Integrated MVG and ERT Survey Over a Shallow Cave
An integrated geophysical MVG (Microgravity Vertical Gradient) and ERT (Electrical Resistivity Tomography) survey was performed over a shallow cave in the Armetta Mountain karst area, close to the Liguria-Piedmont watershed (Tanaro valley). The aim of this study is to test the response of a known shallow karst cave. The cave was developed in the Mesozoic sedimentary cover (dolostones and limestones - CAU : Caprauna Armetta Unit); the shallowest portion of the cave exhibits narrow passages and, at about 30 m below the entrance, a fossil meander which links two large chambers, that represent the target of the geophysical survey. The site was surveyed with five, 235 m long, ERT sections, crosscutting the underground voids; 53 MVG stations was collected in an area of 150 by 50 meters above the cave. The ERT prospecting was performed using a Syscal R1 (Iris Instrument) multielectrode system with sets of 48 electrodes evenly spaced each 5 meters. A Wenner-Schlumberger array was used. The MVG survey was performed using a LaCoste&Romberg D model gravity meters equipped with a digital data acquisition through the Aliod feedback system, GPS tracking and automatic tide corrections, with a nominal resolution of 1 microGal (10 nm/s2). Each MVG station was achieved by two successive readings performed at the bottom and top of a 1.80 meters high tower frame. Differential GPS, with acquisition rate of 1 Hz and sampling time of 120 s, was used for the geographic position. The results of the integrated survey show a clear geophysical response of the hollow which exhibits high resistivity values and a negative gravity anomaly. This paper confirms the effectiveness of the geophysical exploration even in a quite complex environment giving a precise location of the voids. The application of such techniques can be efficient in site surveying where the conceivable presence of hollows may be expected.
A Study of Electrical Resistivity Methods in Monitoring Moisture Fluxes in the Vadose Soil Zone
The nature of dynamic interactions between vegetation and soil moisture remain largely unanswered because it is difficult to monitor and quantify subsurface hydrologic fluxes at relevant scales. Improved methods for monitoring such changes accurately would benefit studies of the impacts of land use and climate change on the hydrologic cycle. Recent studies suggest that electrical resistivity can be used to characterize these processes at different scales. The objective of this study is to test the sensitivities of different 1D inversion approaches to quantify changes in the unsaturated zone related to short-duration rainwater infiltration and seasonal drying. The rainwater infiltration model simulates the downward movement and diffusion of an infiltration pulse in a sandy soil column in daily time steps. The seasonal drying model simulates weekly vadose zone moisture changes for two different vegetation types in the growing season as a result of climatic inputs and root-moisture uptake. Our approach is to first calculate apparent resistivity sounding data from the modeled 1D unsaturated-zone moisture data. We then invert the apparent resistivity data using the minimum number of layers and using a multi-layer approach commonly applied in inversion of 2D resistivity data. We test the sensitivity of the inversion results and soil moisture estimates to the number of layers and their thickness. Also, we analyze the effect of the number of electrode pairs and configuration on the accuracy of the calculation of the apparent resistivity.
Extent of the Ice Lens of the Gardner Pond Rock Glacier, Maine, from Electrical and GPR Surveys
This paper analyzes the presence and extent of the ice lens of the Gardner Pond rock glacier, Maine, by measuring the contrast between rock and underlying ice properties through the use of resistivity and Ground Penetrating Radar (GPR). We acquired two resistivity profiles using Wenner geometry and 3 to 4 m spike spacing: the first along the base of the slope, running west to east, and the second perpendicular to this baseline, running north to south downwards the slope. Using a 100MhZ GPR antenna we collected three profiles: one coincides with the first resistivity line and two run from the top of the slope downward towards the lake. We determined positions and elevations of the resistivity and GPR profiles using total station and Global Positioning System (GPS) measurements. The resistivity results show a contrast in materials beneath the surface. Both the first and the second resistivity line indicate a high contrast of materials in the center of the slope and at the shoreline. The GPR results show a prominent subsurface reflector which appears in the southwest down-slope profile. Though its source is enigmatic, it might mark the top of an ice lens. Based on our results, we strongly suspect the presence of the ice lens of the Gardiner Pond rock glacier, whose extent remains to be inferred from further analysis.
