The DCOS method is based on a statistical analysis of the surface wave energy distribution and does not need any numerical model inversion (there is no hypothesis about the geology of the ground).
The ambient seismic (micro tremor), natural, or produced by the human activity (road, industries …) noise is used to detect the underground anomalies. Dedicated QC software is used to visualize the energy distribution (frequency versus phase velocity) of the surface waves contained in the micro tremor.
A streamer composed of 96 equally spaced geophones is deployed. The DCOS processing is applied to 24 or 48 receivers using a sliding window to extract each group of traces (panels).
Panel extraction using a sliding window
The energy distribution is computed in both directions for each panel: direct (from #1 to #24) and reverse. The energy distribution is analyzed by a statistical approach using half of the geophones from the extracted panel.
When there is no anomaly, the energy received by the two parts of the antenna is similar. On the contrary, the presence of an underground inhomogeneity will modify the energy distribution.
Energy distribution modified by underground anomaly
The windows’ length for the DCOS analysis is adapted depending to the penetration required:
- 24 channels for shallow penetration (up to 11-12 m),
- 48 channels for deep penetration (up to 23-24 m).
For one display composed of 96 receivers the trace extraction gives:
- 73 DCOS analyses using 24 channels,
- 49 DCOS analyses using 48 channels.
The distance between each DCOS analysis is the same as the space between receivers.