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Continuous space based surveying and monitoring for detecting mm movements on the ground

January 19, 2021
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Questions & Answers

What is required to use this software and is there any training that can be provided?

The web-based application is designed to be used by people without any specific skills or training in remote sensing or SAR. The interface is designed to be simple to use for anyone who wishes to receive historical or near real-time ground movement measurements. Feel free to contact us for a personalized demonstration. Training is available on request.

How much depth can InSAR reach?

The land deformation measurements available through CATALYST Insights are capable of detecting small movements of the surface. This can often be used to detect and track underground activity, but the detection of movement is on the surface.

Are the measurement of displacement only available as points or is there any way to make it by area?

Our Insights platform allows you to view the measurements of displacement by both points and fields (area). The measurement fields are derived from the points (persistent scatterers) using an interpolation technique

Is there any additional device that needs to be installed on earth to use InSAR?

For many applications you do not need any additional devices installed on earth to get good measurements. However, some locations and applications (e.g. highly vegetated areas) may benefit from the use of corner reflectors.

What is the availability of historical displacement data?

CATALYST Insights currently supports Sentinel-1 imagery, which goes as far back as 2014. However, we are working with a number of commercial data providers to offer this web-based information using data acquired from their sensors, which date back even further

Is the ground deformation data already processed and available in product form?

The ground deformation measurements over California showcased in the demonstration were already processed with our in-house algorithms and workflow. However, CATALYST insights will process new measurements on demand, based on your area, date range,  and project criteria. 

Did you develop the InSAR/PSI in house?

Yes. The InSAR and Persistent Scatterers workflow were developed by our in-house SAR science team.

Do you use a single reference image from the timeseries? Are the pairs with respect to the first image (date) or to the date before (consecutives images)?

We are not using a single reference file, we use a sequential technique that follows the SBAS approach. Each pair in the time series is made up of a reference and dependent image. The pair is co-registered together in order to extract an deformation map of the time period. To create the time series trend, each new reference in a pair is the dependent image from the most recent pair. This ensures that all images are accurately aligned tighter and generates a time series deformation product. “

So you can combine data from duifferent SAR instruments to create timeseries?

No, we cannot use imagery from different sensors to create an interferogram, so we cannot use images from different sensors in the same time series. However, we can generate multiple time series, each specific to the sensor and calculate the deformation from each to provide deformation measurements at smaller time intervals.

Can we use UAV image data instead of satellite image data ?

Not with CATALYST Insights at this time. Our algorithms and workflows are designed and calibrated to work with satellite SAR imagery

What previous work on the ground must be done to calculate these measurements with SAR images?

This depends on the area and application. In most urban, construction and mining sites were it is easy to find many persistent scatterers, no previous work is required. In more vegetated areas and where specific measurements and precision are required, you may benefit from installing corner reflectors

What is the spatial and temporal sampling rate of the images?

Spatial Resolution: This depends on the sensor used. For Sentinel-1 the resolution is resampled to about 15m GSD. However, many commercial sensors offer sub meter resolution as high as 50cm.

Temporal Resolution: For Sentinel-1 this is as frequent as 6 days. However, new commercial constellations are being launched that will be able to provide same day measurements, even perhaps hourly measurements.

In Amazon areas with big vegetation, Is it possible use this technology without corner reflectors?

This would be very tricky and may not yield reliable results. We would recommend corner reflectors be installed to get more reliable persistent scatterers.

Would you please explain why the displacements trends for different points seems similar in the demo?

The displacement trends for different sampling points were similar was due to the uniform deformation occurring across the area of interest. If non-uniform change occurred,
we would see differences in the displacement trends across the sampling points.

Can we used SAR for groundwater exploration?

Spaceborne InSAR cannot be use for precise estimation of depth and thickness of various subsurface layers and their relative water yielding capacity or to estimate ground water available resources. The SAR signal in the C-band (5.6cm) cannot penetrate the surface for more than a few cm when the soil layer is dry and compose of lossy material.

Spaceborne InSAR can be used to predict and monitor the consequence of aquifer depletion such as land subsidence.

Can we measure change in buildings height in an urban context using sentinel data

Measuring building height changes is possible with InSAR technique but we recommend commercial SAR imagery with a high spatial resolution

You have to rectifiy a lot of effects on the signal and Image. How can you be sure about mm accuracy ?

The mm accuracy is in theory possible and can be sometimes verified using a corner reflector or a well-defined permanent scatterer (i.e.. phase center always at the same position) monitored with precise GNSS measurements. In reality, InSAR data need to be average to reduce the impact of different noise terms and signal decorrelation plus some statistical estimation of the surface movements when computing the surface deformations. The mm accuracy is what is possible to achieve but the accuracy of some persistent scatterers can be in the cm range. This is why it is necessary to use large stacks of data of at least 15-20 images for PSI.

Can we use the results for earthquake prediction?

Yes and no. Earthquake are catastrophic events (i.e. they happed suddenly) that are very hard to predict despite 50 years of research. Using InSAR technique, you can observed gradual terrain deformations (subsidence or uplifting) before the Earthquake but you cannot predict precisely when the earthquake will happened by extrapolating the trends. This paper published in 2017

gives and excellent overview of the problematic, especially the figure 1.

How can movement of slopes be monitored if the terrain is woody?

It would be recommended to install corner reflectors in areas that are not obstructed by the trees.

Can you detect horizontal or vertical changes or both?

The changes are relative from the line of light (LOS) which contains both horizontal and vertical movements that cannot be separated. Using two stacks of data, one ascending and one descending, it’s possible to convert LOS movement to a 3D geodetic coordinate.

Do you have papers published on the techniques used, especially the time series use of InSAR images?

We have implemented a proprietary hybrid PSI workflow that correspond to the industry standard without relying on open-source code. We use a combination of SBAS and techniques similar to Stamps

Is it possible to monitor tree height changes?

Not with typical InSAR methodology, the phase decorrelation over vegetated areas is too high. Could be possible with (stereo) radargrammetry.

Is InSAR change detection valid in mountainous area? 

Yes, but there are limitations if 1) the slope is heavily vegetated and/or 2) the slope is too steep and exceeds the ambiguity height.

What do you think about the accuracy level of Sentinel-1 data for PSI analysis?

The accuracy is good for regional studies due to Sentinel-1’s medium spatial resolution. However, monitoring, for example, a small dam would require a finer spatial resolution