Simultaneous determination of shallow water-table depths and EM wave velocities using multiple-offset GPR profiling data in peatland
Bibai bog in Hokkaido is the only high moor peatland left in the Ishikari peatlands (60,000ha). Almost all bogs except Bibai bog have been reclaimed for farmlands, housing lands, etc. in last century. The total area of Bibai bog is now about 50 ha. The remaining bog is suffering from drying and ground subsidence by the drainage of water. To propose an appropriate management for Bibai bog preservation, deep understanding of the temporal and spatial behaviors of water table is necessary. To obtain water-table depths along a survey line, we used ground-penetrating radar (GPR). In many applications, GPR data are collected with the common-offset profiling (CO), in which the separation distance between a transmitter and a receiver is constant. However, it gives only travel time of reflected EM waves from an unknown reflector. To calculate the velocity of the EM wave, the common-midpoint (CMP) method, a multi- offset acquisition method, is also used. In this study, we collected CO profiling data with different antenna separations along a common survey line in Bibai bog. This approach allows us to directly estimate the EM wave velocity at all measurement locations without performing a common multi-offset acquisition method, such as the CMP method. In this survey, 7 CO data with 7 different antenna separations varying from 60 cm to 120 cm were collected along survey lines in and around the bog. As a result, we could calculate the average EM wave velocity and gain the velocity distribution every 5 cm along the survey lines. In addition, we could determine the shallow water-table depths directly by using multiple offset GPR profiling data.
Roughness of a Layered Geological Media and the Implications for Interpretation of Transient Electromagnetic Response of a Loop Source
Data from controlled-source electromagnetic studies in various geological environments were analyzed as layered models and compared to models with identical thicknesses and a roughness parameter β assigned to the layers to accommodate multiple length scales. For classical diffusion of current, β=1. Values of β not equal to 1 describe anomalous diffusion. Hydraulic conductivity models assume that geological media is inherently rough. The correlation between equations for hydrological diffusion and current diffusion suggests that roughness is appropriate for electromagnetic studies. Plots of zero-crossing time v. loop-source TX-RX separation distance for a half-space have a slope of 1. Traditional layered smooth models have non-linear curves. Data from sites that produce a nearly linear slope not equal to 1 require a complicated classical model. When treated as a case of anomalous diffusion, a model where layers have appropriate β values fits. The parameter β significantly reduces the model complexity, suggesting that anomalous diffusion may be appropriate. Furthermore, late time response curves for classical diffusion have a slope of 5/2. For anomalous diffusion, this slope decreases as β increases. The differences in plots are apparent in the field and can enhance our understanding of subsurface material properties.
Influence of a source line position on results of EM observations applied to the diagnostics of underground heating system pipelines in urban area
The condition of underground constructions, communication and supply systems in the cities has to be periodically monitored and controlled in order to prevent their breakage, which can result in serious accident, especially in urban area. The most risk of damage have the underground construction made of steal such as pipelines widely used for water, gas and heat supply. To ensure the pipeline survivability it is necessary to carry out the operative and inexpensive control of pipelines condition. Induced electromagnetic methods of geophysics can be applied to provide such diagnostics. The highly developed surface in urbane area is one of cause hampering the realization of electromagnetic methods of diagnostics. The main problem is in finding of an appropriate place for the source line and electrodes on a limited surface area and their optimal position relative to the observation path to minimize their influence on observed data. Author made a number of experiments of an underground heating system pipeline diagnostics using different position of the source line and electrodes. The experiments were made on a 200 meters section over 2 meters deep pipeline. The admissible length of the source line and angle between the source line and the observation path were determined. The minimal length of the source line for the experiment conditions and accuracy made 30 meters, the maximum admissible angle departure from the perpendicular position made 30 degrees. The work was undertaken in cooperation with diagnostics company DIsSO, Saint-Petersburg, Russia.
3D modelling of randomly distributed media to investigate EM response of resistive heterogeneous targets
Marine controlled source electromagnetic (CSEM) methods are capable of detecting resistivity anomalies at shallow depths below the seafloor, an environment that has proved challenging for conventional geophysical exploration techniques. While in many cases a uniform resistive anomaly within a known background may be an appropriate modeling approximation, there are situations in which the specific heterogeneity of the target is of interest. For example, coring results show that resistive marine gas hydrate deposits are highly heterogeneous. Monitoring such resistive anomalies for the purpose of studying their time-evolution requires modelling the random nature of these heterogeneities. Using the finite difference 3D EM time-domain code of Druskin and Knizhnerman, we investigate the response of a multi-receiver marine CSEM system to synthetic 3D random media models. The CSEM system is an electric dipole source with an inline array of receivers at multiple offsets. The model is a lattice of cubic elements with different conductivities. A random walk algorithm is used to distribute resistive elements within this lattice. Owing to the diffusive nature of electric field propagation, spatial resolution of the target is limited. We use the arrival time of the electric field at the receivers to characterize the response of various random models. Off- structure (far offset) receivers are sensitive to the overall average resistivity of the model and less sensitive to the details of the distribution of resistive elements. On-structure receivers on the other hand, respond to the statistical distribution of resistive material such as the 'connectivity' of resistive elements. Using random models with different element sizes, we determine the lateral and vertical resolution limit of the instrument array for the target under study.
