What we do
We offer services to help identify and monitor areas undergoing movement. We provide maps of displacements and/or longer term velocity estimates to measure:
- Tectonic motion (that may be used for seismic hazard assessments)
- Movement and flexure estimates on dams, buildings and other infrastructure
We use instrumentation and techniques developed in the field of Space Geodesy. Often in this field, artificial satellites are used to gather information on how far a point on the ground is from the satellite. Depending on the satellite and technique used, a three dimensional position of a ground point can be tracked over time. Tracking the movement of ground points over time is our specialty. Our expertise are in High Precision Global Positioning Systems (GPS), that can measure movement of an individual point over time at the sub-millimeter accuracy level, and Interferometric Synthetic Aperture Radar (InSAR), which measure the satellite Line of Sight (LOS) distance from the satellite to the ground for large regions.
SAR detects millimeter level relative variations in terrain elevation by keeping track of the change in time it takes for the radar signal to hit the target and scatter back in the satellite Line of Sight (LOS) direction. SAR imagery divides the ground surface into individual 'pixels' that have different resolutions depending on the sensor (e.g., 3m for TerraSAR-X stripmap and 9m for ALOS PALSAR-1). Repeated passes can be differenced to observe changes in the terrain elevation at each coherent pixel, and multiple passes can be used to develop a spatially dense array of displacements, velocities and time series. Because the measurements are taken in the LOS direction, both ascending and descending passes are needed to discern vertical and horizontal motion.
Imaging characteristics such as repeat times and radar wavelengths make different satellite platforms more suitable to certain applications. Shorter repeat intervals provide a large number of imagery over a relatively short span of time, and therefore is ideal for fast changing environments. Choosing a sensor that is most sensitive to the observed target is critical for optimal detection of movement and depends on the wavelength. For example, shorter wavelengths (X-band) are more sensitive to features on the order of 3 cm and C- and L-band sensors are sensitive to relatively bigger features, about 6 and 24 cm respectively. Therefore it is important to select imagery that will provide the best analysis and results for the project. Multi-temporal SAR imagery can be used to detect small deformations, thus providing means to detect cm/year subsidence over large areas. Two major algorithms exist for this purpose: (1) Small Baseline Subset (SBAS) Interferometry and (2) Persistent Scatterer InSAR (PSI). The results from both methods have improved signal-to-noise ratio (SNR) compared with conventional InSAR that only measures the displacement between two discrete periods of time (i.e., between two different SAR acquisitions). SBAS relies on a distributed scattering mechanism and uses filtering and averaging of the interferograms to improve the SNR. The SBAS method relies on the redundancy involved with inverting a large number of interferograms to calculate the displacement though time for each coherent pixel covered by InSAR. PSI relies on identifying pixels in the interferogram that exhibit a stable, point-like scattering mechanism. With PSI, interferograms are generated in reference to a single master, and persistent scatterers that are observable in all interferograms are analyzed. Because natural terrain (i.e. rural or undeveloped areas) tends to be dominated by distributed scattering mechanisms, SBAS is the preferred method for this analysis. A large number of acquisitions are required for the analysis, which can be obtained from archived imagery and ongoing monitoring.
A GPS system consists of three parts: the satellite constellation, the GPS ground stations, and the GPS control station, which controls the satellite positions. GPS satellites emit two carrier frequencies, L1 and L2, that have wavelengths of 19 and 24 cm, respectively. Knowing the time these signals are emitted by the satellite and detected on the ground with a GPS antenna, one can calculate their distance. A three-dimensional position of a ground point can be estimated with at least 4 distance measurements, each from a different satellite. A minimum of 24 GPS satellites centered on 6 orbital planes are needed so that at least 4 satellites can be seen at any place on the surface of the earth, except maybe at the poles.
GPS that are permanently installed can be used to monitor long term motion of the ground point. GPS can also be used in 'kinematic' surveys that can be used in mapping. GPS is also a useful and complimentary tool that can help calibrate and provide additional observation when used with InSAR products.