Geological environment disturbance monitoring

The disturbance of CO2 perfusion project to geological environment is reflected in inducing surface deformation and earthquake, and earthquake monitoring can be included in the monitoring of National Seismological Network. This book focuses on the monitoring technology of surface deformation. At present, any technology can't effectively identify the surface deformation, so it usually needs a variety of technologies to cross-use, and comprehensively analyze and judge the surface deformation characteristics with reference to the measurement results.

(A) D-InSAR monitoring surface deformation.

The monitoring principle of 1. In D.C.

InSAR technology is a technology to extract three-dimensional spatial information of ground targets from the phase data of complex radar images. The basic idea is: using two antennas to image at the same time or one antenna to repeatedly image at a certain time interval, the complex radar image pairs in the same area can be obtained. Because the distance between the two antennas and the ground target is not equal, the phase difference occurs between the same name image points of complex radar image pairs, forming interference patterns. The phase value in an interferogram is a measure of the phase difference between two images. According to the geometric relationship between the phase difference of two images and the three-dimensional spatial position of the ground target, the three-dimensional coordinates of the ground target can be determined by using the parameters of the flight orbit, which can be used to provide a large-scale high-precision digital elevation model (DEM). Let's take the satellite repeated orbit interference mode as an example, and its imaging geometry diagram is shown in Figure 10-4.

Fig.10-4 schematic diagram of geometric relationship of InSAR imaging

S 1, S2 is the position where the satellite images the same area twice (that is, the position of the antenna). The orbital height of position S is h, the baseline length (S 1 distance to S2) is b, the baseline horizontal angle is a, the incident angle is θ, the height of ground target P is h, the distance from S 1 to ground target P is γ, and the distance from S2 to ground target P. Combined with the above figure, the height h of ground target P can be expressed as:

Introduction of geological storage technology and method of carbon dioxide

According to cosine theorem:

Introduction of geological storage technology and method of carbon dioxide

So there are:

Introduction of geological storage technology and method of carbon dioxide

Finishing formula (10-3):

Introduction of geological storage technology and method of carbon dioxide

In In In SAR, the interference phase refers to the echo phase difference △φ received by ground target P after γ, r+δ, ray γ reaching S 1 and S2, and the phase difference △φ has the following relations with distance difference △γ and microwave wavelength λ:

Introduction of geological storage technology and method of carbon dioxide

Considering that the echo signals received by repetitive orbit radar are all transmitted and returned signals, there are:

Introduction of geological storage technology and method of carbon dioxide

Substituting Formula (10-6) and Formula (10-4) into Formula (10- 1) to obtain the following expression:

Introduction of geological storage technology and method of carbon dioxide

The above formula is the principle of getting the ground elevation from the interference phase, and the parameters are explained as follows: θ and H are known, and H can be calculated and measured by the radar height on the satellite. The baseline distance b and included angle α between the antenna connecting line and the horizontal line can be determined by satellite orbit parameters, but the accuracy is not high. According to the imaging principle, the orbit parameters can be calculated by a certain number of known ground points (control points) to improve the accuracy of B and α. There are usually two methods to calculate the phase difference △φ: direct phase subtraction of two complex-valued images and conjugate multiplication of complex-valued images-interference processing. These two methods are completely equivalent, but the second method is more commonly used. The phase principal value between [-π, π] is obtained by interference processing, and it must be expanded to get the total phase value of △φ.

2. The basic principle of D-InSAR

Suppose we have obtained two interferograms in the same area, one of which is obtained by interferometric processing of two SAR images before the surface deformation event, and this interferogram contains the information of the surface and topography; The other is obtained by SAR interference processing of two scenes before and after the deformation event, which contains the information of the earth's surface and topography and the micro-deformation information caused by it. On this basis, the two interference images are processed by difference to remove leveling effect, noise and atmospheric delay, and finally the accurate information of ground deformation is obtained.

The phase difference obtained by D-In SAR is a mixed phase, which consists of five parts, as shown in formula (10-8):

Introduction of geological storage technology and method of carbon dioxide

Among them, Topog φ geography is the phase contributed by topographic factors, and φ φ displacement is the phase caused by topographic changes; Φ Atmospherer, e is the atmospheric delay phase; φφflat is the phase caused by the reference plane; φφnsise is the phase caused by noise. DInSAR differential interferometry eliminates four terms through a series of processing methods, namely, φ topography, φ atmosphere, φ flatness and φ noise, leaving only the phase caused by φ topography.

3.d- In the process of SAR monitoring

1) Determine the monitoring area, monitoring scheme and monitoring period;

2) Acquisition of radar data and DEM data;

3) Two-track method and three-track method can be used for differential interferometry of synthetic aperture radar. Two-track method uses two SAR images before and after the surface change in the work area to generate the interference fringe pattern φ d, and then uses the DEM data φ obtained in advance to simulate the terrain phase pattern φ sim, and removes the terrain information from the interference fringe pattern to obtain the surface deformation information△ r (formula 10-9).

Introduction of geological storage technology and method of carbon dioxide

The data processing flow is shown in figure 10-5.

