately predict soil strength as measured by SPT...

Reinterpretation of a vintage 4.5-km resistivity line through Sulphur Springs, St. Lucia

Saint Lucia is part of the Lesser Antilles volcanic island arc in the eastern Caribbean (Figure 1). Volcanic activity has been concentrated in the southern half of the island for the last several million years (Wohletz et al., 1986). The Sulphur Springs geothermal area lies within what has been identified as the Qualibou Caldera. Geologic studies have shown that recent volcanic activity in the area has been of a type that is likely to emplace a magma heat source for high temperature geothermal systems to naturally develop. Any evidence for a supposed intrusion beneath Sulphur Springs comes from temperature measurements of the superheated steam and wells, and from microseismic data (GENZL, 1992). Aspinall et al. (1976) suggest two fluid bodies, one near Sulphur Springs at a depth between 0.5 and 2 km and a second on the northern caldera rim at depths greater than 1 km.

Self Potentials In Cave Detection

been in mineral exploration and in recent years increasingly in environmental and engineering investigations. The SP method simply measures a naturally occurring potential between electrodes on the surface or in boreholes. There are three mechanisms that generate self potentials: streaming potentials due to fluid flow, electrochemical potentials generated by concentration differences of electrolytes, and thermoelectric potentials from temperature gradients. Self potential, SP, and streaming potentials will be used interchangeably as we are only interested in potentials generated from fluid flow in this study. Deformation of groundwater flow by preferential drainage paths increases water flow velocity and intensifies streaming potentials caused by filtration. In the case we are studying, the drainage paths are defined by flow into cavities. SP anomalies can be from tens to hundreds of millivolts depending on the pressure drop, lithology, and chemical composition of the water. On the boundary of solid and liquid phases, an electrically charged electrochemical layer is formed between the mineral and pore moisture. Streaming potentials are directly proportional to the potential difference between the immobile part of the electrochemical layer and the free solution. Streaming potentials decrease as the electrolyte concentration increases. Most mineral surfaces are negatively charged, hence an excess of positive ions is drawn to the surface to form the electrochemical boundary layer. The positive ions are carried in the direction of the water flow—hence areas of water inflow are characterized by positive anomalies and those of flow sources by negative ones. In a permeable and homogeneous medium, the streaming potentials show the contours of groundwater flow. The slope from a line plot of SP data is proportional to the hydraulic gradient in the direction of the survey line. The theoretical basis of the streaming potential was first worked out by Helmholtz. He proposed that streaming potentials are present when a conduction current balances the convection current caused by the preferential transport of positive ions (Figure 1). As the fluid moves under a pressure difference, P, it drags positive charge with it producing the convection current. The conduction current is simply an Ohm’s law equilibrium balancing current. At steady state Iconv = Icond, leading to the Helmholtz-Smoluchowski equation