BACKGROUND Sinkholes (also known as dolines) are a hazard across many parts of the world. They are commonly the result of karst dissolution processes in areas underlain by carbonates or evaporites. However, they may also be caused by suffosion or piping of sediment, or collapse of existing void spaces. Although most sinkholes occur naturally, others can be caused or infl uenced by human activity, for example water leaks, collapse of abandoned mine workings, and oil and gas production activities.
The Permian Basin, spanning Texas and New Mexico, is currently the highest oil-producing region in the United States1. It contains large deposits of Palaeozoic evaporites; dissolution of these in contact with groundwater can result in a susceptibility to sinkhole formation2.
CHALLENGEPrediction of sinkhole locations and occurrences is extremely challenging. However, owing to potential risks to lives and property there are obvious benefi ts to any advanced warning of these hazards. This is certainly true within the oil and gas industry, where subsurface cavities that lead to sinkholes can cause wellbore integrity issues, loss of drilling mud and can contain fl ammable gas.
Sinkholes are diffi cult to map using conventional survey techniques due to uncertainties over their location, as well as the fi nancial and logistical risks associated with fi eld work. They can also be challenging to detect using InSAR, particularly those occurring over small scales, or with fast or strongly variable subsidence rates. Furthermore, some sinkholes have been shown not to exhibit precursory subsidence, instead occurring as a sudden collapse. However, high-resolution synthetic aperture radar (SAR) sensors introduced over the past decade have expanded capabilities to enable monitoring of smaller sinkholes. Along with more frequent sampling, and longer wavelength radar data, they also assist in capturing high deformation gradients over short distances.
Understanding ground deformation signals, and the processes which affect the progression and timing of sinkhole development, is key for understanding and mitigating these hazards.
This case study explains how InSAR has been used to map sinkhole subsidence in the Permian Basin town of Wink.
CHALLENGESinkholes pose an ever-present danger to communities, infrastruc-ture and oil and gas operations in the USA. A visible sinkhole is often the tip of the iceberg compared to the wider region that has the potential to collapse. A solution is required to map both precursor and post-collapse deformation associ-ated with these hazards.
SOLUTIONUsing a combination of optical satellite imagery and InSAR, we map sinkhole features and localized deformation to infer the extent and magnitude of the hazard, provid-ing an essential input into planning, risk assessment and remediation activities.
CONCLUSIONThe results show subsidence of surrounding areas can continue for at least 35 years after sinkhole formation, underlining the ongo-ing risk of further impacts to land, infrastructure and potentially life. InSAR can help to identify areas at risk of future sinkhole forma-tion, with precursor signals at Wink detectible at least nine years before collapse occurred.
This case study provides evidence of how low-cost InSAR mapping can be used to remotely monitor large regions for sinkhole hazards, reducing the financial, logistical and safety risks associated with conventional surveying.
REVEALING HIDDENSINKHOLE HAZARDSRegions prone to sinkhole formation can often be identi� ed. However, prediction of exact sinkhole locations and timing of formation are exceedingly dif� cult. This case study from Wink, in the Permian Basin of Texas, demonstrates how InSAR can measure both precursor and post-collapse deformation associated with sinkhole hazards to support risk assessment and mitigation.
SOLUTIONInSAR has previously been used to successfully detect subsidence around a number of existing sinkholes3,4. In a number of locations, archive InSAR studies have also revealed precursor subsidence signals, prior to sinkhole formation. InSAR therefore has the potential to detect precursor subsidence which may correlate with sinkhole formation, identifying locations of high potential risk.
Two sinkholes occurred around 3.5km to the northeast of the town of Wink; one in June 1980, and a second in May 2002 (Figs. 1a & 1b). The 1980 sinkhole has a diameter of approximately 100m. The 2002 sinkhole opened around 1.5km to the south, with initial dimensions of 150m by 100m, and has since expanded to 250m by 220m. The area is underlain by the Salado salt formation, which is susceptible to natural dissolution. Between the 1920s and 1960s, oil production and water injection took place at a number of wells across the Hendrick oil fi eld, in the area later affected by the two sinkholes. It has been suggested that the older drilling technology used during that period may have resulted in well casing integrity issues, which could have accelerated the formation of dissolution cavities5.
An archive of SAR data collected over a period of more than 20 years enables a long term perspective on surface deformation across Wink. Unfortunately no suitable archive SAR imagery is available spanning the formation of the sinkholes in 1980 or 2002. However, datasets are available for periods between the two events (ERS, 1993-2000), and following the formation of the second sinkhole (ALOS PALSAR, 2007-2011). A small stack of recent SAR images from Sentinel-1A are also available, allowing an up-to-date assessment of ongoing deformation, and providing an opportunity for ongoing monitoring.
Figs. 2a, 2b and 2c show wrapped interferograms from ERS, ALOS and Sentinel-1A data, showing the variations in observed deformation (shown as colored contours) across a range of different time periods. Three main areas of subsidence are visible; two surrounding the existing sinkholes, and an area further to the south. Around the 1980 sinkhole, persistent subsidence features are observed in all three periods. These are centred on the sinkhole, with a diameter of 200-600m. The 2002 sinkhole is located towards the southwest corner of a wider area of subsidence, with extents of approximately 1 by 2km. In addition to the moderate subsidence immediately surrounding the sinkhole, there are several locations where even stronger subsidence is concentrated, to the north and east. Overall, rates of subsidence in some parts of this area are up to ~80cm/yr.
Around 1.3km to the south of the 2002 sinkhole is a third area of subsidence, with an extent of around 400m. This is of lower magnitude than that surrounding the sinkholes; and the deformation appears to be somewhat sporadic in nature. This subsidence signal is most clearly visible in the ALOS data (Fig. 2b), with around ~14cm of subsidence observed between July 2007 and January 2011. This area of subsidence does not correspond to any existing sinkhole, but shows temporal persistence over a timespan of at least 15 years. It could potentially represent an area of subsurface dissolution, and therefore be at higher risk of future sinkhole formation. However, there could be other causes for such a signal, including ongoing oil production or water abstraction.
These results provide a clear demonstration of the ability of InSAR to measure ground deformation related to salt dissolution and sinkhole formation. The results show subsidence of surrounding areas can continue for at least 35 years after sinkhole formation, underlining the ongoing risk to land, infrastructure and life. Furthermore, InSAR can help to identify areas at risk of future sinkhole formation, with precursor signals at Wink detectible at least nine years before collapse occurred. This offers the potential for mitigation of sinkhole hazards and the optimization of on-site investigations.
- 2. Paine, J. G., Buckley, S. M., Collins, E. W., & Wilson, C. R. (2012). Assessing collapse risk in evaporite sinkhole-prone areas using microgravimetry and radar interferometry. Journal of Environmental and Engineering Geophysics, 17(2), 75-87.
- 3. Nof, R. N., Baer, G., Ziv, A., Raz, E., Atzori, S., & Salvi, S. (2013). Sinkhole precursors along the Dead Sea, Israel, revealed by SAR interferometry. Geology, 41(9), 1019-1022. doi: 10.1130/G34505.1
- 4. Jones, C. E., & Blom, R. G. (2014). Bayou Corne, Louisiana, sinkhole: Precursory deformation measured by radar interferometry. Geology, 42(2), 111-114.
- 5. Kim, J. W., Lu, Z., & Degrandpre, K. (2016). Ongoing deformation of sinkholes in wink, texas, observed by time-series Sentinel-1a SAR interferometry (preliminary results). Remote Sensing, 8(4), 313.
