Maksim Bano

Electrical structure of the Himalaya of central Nepal: High conductivity around the mid-crustal ramp along the MHT

Twelve broadband magnetotelluric (MT) soundings were performed across the Himalaya of Central Nepal in 1996 in order to determine the electrical structure of the crust and its relation to geological structures and active tectonics. The MT impedance tensors were obtained for frequencies between 0.001 and 500 Hz. The 2‐D section, derived from joint inversion of TE‐ and TM mode after RRI and Groom/Bailey decomposition, shows high conductivity in the foreland basin (∼30 Ω.m) that contrasts with the resistive Indian basement (>300 Ω.m) and Lesser Himalaya (>1000 Ω.m). In addition, our MT sounding reveals a major conductive feature beneath the front of the Higher Himalaya, also characterized by intense microseismic activity, and the position of a mid‐crustal ramp along the major active thrust fault (MHT). This high conductivity zone probably reflects metamorphic fluids, released during underthrusting of the Indian basement and pervading well connected microcracks induced by interseismic stress build‐up, or distributed brittle deformation around the ramp.

Ground Penetrating Radar Measurements in a Controlled Vadose Zone: Influence of the Water Content

Ground penetrating radar (GPR) is a nondestructive method, which, as with other geophysical methods, has been successfully used to estimate the water content or hydraulic properties of soils. We performed GPR measurements to calibrate and compare water content estimates with actual water contents in a sand box. A vadose zone was simulated by injecting water in a sand box. We obtained four GPR data sets: for dry sand, for sand with water tables at 72- and 48-cm depths, and for sand after drainage. Using the reflections (or diffractions) from the bottom of the sand box (or objects buried in the sand), mean relative dielectric permittivities were determined at several depths in the sand box. These relative dielectric permittivities were used to calculate “real” mean relative dielectric permittivities of a sand box made up of three layers (dry sand, unsaturated sand, and fully saturated sand), knowing that a layer can be subdivided into more layers depending on the depth of the reflections (or diffractions) recorded. We used three relationships between relative dielectric permittivity and the water content to estimate the mean water content for each layer. From these water contents and the known volume of sand considered, we estimated the amount of water in the sand box for each water table. Subtracting the volume obtained for dry sand from the volume obtained for the different water tables gave estimates of the variations in water quantities in the sand box; these were compared with the quantities injected in the sand box. Despite uncertainties in the determination of the mean relative dielectric permittivities, the calculated variations in water quantities were very similar to those injected in the sand box.

Influence of grain size, shape and compaction on georadar waves: examples of aeolian dunes

SUMMARY Many ground penetrating radar (GPR) profiles acquired in dry aeolian environment have shown good reflectivity inside present-day dunes. We show that the origin of this reflectivity is related to changes in grain size distribution, packing and/or grain shape in a sandy material. We integrate these three parameters into analytical models for bulk permittivity in order to predict the reflections and the velocity of GPR waves. We consider two GPR cross-sections acquired over aeolian dunes in the Chadian desert. The 2D migration of GPR data suggests that dunes contain different kinds of bounding surfaces. We discuss and model three kinds of reflections using reasonable geological hypothesis about aeolian sedimentation processes. The propagation and the reflection of radar waves are calculated using the 1D wavelet modelling method in spectral domain. The results of the forward modelling are in good accordance with real observed data.

Radar reflections and water content estimation of aeolian sand dune

Many Ground-penetrating radar (GPR) profiles acquired on the dry aeolian environment have shown a strong reflectivity inside the present-day dunes. Changes in water content are believed to be the origin of this reflectivity. In this paper we try to model the radar reflections coming from the bottom of a dry aeolian dune by using the 1D wavelet modeling method. The principle of this method is to calculate the effects of the propagation (absorption, dispersion and geometrical spreading) and reflection on a reference wavelet for different values of various parameters, and the best fit between the observed and calculated data leads to the optimum set of parameters. By combining the wavelet modeling method with a semi-empirical mixing formula for the bulk permittivity of moist sandy soils we estimate the water content and explain the importance of this latter parameter on reflection coefficient and on wavelet modeling.

Modeling and inverse Q imaging of ground penetrating radar waves in 1 and 2d

As in the case of seismic attenuation, dielectric loss or radar attenuation may be related to the quality factor Q which is given by the ratio of the real to imaginary parts of effective (or total) dielectric permittivity. The frequency dependence of both parts of the effective permittivity would imply the broadening (absorption) and distortion (dispersion) of the radar pulse when it travels into the earth, so the radar pulse is not stationary. Therefore the classical deconvolution and migration methods, used currently in seismic processing, do not always work well in the case of GPR data. In order to compensate for radar attenuation (absorption and dispersion) two methods (in ID and 2D) are proposed. These methods are based on the weighted downward extrapolation of the electric field in the frequency domain. The results are tested and illustrated using synthetic and real radargrams in 1 and 2D.