New tool predicts geological movement and the flow of groundwater in old coalfields

Date:
November 16, 2020
Source:
University of Nottingham
Summary:
A remote monitoring tool can help authorities manage public safety
and environmental issues in recently abandoned coal mines. The
tool uses satellite radar imagery to capture millimeter-scale
measurements of changes in terrain height. Such measurements can
be used to monitor and forecast groundwater levels and changes
in geological conditions deep below the earth's surface in former
mining areas.



FULL STORY ==========================================================================
A remote monitoring tool to help authorities manage public safety and environmental issues in recently abandoned coal mines has been developed
by the University of Nottingham.


==========================================================================
The tool uses satellite radar imagery to capture millimetre-scale
measurements of changes in terrain height. Such measurements can be used
to monitor and forecast groundwater levels and changes in geological
conditions deep below the earth's surface in former mining areas.

With a long history of coal mining, the project was tested in the UK at
a regional scale, but has global implications given the worldwide decline
in the demand for coal in favour of more sustainable energy sources.

The method was implemented over the Nottinghamshire coalfields, which
were abandoned as recently as 2015, when the last deep mine, Thoresby
Colliery, shut its doors for good.

When deep mines are closed, the groundwater that was previously pumped
to the surface to make mining safe, is allowed to rise again until it
is restored to its natural level in a process called rebound.

The rebound of groundwater through former mine workings needs careful monitoring; often containing contaminants it can pollute waterways
and drinking water supplies; lead to localised flooding; renew mining subsidence, land uplift and reactivate geological faults if it rises too
fast. Such issues can cause costly and hazardous problems that need to
be addressed prior to the land being repurposed.



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The Coal Authority therefore needs detailed information on the rebound
rate across the vast mine systems it manages so it knows exactly where
to relax or increase pumping to control groundwater levels.

Measuring the rate and location of mine water rebound is therefore
vital to effectively manage the environmental and safety risks in
former coalfields, but difficult to achieve. Groundwater can flow in unanticipated directions via cavities within and between neighbouring collieries and discharge at the surface in areas not thought to be
at risk.

In the past, predicting where mine water will flow was heavily-reliant on
mine plans; inaccurate or incomplete documents that are sometimes more
than a century old; and borehole data. Costing approximately -L-20,000
to -L-350K each, boreholes are expensive to drill and are often sparsely situated across vast coalfields, leaving measurement gaps.

More recently uplift, subsidence and other geological motion has been
monitored by applying Interferometric Synthetic Aperture Radar (InSAR)
to images acquired from radar satellites. However, this interferometry technique has historically worked only in urban areas (as opposed to rural ones), where the radar can pick up stable objects, such as buildings or
rail tracks, on the ground to reflect back regularly to the satellite.

This study uses an advanced InSAR technique, called Intermittent Small
Baseline Subset (ISBAS), developed by the University of Nottingham and its spin-out company Terra Motion Ltd. InSAR uses stacks of satellite images
of the same location taken every few days or weeks which makes it possible
to pick up even the slightest topographical changes over time. Uniquely,
ISBAS InSAR can compute land deformation measurements over both urban and
rural terrain. This is beneficial when mapping former mining areas, which
are often located in rural areas. Over the Nottinghamshire coalfields,
for example, the land cover is predominantly rural, with nearly 80 per
cent comprising agricultural land, pastures and semi-natural areas.



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Such a density of measurements meant study lead, University of Nottingham
PhD student David Gee could develop a cost-effective and simple method
to model groundwater rebound from the surface movement changes.

The study found a definitive link between ground motion measurements
and rising mine water levels. Often land subsidence or uplift occurs as
a result of changes in groundwater, where the strata acts a little like
a sponge, expanding when filling with fluid and contracting when drained.

With near-complete spatial coverage of the InSAR data, he could fill in
the measurement gaps between boreholes to map the change in mine water
levels across the whole coalfield. The model takes into account both
geology and depth of groundwater to determine the true rate of rebound
and help identify where problems associated with rebound may occur.

The findings have been published in a paper 'Modelling groundwater
rebound in recently abandoned coalfields using DInSAR' in the journal
Remote Sensing of Environment.

David Gee, who is based in the Nottingham Geospatial Institute at the University, said, "There are several coalfields currently undergoing mine
water rebound in the UK, where surface uplift has been measured using
InSAR. In the Nottinghamshire coalfields, the quantitative comparison
between the deformation measured by the model and InSAR confirms that the
heave is caused by the recovery of mine water." At first a forward model
was generated to estimate surface uplift in response to measured changes
in groundwater levels from monitoring boreholes. David calibrated and
validated the model using ISBAS InSAR on ENVISAT and Sentinel- 1 radar
data. He then inverted the InSAR measurements to provide an estimate of
the change in groundwater levels. Subsequently, the inverted rates were
used to estimate the time it will take for groundwater to rebound and
identify areas of the coalfield most at risk of surface discharges.

"InSAR measurements, when combined with modelling, can assist with the characterisation of the hydrogeological processes occurring at former
mining sites. The technique has the potential to make a significant contribution to the progressive abandonment strategy of recently closed coalfields," David said.

The InSAR findings offer a supplementary source of data on groundwater
changes that augment the borehole measurements. It means monitoring
can be done remotely so is less labour-intensive for national bodies
such as the Environment Agency (which manages hazards such as flooding, pollution and contaminated land) and the Coal Authority (which has a
mandate to manage the legacy of underground coal mining in terms of
public safety and subsidence).

The model has already flagged that some parts of the coal fields that
are not behaving as previously predicted, which could influence existing remediation plans.

David explains, "The deepest part of the North Nottinghamshire coalfield,
for example, is not rebounding as expected which suggests that the mine
plans here might not be completely accurate. The stability is confirmed
by the InSAR and the model -- future monitoring of this area will help
to identify if or when rebound does eventually occur.

"Next steps for the project are to integrate our results into an existing screening tool developed by the Environment Agency and Coal Authority
to help local planning authorities, developers and consultants design sustainable drainage systems in coalfield areas. The initial results,
generated at a regional scale, have the potential to be scaled to all coalfields in the UK, with the aid of national InSAR maps," adds David.

Luke Bateson, Senior Remote Sensing Geologist from the British Geological Survey, said, "InSAR data offers a fantastic opportunity to reveal how
the ground is moving, however we need studies such as David's in order to understand what these ground motions relate to and what they mean. David's study, not only provides this understanding but also provides a tool
which can convert InSAR ground motions into information on mine water
levels that can be used to make informed decisions." Dr Andrew Sowter,
Chief Technical Officer at Terra Motion Ltd, explains, "Studies like
this demonstrate the value to us, as a small commercial company,
in investing in collaborative work with the University. We now have a remarkable, validated, result that is based upon our ISBAS InSAR method
and demonstrably supported by a range of important stakeholders. This will enable us to further penetrate the market in a huge range of critical applications hitherto labelled as difficult for more conventional InSAR techniques, particularly those markets relating to underground fluid
extraction and injection in more temperate, vegetated zones."

========================================================================== Story Source: Materials provided by University_of_Nottingham. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. David Gee, Luke Bateson, Stephen Grebby, Alessandro Novellino,
Andrew
Sowter, Lee Wyatt, Stuart Marsh, Roy Morgenstern, Ahmed
Athab. Modelling groundwater rebound in recently abandoned
coalfields using DInSAR. Remote Sensing of Environment, 2020; 249:
112021 DOI: 10.1016/j.rse.2020.112021 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/11/201116112934.htm

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