3D DAS VSP for Coal Seam Exploration: A Case Study from Queensland, Australia

Abstract

Seismic methods are extensively used in coal mining for expanding resource discoveries and definition as well as for mine monitoring. However, the use of borehole seismic methods is relatively uncommon due to the high cost of borehole seismic acquisition using conventional downhole tools. The introduction of distributed acoustic sensing (DAS), which uses optical fibres to record seismic data, has dramatically increased the cost-effectiveness of borehole seismic methods. Fibre-optic cables are inexpensive and, once deployed in a borehole, can be abandoned or used later for further monitoring of the subsurface. The case study presented here concerns the use of DAS to record a 3D VSP (vertical seismic profiling) for coal seam exploration in Queensland, Australia. This study trialled effective strategies for deploying cables into boreholes and demonstrated how this technology could be incorporated into the standard coal exploration process. The final processing results produced a high-resolution 3D seismic cube where the coal seams below the basalt cover are clearly identifiable around the boreholes. Permanent installation of the fibre-optic cables into a set of boreholes provides immediate benefits of 3D seismic imaging and can create additional value in utilising these sensors for further discrete or continuous subsurface measurements, including stability monitoring of underground workings and detection of methane accumulations.

Keywords: 3D VSP; DAS; borehole; reflectivity method.

Figure 1

Acquisition map. Red dots show the locations of the boreholes. Green shot points were recorded with fibre optics in Boreholes 2 (east) and 3 (west) (connected together); blue shot points were recorded with fibre optics in Borehole 1 (south).

Figure 2

Installation of the fibre optic cable on a poly pipe to Borehole 2. The polyethylene pipe is attached to a grouting truck that was used to cement the borehole through the pipe.

Figure 3

(a) Schematic picture of the fibre optics installation in the bores; (b) The wellhead of Borehole 3 with a coiled excess of the fibre optic cable; (c) The recording vehicle housing the fibre optic interrogator and a vibroseis truck during the survey acquisition at Borehole 2.

Figure 4

Log/log plot of the estimated relationship between P-wave velocity and density based on the log data from the bottom of the holes. The constant velocity datapoints correspond to the velocity of the steel casing, as indicated by the dark colour corresponding to the shallow depths. The datapoints were fitted by two curves (solid and dashed lines in the log/log plot) for the different density ranges, since it is clear that one line would not produce a good fit.

Figure 5

P-wave velocity computed from the density log compared to the logged sonic velocity in Borehole 1.

Figure 6

Examples of field raw and synthetic seismograms: (a) raw common source gathers at 30, 180 and 300 m offset from Borehole 2; (b) respective synthetic common source gathers at 30, 180 and 300 m. Red dashed lines indicate primary reflections; yellow dashed lines indicate multiple reflections.

Figure 7

(a) The synthetic common receiver gathers at 200 m depth for the source line 5352. The yellow dashed lines indicate multiples; (b) a section of the migrated 3D synthetic volume. The artefacts that originated from multiples are highlighted by red dashed polygons; (c) P-wave velocity computed from the density log.

Figure 8

Examples of raw DAS data. Raw seismograms recorded with the continuous fibre-optic connecting Boreholes 2 and 3 to their respective nearest offset: (a) shot point nearest to Borehole 2 (~30 m); (b) shot point nearest to Borehole 3 (~30 m).

Figure 9

Example of the application of a model-based static correction for both boreholes at 200 m depth. Source line 5384. (a) Data from Borehole 2 before and (b) after static was applied; (c) Data from Borehole 3 before and (d) after static was applied.

Figure 10

Example of the wavefield separation for Borehole 2 data (left panels show data after wavefield separation) and application of non-stationary predictive deconvolution. Right panel spectra before (blue) and after (orange) deconvolution.

Figure 11

Fold maps at the target interval at the deepest coal seam of 380 m for the source point range selected for migration: (a) for Borehole 3; (b) for Borehole 2. Coordinate grids are relative and independent for each map.

Figure 12

The 3D DAS VSP migrated volumes for Borehole 2 and Borehole 3. The density log is overlayed on the Borehole 2 seismic volume.

Figure 13

The 2D transect across the two boreholes. Density log is overlayed on the Borehole 2 seismic volume.