As a compact wave packet travels through a dispersive medium, it becomes dilated and distorted. As a result, ground-penetrating radar (GPR) surveys over conductive and/or lossy soils often result in poor image resolution. A dispersive migration method is presented that combines an inverse dispersion filter with frequency-domain migration. The method requires a fully characterized GPR system including the antenna response, which is a function of the local soil properties for ground-coupled antennas. The GPR system response spectrum is used to stabilize the inverse dispersion filter. Dispersive migration restores attenuated spectral components when the signal-to-noise ratio is adequate. Applying the algorithm to simulated data shows that the improved spatial resolution is significant when data are acquired with a GPR system having 120 dB or more of dynamic range, and when the medium has a loss tangent of 0.3 or more. Results also show that dispersive migration provides no significant advantage over conventional migration when the loss tangent is less than 0.3, or when using a GPR system with a small dynamic range.
The resistivity, dielectric constant, and loss tangent of natural clay permafrost samples that have never been thawed have been measured as functions of temperature, applied uniaxial confining load, and applied electric field strength. DC resistivities are on the order of 10 5 ohm-m at −10 °C, with the complex resistivity becoming strongly frequency dependent within and above the range of 10 to 10 3 Hz (resistivity decreasing with increasing frequency). Below 10 3 Hz, the electrical properties are slightly dependent upon applied electric held, and below 10 5 Hz, the electrical properties are very strongly dependent upon applied uniaxial confining load. Several different mechanisms are responsible for the observed properties, including ionic conduction, a colloidal response that is similar to a Maxwell-Wagner type of effect, the relaxation of Bjerrum defects in ice, the relaxation of the unfrozen water molecules, and a possible relaxation of organic molecules in the unfrozen water sheath surrounding clay particles.
A ground-penetrating radar survey was made on a large complex aeolian dune along the margin of Great Sand Dunes National Monument, Colorado, to delineate the internal structures formed by dune migration in a complex wind regime. Radar waves were partially reflected from sediment interfaces that had differing densities or moisture contents. In this way bounding surfaces between sets could be interpreted from changes in the attitude of sets of reflectors. The radar reflectors were recorded to depths of 15 m, but the best resolution of bounding surfaces was obtained in the upper 5 m of the dune sand. Bounding surfaces interpreted from reflectors define a main dune set 5–8m thick, with foresets up to 23 m long. Thicknesses of other wedge-shaped and tabular planar sets range from 0.75 to 1.5 m, averaging 1 m; set lengths range from 6 to 12 m, averaging 8.5 m. Trough-shaped sets range in thickness from 0.5 to 3m, averaging 1.1m, and range in width from 5 to 22 m, averaging 10 m. These trough structures may have been caused by the migration of scour pits associated with small superimposed dunes, or may be the result of scour fills formed during reversing winds. Reversing winds also formed numerous subtle bounding surfaces (reactivation surfaces) along the leading edge of the dune as it migrated, defining sets ranging in thickness from 0.5 to 2 m, averaging 1 m, and with foreset lengths ranging from 15 to 23 m, averaging 20 m. This study demonstrates the usefulness of ground-penetrating radar in resolving the internal structures of damp to dry, clay-free, aeolian dune sand. The numerous sets and bounding surfaces resolved with radar indicate a relationship between the complexity of internal structure and the multiple directions of sand-carrying or sand-scouring winds. In a tectonically active basin such as the San Luis Valley, these complex aeolian dune sand bodies have excellent preservation potential.
The electrical properties of granite appear to be dominantly controlled by the amount of free water in the granite and by temperature. Minor contributions to the electrical properties are provided by hydrostatic and lithostatic pressure, structurally bound water, oxygen fugacity, and other parameters. The effect of sulfur fugacity may be important but is experimentally unconfirmed. In addition to changing the magnitude of electrical properties, the amount and chemistry of water in granite significantly changes the temperature dependence of the electrical properties. With increasing temperature, changes in water content retain large, but lessened, effects on electrical properties. Near room temperature, a monolayer of water will decrease the electrical resistivity by an order of magnitude. Several weight‐percent water may decrease the electrical resistivity by as much as 9 orders of magnitude and decrease the thermal activation energy by a factor of 5. At elevated temperatures just below granitic melting, a few weight‐percent water may still decrease the resistivity by as much as 3 orders of magnitude and the activation energy by a factor of 2. Above the melting temperature (650° to 1100°C depending upon water pressure), a few weight‐percent water will decrease the resistivity by less than an order of magnitude and will barely change the activation energy. Remarkably, the few weight‐percent water must be present as free water. Experiments with hydrated hornblende schist (with structural water) indicate an electrical resistivity very similar to that for dry granite. The implications of these results, together with the findings of deep magnetic sounding and magnetotelluric surveys, suggest much more free water than is commonly associated with the lower crust and possibly into the upper mantle.
