HR: 0800h
AN: H51E-03    [Abstracts]
TI: Characterization of radar (GPR) signatures and physical properties of a near-surface granular stratigraphy in the Valley of Queretaro, Mexico
AU: * Cerca, M
EM: mcerca@geociencias.unam.mx
AF: Centro de Geociencias, UNAM, UNAM campus Juriquilla, Queretaro, QRO 76230, Mexico
AU: Carreon-Freyre, D
EM: freyre@geociencias.unam.mx
AF: Centro de Geociencias, UNAM, UNAM campus Juriquilla, Queretaro, QRO 76230, Mexico
AB: Coupled use of GPR surveying with three different frequencies (300, 900, and 2000 MHz) and physical attributes determinations in a granular near-surface sequence gives insights into the effects of water content and type to the velocity of EM wave propagation (permittivity). Using known depth reflectors in the estimation of velocities for each frequency allowed obtaining vertical differences in velocity. Characteristic radar signatures were obtained for each of the 5 layers composing the horizontally stratified near-surface sequence studied. The sequence is composed of a cover of organic (0 - 0.5 m) and carbonated (0.5 - 1.0 m) soils over a pyroclastic (1 - 4.5 m), sandy gravel (4.5 - 5.6 m), silty sand (5.6 - 7.5 m), and welded tuff (7.5 - ? m). Selected physical attributes measured in detail each 15 cm of the sequence included grain size, relative strength, bulk dry density, water content (gravimetric and volumetric), specific density, and electrical conductivity. The correlation of physical properties with radargrams show the potential of GPR for characterizing layers with a similar set of interrelated physical attributes. However, the analysis of radar signatures assumes a certain degree of homogeneity in the attributes of a layer and the record of certain attributes depends, at least partially, of the frequency. For instance, small voids at the base of the carbonate soil were recorded as air wave hyperboles only with the 900 MHz frequency. Since no significant water content contrasts are observed in the sequence, the difference in the estimation of velocities of propagation from water content measurements using the Topp equation and the velocities obtained with the known reflector method can be associated to the contribution of dielectric losses. Thus, any model representing the bulk permittivity of clay bearing sequences should consider the effects of conductive losses. In our case, these effects can be related to the presence of different clay materials that have an effect in the type of water. Furthermore, bulk velocities of propagation in a simple layered near-surface sequence may be strongly frequency dependent. Valuable information can be obtained if the depositional history of the sequence and its physical attributes are determined experimentally and analyzed in detail.
DE: 1822 Geomechanics
DE: 1895 Instruments and techniques: monitoring
SC: Hydrology [H]
MN: 2007 Joint Assembly