Pipemon.com - Ground motion and structure monitoring

Case studies: Ground monitoring

Ground motion monitoring case studies

The PIPEMON ground and structure motion monitoring services have been applied to some typical application areas for pipeline-related ground motion monitoring as investigated within the PIPEMON project. The preliminary case study results can be summarised below: In general, satellite-based ground motion monitoring can be very attractive when compared to conventional approaches to monitoring ground movement along pipelines. The process can be conducted remotely (following CR installation, if required), and may be particularly cost-effective when applied to remote geographical areas. It can be used in a non-invasive manner to accurately and effectively survey large remote areas. It can also be used to further confirm provisional findings regarding ground movement as measured by ground-based measurement tools, or to help establish spatial limits around areas that are suspected to be moving over time. The following bullet points lead to the individual (preliminary) results of the trials: 1. Underground Storage Areas 2. Coal Mining Activity Areas 3. Landslides Underground Storage Areas The PSI technique was applied over a salt cavern field in Germany, which is used for storage of crude oil and natural gas. Subsidence of the cavern field has been monitored since 1975 with a yearly ground-leveling survey over the entire field area. PSI processing was carried out using a dataset of 70 descending ERS-SAR images, covering a time span of 12 years, from 9 May 1992 to 25 January 2005. An extract of the final estimated velocity field in the direction of the satellite line-of-sight is shown in the figure below.


Extract of the estimated ground movement velocity (mm/yr) over an underground storage cavern, Germany (PSI data copyright TRE 2005. ERS data copyright ESA 1992-2005. Background image copyright Landesvermessungsamt Nordrhein-Westfalen).

Measurement points were obtained from pipelines and related infrastructure as well as farmhouses and outbuildings on the site, which individually provided a strong reflection of the radar signal back to the satellite. A close-up of the point coverage is shown in the figure below.


Close-up of point coverage for an underground storage cavern, Germany (PSI data copyright TRE 2005. ERS data copyright ESA 1992-2005. Background image copyright Landesvermessungsamt Nordrhein-Westfalen).

The sampling involves a non-invasive monitoring technique that can measure ground movement over a wide area with sub-millimetre precision. For each positioned point that is mapped in the above figure, a motion history dating back to 1992 can be derived from archival satellite radar data (see figure below).


Example of time series of ground displacement of a single measurement point. The displacement values are relative to a chosen reference point.

An example of the surface pipeline infrastructure for the gas storage cavern is shown in the next figure.


Surface infrastructure associated with an underground gas storage cavern, Germany.

The network of ground leveling measurements was compared to the PSI processing result. For this, the leveling measurements had to be interpolated to the PSI results, both spatially and temporally. The two figures below represent the average annual velocities over the entire cavern field, as derived from leveling and PSI measurements, respectively.


Average annual velocities derived from leveling measurements.	Average annual velocities derived from the PSI result (PSI data copyright TRE 2005. ERS data copyright ESA 1992-2005).

The operator of the German gas storage facility indicated that the PSI measurements corresponded to the leveling survey results, despite some differences in the centre of the bowl. The subsidence bowl was however clearly defined by the PSI data. A strict direct comparison between ground survey data and PSI data was not possible, as measurements were interpolated spatially and temporally. The operator indicated that more measurement points might have further improved results; in such instances, where natural scatterers for PSI are insufficient or lacking, CRs can be installed. The operator sees InSAR as being particularly useful for providing important information on areas, such as wetlands, that are typically inaccessible for ground survey teams. The PSI technology has the advantage of exploiting a satellite data archive from potentially 1992 onwards, providing historical ground movement data that is not possible to replicate with conventional ground movement methods. Especially slow ground movements can be detected and spatially defined, in particular movements that might be overlooked using conventional ground-based methodologies. PSI’s high point density over urban areas allows the identification of unstable areas at a glance.

Coal Mining Activity Areas A coal mining subsidence test site associated with a gas pipeline in the UK was processed for the PRESENSE project and its utility further studied in the PIPEMON project. The figure below shows a prominent displacement contour amounting to at least 12 cm within a 2-month period, centered on an elongated subsidence pattern of 1.5 km ? 1 km. Subsidence in this figure is directly attributable to mining activity along the block highlighted in red. The area of subsidence extends in width over a previously mined block (in yellow) and onto a section of the pipeline (blue).


Displacement of pipeline related to subsidence associated with old subsurface coal mining activities (InSAR data copyright NPA 2003. ERS data copyright ESA 1999-2000).

DifSAR has proven to be useful to measure fast ground movement over short intervals, for example over coal mining activity areas. While both DifSAR and PSI cover wide areas, CRs can be installed in arrays within relatively localized areas, to monitor the movement of specific sites or structures (see below). Landslides A cluster array of 6 metallic corner reflectors (CRs, see figures below) was placed within the pipeline right-of-way to measure ground movement rates over a known landslip area associated with a pipeline river crossing in north-central Canada. CRs guarantee a clear, strong and time persistent target response to the satellite radar sensor, which is necessary especially in vegetated areas where few or no natural reflectors are present in the target area. CRs were deployed in March 2006 at the Canadian test site, and data collection has been continuing since this time.


Corner Reflectors: a) ground view of a deployed CR (copyright NPA), b) CR and protective wooden fencing installed at the Canadian site (copyright EBA).

The underground pipeline is subjected to deep seated and slow land creep on a slope just before the river crossing. The slope drops 100m in height over a distance of 600m (see figure below).


Profile of the slope before a pipeline river crossing in Canada, showing the locations of the six Corner Reflectors (A to F) that were installed. Corner Reflectors A and F act as references on stable locations.

Four satellite images have been acquired so far and successfully processed with DifSAR. An example of the radar amplitude response of the six Corner Reflectors is shown in the figure below for the first image acquired. All reflectors were orientated correctly towards the satellite and returned a sufficiently strong signal back to the satellite for all four acquisitions.


Interferometry response from the six deployed Corner Reflectors (encircled in red) (Envisat ASAR data, copyright ESA 2006).

Preliminary results suggest that the site is relatively stable, but further acquisitions need to be made to confirm this trend. A comparison will be made between the DifSAR measurements and the readings of the slope inclinometers installed at the site.