# First Results from Sentinel-1A InSAR over Australia: Application to the Perth Basin
## 6 Future Monitoring of the Perth Basin  
InSAR data are being used increasingly to complement or replace ground-based geodetic surveys for deformation monitoring due to a broader spatial coverage and lower costs. Applying these data to the Perth Basin, we have been able to investigate a larger spatial extent of the deformation field, albeit over a limited time interval (0.7 y) that was solely governed by the availability of Sentinel-1A data. Whilst it is not possible to determine whether the displacements we have measured are representative of long-term deformation or are seasonal variations, the observations do indeed provide new insights into the spatial distribution of ground displacements in the Perth Basin that can be used to coordinate subsequent focused subsidence monitoring over targeted regions [2]. To guide future InSAR studies in the Perth Basin, we estimate the time period required to measure subsidence at linear rates obtained from previous time series analysis at GPS stations HIL1 and PERT [1] using Sentinel-1 and TerraSAR-X. Between 1994/1998 and 2012, time series at the two GPS stations show non-linear subsidence that correlates with groundwater levels from the deeper Leederville and Yarragadee aquifers [1]. After removal of annual and semi-annual seasonal signals, this was modelled by three piecewise linear trends spanning 1994/1998-2000, 2000-2005 and 2005-2012 (Table 1). The largest subsidence rate (6.97 \(\pm\) 1.30 mm/y) was at HIL1 between 2000 and 2005. The smallest reliable GPS rate (i.e., that with an estimated error smaller than the estimated rate) (2.51 \(\pm\) 1.89 mm/y) was at PERT between 2005 and 2012 [1]. For each of the GPS epochs, we estimate the error on InSAR measurements using the method described in [PERSON] et al. (2016). This takes into consideration the length of the observation period, the frequency of the acquisitions and the standard deviation of the atmospheric noise. We estimate the standard deviation of the atmospheric noise by masking the regions identified to be deforming in Figure 4A,B and assuming that residual phase signals are due to uncorrected atmospheric noise. The average standard deviation of the ERA-I corrected interferograms is \(\sim\)9 mm for Sentinel-1A and \(\sim\)5 mm for TerraSAR-X. For each satellite mission, we assume there are no gaps in acquisitions. We include results for Sentinel-1A at the acquisition frequency of data in this study (24-day repeat intervals), the acquisition frequency implemented for Sentinel-1B since October 2016 (12-day repeat intervals) and the current combined Sentinel-1A and -1B acquisition frequency over Europe (six-day repeat intervals) assuming the same value of atmospheric noise. In each case, we consider subsidence to be detectable when the cumulative deformation exceeds the estimated error. The results for each GPS epoch are shown in Table 1. Over the period of this study (0.7 y), TerraSAR-X may be able to detect deformation at HIL1 and PERT at the 2000-2005 rates and at HIL1 at the 1998-2000 rate. However, neither TerraSAR-X nor Sentinel-1A are able to detect deformation at the smaller 2005-2012 rates at PERT or HIL1, with Sentinel-1A requiring 1.38 years to detect the smallest magnitude signal identified by [PERSON] et al. (2016) at PERT. This is approximately double the current period of continuous Sentinel-1A acquisitions over the Perth Basin. For the newly-implemented Sentinel-1B acquisition strategy (12-day intervals), the estimated detection periods reduce by an average of 14% (Table 1). In the case of Sentinel-1A and -1B at six-day repeat intervals, the theoretical detection period decreases by 25%. At this level of repeat, the results suggest that Sentinel-1 may detect deformation at both HIL1 and PERT at the 2000-2005 rates over the period of this study. Estimating the minimum observation length or repeat frequency to measure small-magnitude deformation in this way is informative for planning future monitoring strategies.