The portable seismometer sits on weathered limestone and clay, recording ambient vibrations for about 40 minutes per station. That brief window captures enough microtremor data to define the fundamental resonance frequency of the soil column beneath an Austin site. Unlike active seismic methods that require a hammer or weight drop, the HVSR (horizontal-to-vertical spectral ratio) technique relies on natural background noise — wind, traffic, distant machinery. The field team deploys the instrument in a grid pattern, covering the project footprint systematically. Each measurement point yields a spectral ratio curve that reveals the impedance contrast between softer overburden and the underlying bedrock. For projects along the Balcones Fault Zone, this passive approach provides a non-invasive first pass at site classification without drilling. Before committing to borehole locations, the engineer can review these profiles to target critical zones, then follow up with a MASW Vs30 survey for shear-wave velocity confirmation.
The HVSR microtremor survey identifies the fundamental resonance frequency where the site will amplify seismic motion — critical data for Austin's variable soil-over-limestone geology.