PERMEABILITY AND FREQUENCY DEPENDENCE OF THE SEISMOELECTRIC EFFECT ASSOCIATED WITH COMPRESSIONAL WAVES IN UNCONSOLIDATED SEDIMENTS
The propagation of seismic waves through saturated poroelastic media can generate electric fields, known as seismoelectric effects, as a consequence of electrokinetic coupling associated with movement of pore fluid relative to solid matrix. There has, as a result, been a longstanding interest in using the seismoelectric effect to estimate permeability. Indeed, recent analyses and borehole logging experiments indicate that the amplitudes of seismoelectric signals induced by Stoneley waves may be used to estimate permeability. Curiously, however, theoretical models for the co-seismic electric fields that travel with compressional waves in homogeneous poroelastic media indicate that there is no explicit dependence on permeability at low frequencies. Rather, it would appear that the estimation of permeability from such co-seismic seismoelectric effects may require broadband measurements and the identification of a critical frequency above which the frequency dependence changes. Theoretical seismoelectric transfer functions for the case of compressional waves, and equations for the above-mentioned critical frequency (expressed in terms of permeability and other physical properties) have been derived independently by at least two groups. These equations have not been fully reconciled with each other and have not yet been extensively tested by comparison to field or lab measurements. We have analysed the frequency response of co-seismic seismoelectric effects induced by compressional seismic waves during vertical seismoelectric profiling experiments in a borehole penetrating glaciofluvial sediments. The measurements were made using electrodes and a clamped geophone in the borehole, and a special seismic source at surface that produced pulses with a bandwidth of several hundred Hz. Measurements within a sand layer confirmed that seismoelectric amplitudes scaled approximately linearly with the product of frequency and seismic particle velocity as predicted by theoretical models in the low frequency regime. We consider the range of frequencies that would be required (according to models) to identify the critical frequency needed for estimation of permeability in a variety of porous sediments. Future borehole experiments are planned to try to confirm the existence of the low frequency and high frequency regimes predicted by theory and to assess the feasibility of making in-situ permeability estimates.
Effects of Pressure on Attenuation of Seismic Waves Through Sedimentary Rocks
The research focuses on the effects of pressure on the Attenuation of Seismic waves propagating three different rock samples from the upper crust. The frequency used ranges from 1Hz to 1000Hz while the pressure was varied from atmospheric to 27,960 Nm -1. The research was carried out in Year 1 Laboratory of the Department of Physics of the Lagos State University Ojo, Nigeria. The Continuous wave Transmission and spectral Amplitude wave ratio technique was employed to determine the Attenuation Coefficient K for each rock sample. Attenuation Coefficients were plotted against frequency on scatter diagram and bar charts were constructed. Result shows sandstone attenuated most of the three rock types used.
Locating the Toronto Transmit Subway Tunnel below Queens Park in downtown Toronto
The principle objective of this project was to trace the location and depth of the Toronto Transit Commission subway tunnel running below Queen's Park between Museum Station and the Legislature. A Proton Precession Magnetometer equipped with a GPS system was used to obtain 3,200 data points of total field strength along with their precise location. The resulting map shows a clear magnetic high running North South. The anomaly has a value of 10,000 nT on the North Side of the Park and weakens to 7,000 nT on the South side. We attribute this change to an increase in the depth of the tunnel. We compared our results to synthetic data for a tunnel modelled as a two-dimensional body with a distorted octagonal cross-section about 10 m wide and 3 m thick. Our field data is consistent with a model for a tunnel that is 8m deep in the North, and increases to a depth of 10 m in the South. For ongoing analysis we would want to test whether our data can resolve two subway tunnels running parallel to each other.
Seismic Noise in the Earth Science Building at the University of Toronto
The Earth Science Building at the University of Toronto, Downtown Campus, has architecturally separable sections, including the South Wing (which houses the Geology Department) and the adjacent Library Wing. To compare the seismic noise in these two sections, we conducted a passive seismic survey. Two seismic lines, one in the basement and one on the second floor, each with eight triaxial 14 Hz geophones, were set up such that four geophones were located in each wing. Eight-second recordings of ground motion were collected. Their power spectra revealed that the Library Wing contained a distinct maximum peak amplitude at 30 Hz that was not present in the South Wing. Moreover, the amplitudes were greatest on the second floor in the Library Wing of the building. We attributed these differences to the presence of unique features of the Library Wing: more people, and a rounded structure that may have a specific resonance frequency and may trap seismic energy. The peak amplitudes were hypothesized to be caused by the air conditioning in the building.