Determinación de heterogeneidad y anisotropía en aluvión por micro-tomografía sísmica de cross-hole

in order to analyze the seismic behavior of soil samples at laboratory scale, it was developed and implemented a tomographic system based on wave transmission on the acoustic range in bi-dimensional domains with limited spatial coverage (cross-hole), to determine so different properties of granular materials in general, as anomaly detection, as well. the goal was to detect, from the measurement of compressional waves, variations in the degree of soil compaction, seeking for the determination of heterogeneity and anisotropy. for the tests, 14 general purpose 6khz resonant frequency piezo crystals were used; 7 as sources and 7 as receivers of mechanical waves in the acoustic range, mounting them on opposite sides of a rectangular rigid frame of soft wood. the whole set of frame-sensors was placed inside a cylindrical-shaped metallic container filled later with classified river sand. the emission was generated by a pulser with rectangular pulses of up to 400v amplitude and a 10ms width and 10ms repetition applied directly on the transducers. the receiver consisted of a two-channel low noise amplifier with high-pass filters of a 100hz at the input, with adjustable gains ranging from approximately 40 to 80 db. the system was connected to a tektronix dso tds 210 with a rs232 interface for storing data on a pc. the results made it possible to detect both vertical heterogeneity and anisotropy in the material due to the weight of the overlying layers that produced an increase in material density and; therefore, an increase in the speed and curvature of the rays. with the data obtained, it could be established a law of increasing speed as a function of depth.