In the interpretation of induced polarization data, it is commonly assumed that metallic mineral polarization dominantly or solely causes the observed response. However, at low frequencies, there is a variety of active chemical processes which involve the movement or transfer of electrical charge. Measurements of electrical properties at low frequencies (such as induced polarization) observe such movement of charge and thus monitor many geochemical processes at a distance. Examples in which this has been done include oxidation‐reduction of metallic minerals such as sulfides, cation exchange on clays, and a variety of clay‐organic reactions relevant to problems in toxic waste disposal and petroleum exploration. By using both the frequency dependence and nonlinear character of the complex resistivity spectrum, these reactions may be distinguished from each other and from barren or reactionless materials.
A headline in The Rocky Mountain News on 2 January 2006 read “Avalanche kills 2 snowmobilers.” A full-page color photo accompanied the article which, unfortunately, was not describing an uncommon event. Snow slides or avalanches kill between 150 and 200 people every year around the world, and many more are injured. Combine an average of 2300 snow avalanches in the Colorado mountains every winter with increasingly popular winter tourism, and it is no surprise that Colorado has more avalanche fatalities than the next two states (Alaska and Washington) combined. Avalanches not only bury people, but they close roads (sometimes with snow more than 50 ft deep for 1000 ft along the road) for days at a time, and they damage facilities (such as the 2003 avalanche that destroyed a water treatment plant and closed a major interstate highway for several days). During the 2004–2005 winter in Colorado, 2985 avalanches buried nearly six miles of highway; 76 people were reportedly caught in the snow slides, resulting in ...
Introduction The electrical resistivity of dilute aqueous salt solutions has been studied for a number of years, but very few data exist on concentrations above 0.1 molar. Normal groundwaters commonly are near 0.1 molar, while most geothermal and oilfield fluids are at least several molar (Table 1). Thus, the interpretation of electrical measurements in geothermal areas at present is based mainly on extrapolation of lower temperature and lower concentration data. Such extrapolation may introduce serious errors into the interpretation of geothermal reservoir characteristics determined from electrical measurements. This paper presents new experimental data and an improved descriptive model of the electrical properties of brines as a function of temperature properties of brines as a function of temperature from 22 to 375 deg. C and concentration from roughly 3 to 26 wt% while under 31 MPa hydrostatic pressure. Data and models are given for brines composed of the chlorides of sodium, calcium, and potassium, and their mixtures. Comparison of the older log interpretation formulas to the new models illustrates an order of magnitude improvement in accuracy with an overall fit to within 2%. Resistivity Dependence Upon Temperature Some researchers have postulated that the electrical resistivity of fluid saturated rocks follows the temperature dependence of the saturating fluid in the absence of conducting minerals or significant surface conduction along altered pore walls. This assumption resulted from the success of a simple empirical formula relating the resistivity of a rock to the resistivity of the fluid filling the pores of the rock: Pr=FPw, where Pr= resistivity of clay-free, nonshale material that is 100% saturated, Pw= resistivity of saturating solution, and F= formation resistivity factor. A number of investigators have derived formulas that add the temperature of the saturating fluids. Experimental observations have shown that some rocks obey these formulas while others do not. 17–19 Part of the problem is the inadequate knowledge of Part of the problem is the inadequate knowledge of the resistivity dependence on temperature for the solution that fills the rock pores. We have found empirically that the best fit of the resistivity data to temperature is pw=bo+b1T-1 +b2T+b3T2+b4T3, where T is temperature and coefficients b are found empirically.
Ground‐penetrating radar (GPR) has the potential to image the Martian subsurface to give geological context to drilling targets, investigate stratigraphy, and locate subsurface water. GPR depth of penetration depends strongly on the electromagnetic (EM) properties (complex dielectric permittivity, complex magnetic permeability, and DC resistivity) of the subsurface. These EM properties in turn depend on the mineralogical composition of the subsurface and are sensitive to temperature. In this study, the EM properties of Martian analog samples were measured versus frequency (1 MHz‐1 GHz) and at Martian temperatures (180–300 K). Results from the study found the following: gray hematite has a large temperature‐dependent dielectric relaxation, magnetite has a temperature‐independent magnetic relaxation, and JSC Mars‐1 has a broad temperature‐dependent dielectric relaxation most likely caused by absorbed water. Two orbital radars, MARSIS and SHARAD, are currently investigating the subsurface of Mars. On the basis of the results of our measurements, the attenuation rate of gray hematite is 0.03 and 0.9 dB/m, magnetite is 0.04 and 1.1 dB/m, and JSC Mars‐1 is 0.015 and 0.09 dB/m at MARSIS and SHARAD frequencies, respectively, and at the average Martian temperature of 213 K. With respect to using GPR for subsurface investigation on Mars, absorbed water will be a larger attenuator of radar energy as high concentrations of magnetite and gray hematite are not found globally on Mars.
