Avalon Sciences (ASL) is a manufacture of borehole seismic instrumentation for use with both hostile deep well and shallow-hole environments. This article introduces this technology, including recent an example of its application within the characterization and monitoring of mining fracture networks.
Why Borehole Seismic?
The domain of borehole seismic exploration is fast approaching a new era in technological advancement. Today, the dynamic and challenging requirements for mapping and monitoring are continually broadening to encompass surveys within increasingly complex and hostile well environments, whilst ensuring the technology remains versatile and economic for shallower engineering and mining applications.
The deployment of seismic recording systems downhole to acquire high resolution data has become common practice over the last few decades. This technology has been dominantly used to complement surface seismic surveying bringing advantages such as noise reduction and immunity from the distorting effects of near-surface rocks, especially sediments. The well-established method of downhole Vertical Seismic Profiling (VSP) thus allows for much higher resolution imaging between lithologies proximal to the well whilst even revealing subtle impedance changes beyond total depth of the exploration well. At its most basic application, ‘check shot’ borehole seismic calibrates for any inferred depth uncertainty of reflectors presented by surface seismic, giving an enhanced velocity model around a well.
This technology also provides a unique opportunity within both O&G and mining environments for determining quality and integrity of reservoirs/fracture networks that remain below surface seismic resolution can be achieved if the seismic receiver can be located downhole proximal to the area of interest.
Strong yet Sensitive!
Borehole seismic arrays are deployed as a series of satellites on a wireline string. The current industry standard borehole seismic receiver systems in general provide a downhole pressure housing for the seismic sensor and coupling mechanism (usually in the form of a retractable arm) to the borehole casing or formation. Most modern sensor packs are three-component, allowing for full 3-D particle motion analysis of the seismic wave.
The quality of the geophone receiver plays a fundamental part in how accurately the fracture progression is mapped. Excellent sensor sensitivity is a vital characteristic when trying to pick low-amplitude microseismic arrivals. Even when deployed in relatively quiet borehole conditions, sensors featuring significant electronic noise (especially at temperature) can be enough to mask a microseismic arrival.
To improve on
this signal to noise ratio, technology has evolved to stack the phones within
the sensor pack component of a downhole receiver, with the latest technology
achieving four phones on each component, with up to 12 in each receiver
satellite.
Borehole Seismic for Mining
Figure 3 Shallow Geophone with slickline deployment
ASL has extensive experience in supplying bespoke retrievable, semi/permanent deployment of multicomponent seismic sensors for geotechnical engineering projects and mining stability studies, including fracture network analysis within mining environments. ASL’s shallow borehole solutions have gained popularity due to ease of use, low maintenance overhead being a priority whilst still delivering high performance with top quality data whilst minimizing rig time. The bespoke installation of a network of geophones is often necessary for the long term monitoring of seismic activity from block cave mining. In recent years, Avalon has worked closely with mining operators to supply and deploy permanent systems with projects involving slick-line wireline installation and then cemented permanently to a depth of up to 1.5km within 15 cased near-vertical wells located around the mine circumference.
Analog cabling was coupled to the wireline slick-line with the sensors pods located within individual pressure housings. The system produced excellent broadband data with very low back ground noise. This, combined with an uncomplicated deployment strategy employing simple but robust analog electronics, delivered a fast and cost effective seismic solution for passive seismological study within a mining environment.W
