Distorting effects of the near-surface layer on seismic imaging - Physical model investigations

The 2D ultrasonic model facility at ETH Zurich was used to investigate how a variable near-surface weathered layer can adversely affect the imaging of reflection data. The simulated geological structures are constructed from 2 mm thick metal and plastic sheets, cut and bonded together into various shapes. The experiments entailed the use of a piezoelectric source driven by a pulse amplifier at ultrasonic frequencies to generate Lamb waves in the plate, which are detected by piezoelectric receivers and recorded digitally on a National Instruments recording system, under SignalExpress software control. High quality reflection seismic data were collected. However, the pre-stack migrated image for the deeper reflectors deteriorates as the near-surface becomes more complex and as block anomalies are inserted within the layers. Crosshole and VSP seismic data were also acquired, and first arrival travel-time tomography used to reliably reconstruct the velocity distribution. The 2D physical scale model system provides a valuable complement to numerical modeling to study various aspects of wave propagation and subsurface imaging.