Underground cavities detection and localization – DCOS

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).

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  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.

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  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.

• Detection and localization of several types of underground inhomogeneities: natural cavities, karsts artificial man-made cavities, under consolidated soils, break of the tabularity…
• Cavities might be dry, air filled, soft sediment filled or partially water filled.
• One DCOS analysis for each and every panel extraction.
• DCOS analysis isn’t time consuming and is performed without any numerical model inversion (no hypothesis about the geology).
• The natural or anthropic seismic noise is used to perform the DCOS analysis. This process allows working in industrial environments.
• Noise generated by a car running along the streamer can also be used.

Operations (clear space)

• Deployment of the equipment: 1h00 – 1h30 (first display), 30 min to 1h00 for the others.
• Acquisition for one display: 10 – 30 min

DCOS Analysis
Processing 1 display # 3 h – 4h

	Penetration investigated	#DCOS Analysis	Analysis length
Shallow	11-12 m	73	144 m
Deep	23-24 m	49	96 m

Land equipment

• Multi-channel streamer from 24 to 192 receivers,
• Seismic source : seismic noise (micro tremor),
• Seismic recorders (8 Geometrics GEODES),
• On-line Quality-control Software,
• Laptop computer.

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  Enregistreur et PC
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  Géophones

DCOS Software processing running on PC workstation

Studies of construction sites:

• Site qualification for rough works,
• Karsts detection (air or water filled),
• Artificial cavities detection (old mining shafts, underground buildings, tunnels, buried tanks, drums, pipelines…),
• Local under-compressed sediment detection,
• Underground anomalies cartography

Embankments study:

• Embankments evaluation,
• Anomalies localization (void, cavities, under consolidated areas …).

Linear works study:

• Evaluation of the roadways, railways, tracks and tunnels,
• Localization of voids to prevent soils subsidence

With the same data collected, a classical MASW interpretation analysis can be performed (see SismOcean’s land surface wave – MASW section).

The combination of both MASW and DCOS interpretations enables to refine some DCOS’s output: under consolidated areas, local variation of the geology.

Shallow investigation: man-made cavities

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  The depth is expressed according to the
  surface wave wavelength (depth ≈1/2 wavelength)

Shallow and deep investigation: Geotechnical correlation

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  Geophones

2D results at constant depth

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DCOS analysis:

• is performed for every extracted panel (73 for shallow investigation, 49 for deep investigation),
• spacing is equal to the receiver spacing (one analysis every two meters for example). The high density of results gives access to thin analyse for the survey.
• Isn’t time consuming and does not require numerical inversions.

DCOS analysis is well adapted for the detection and localization of any underground inhomogeneity such as: natural or man-made cavities, under-consolidated areas, local variation of the geology…

Running the DCOS analysis can be performed on any place with accessible clear surface.

In an industrial environment the method takes advantage using the ambient seismic noise (micro seismic) with an average maximum penetration about 20 m.

At this time, about 40 kilometres of studies were carried out using DCOS analysis (karst, urban site, 4 validation test sites known proposed and checked with success by French railways).