Seismic imaging of sandbox experiments – laboratory hardware setup and first reflection seismic sections

摘要

With the study and technical development introduced here, we combineanalogue sandbox simulation techniques with seismic physical modelling ofsandbox models. For that purpose, we designed and developed a newmini-seismic facility for laboratory use, comprising a seismic tank, aPC-driven control unit, a positioning system, and piezoelectric transducersused here for the first time in an array mode. To assess the possibilities andlimits of seismic imaging of small-scale structures in sandbox models,different geometry setups were tested in the first 2-D experiments that alsotested the proper functioning of the device and studied the seismo-elasticproperties of the granular media used. Simple two-layer models of differentmaterials and layer thicknesses as well as a more complex model comprisingchannels and shear zones were tested using different acquisition geometriesand signal properties. We suggest using well sorted and well rounded grainswith little surface roughness (glass beads). Source receiver-offsets lessthan 14 cm for imaging structures as small as 2.0–1.5 mm size have provenfeasible. This is the best compromise between wide beam and high energyoutput, and is applicable with a consistent waveform. Resolution of theinterfaces of layers of granular materials depends on the interfacepreparation rather than on the material itself. Flat grading of interfacesand powder coverage yields the clearest interface reflections. Finally,sandbox seismic sections provide images of high quality showing constantthickness layers as well as predefined channel structures and indications ofthe fault traces from shear zones. Since these were artificially introducedin our test models, they can be regarded as zones of disturbance rather thantectonic shear zones characterized by decompaction. The multiple-offsetsurveying introduced here, improves the quality with respect to S / N ratio andsource signature even more; the maximum depth penetration in glass-beadlayers thereby amounts to 5 cm. Thus, the presented mini-seismic device isalready able to resolve structures within simple models of saturated porousmedia, so that multiple-offset seismic imaging of shallow sandbox models,that are structurally evolving, is generally feasible.