Passive Seismic Data on Campus (ANT)
Objective
Ambient noise tomography is a seismic imaging technique that uses continuous background noise to probe the Earth's shallow structure. This method enables the study of surface wave propagation in urban environments where traditional active surveys are impractical. Using wireless geophones deployed across the IISER Pune campus, we recorded passive seismic data to map the local subsurface velocity structure.
Methodology
The data processing pipeline involves three main phases. First, we remove the instrument response, mean, and trend from the raw data, apply a band-pass filter, and cut it into 1-minute segments. We then apply time-domain normalization and spectral whitening.
In the second phase, we compute cross-correlations and stack these day-correlations over the desired number of days. Finally, we measure the group and phase velocity from the resulting dispersion curves.
Observations & Results
When analyzing the energy distribution by azimuth, we found a clear concentration of energy directed towards the two main roads bordering the campus, indicating that the majority of the background noise is generated by ongoing traffic.
Comparing our small dataset with legacy Deccan data (where stacking was done for over 1.5 years on broadband stations 80km apart), we observed distinct trends in our cross-correlations. This indicates that using wireless geophones is highly feasible for local tomography, even with a relatively small dataset.
Future Work
The next step is to improve the resolution of the dispersion curves, either through better pre-processing or by isolating different frequency ranges. Ultimately, the goal is to invert these curves to generate a complete 2D subsurface image of the surface velocities beneath the campus.