Figure 10-5 dual-track D-InSAR data processing flow

(2) Monitoring surface deformation by PS-InSAR.

The monitoring principle of 1. PS in SAR

PS technology uses multi-scene SAR images in the same area (generally more than 25 scenes) to find permanent scatterers that are not affected by temporal and spatial baseline decorrelation and atmospheric effects by statistically analyzing the amplitude information of all images. Using the interpolation of these permanent scatterers to fit the surface, the terrain error, the deviation value of the target in the line of sight direction and the atmospheric delay value are calculated, so as to estimate and remove the contribution value of the atmospheric delay phase and improve the deformation monitoring accuracy.

The phase of each pixel in the differential interferogram can be expressed by the following formula:

Introduction of geological storage technology and method of carbon dioxide

Where φφdiff is the differential interference phase; φφdef is the surface deformation phase; φφtopo is the elevation correction stage; φφatmo is the atmospheric delayed phase; φ noise is the noise phase.

2.PS-In SAR monitoring process

1) Determine the monitoring area, monitoring scheme and monitoring period.

2) Acquisition of radar data and DEM data.

3) The data processing flow is shown in figure 10-6.

Figure 10-6 PS-lnSAR data processing flow

Mainly comprises the following processing steps:

① Registration and radiometric calibration. In the process of synthetic aperture radar imaging, the amplitude of each pixel in synthetic aperture radar image will change with the incident angle, orbit position, atmospheric conditions and so on. Therefore, it is impossible to directly compare and analyze the amplitude information in time series SAR images, and image registration and radiometric calibration (radiometric calibration) are needed to unify the pixel position and radiation intensity of sequence SAR images.

2 PS selection point. From N+ 1 calibrated SAR images, the permanent scatterers in the study area are identified by using the algorithms of coherence coefficient threshold, phase deviation threshold and amplitude deviation threshold.

③ Generate differential interferogram. Making the selected PS points involves differential processing. N differential interferograms are obtained.

④ Sparse mesh expansion. The differential interference phases of all PS points can be obtained from N differential interferograms.

Introduction of geological storage technology and method of carbon dioxide

The oblique distance ρ from PS point to satellite, spatial baseline B⊥, time baseline t, radar wavelength λ and φ differential interference phase φ diff of PS point are known values, and the elevation correction△ h and linear deformation velocity of PS point are obtained. And φφatmo is the value of the required solution. The equation (10- 1 1) cannot be solved directly. According to the spatial correlation between PS phase components, the differential phase model of PS point domain can be established for indirect solution.

Due to the lack of prior conditions, it is impossible to solve the interference phase of a single PS point. We must first estimate the winding phase gradient of adjacent points, and then integrate the phase gradient. It is assumed that the phase residuals of two adjacent points satisfy:

Introduction of geological storage technology and method of carbon dioxide

Then, the phase expansion can be carried out in space, and the elevation correction φφtopo and the linear deformation velocity ν of each PS point can be calculated.

⑤ Extracting atmospheric delay phase and nonlinear deformation. After calculating the linear deformation and DEM correction of each PS point, the residual phase can be obtained by subtracting them from the initial differential interferogram. The residual phase is mainly composed of nonlinear deformation phase, atmospheric phase and noise. In the remaining stage, the frequency characteristics of atmospheric stage and nonlinear deformation stage are different in time domain and space domain. Because the spatial correlation length of the atmosphere is about 1km, the atmospheric disturbance in the interferogram is a low-frequency signal in the spatial domain, but for a pixel, the atmospheric condition can be regarded as a random process at different radar imaging times, and the atmospheric phase is a white noise in time. However, nonlinear deformation has small correlation length in space and low frequency characteristics in time domain. Therefore, nonlinear deformation and atmospheric phase can be separated by filtering in time domain and space domain. Iterate steps ④ and ⑤ until accurate elevation correction φφtopo and linear deformation velocity ν are obtained. At the same time, the final phase noise can be obtained in this step, and more reliable PS points can be identified according to the time correlation of PS points.

⑥PS point time series analysis. Knowing the linear deformation rate and nonlinear deformation variables of PS, the deformation variables of time series of each PS can be obtained, and the deformation field can be analyzed by using other software (such as Arcgis).

4) Spot sampling investigation and verification.

5) Submit the monitoring results.

(3) Inclinometer

Tiltmeter is a tool that can measure very small (one billionth) tension changes on the surface or deep of the earth. Inclinometer is usually used to monitor oilfield development, including water flooding, CO2 flooding and hydraulic fracturing. Usually, radio or satellite is used to collect measurement data remotely. The surface deformation related to CO2 injection and migration can be accurately measured by a series of inclinometers.

(4) Global Navigation Satellite System

GNSS is the abbreviation of Global Navigation Satellite System, that is, Global Navigation Satellite System. GNSS includes American GPS, Russian GLONASS, China compass and EU Galileo system, and the number of available satellites reaches more than 65,438+000. From the beginning, GNSS was not a single constellation system, but a comprehensive constellation system (called GNSS- 1 at that time, and later built as EGNOS) including the GPS system of the United States and the GLO-NASS system of Russia. Global navigation satellite system can accurately determine any place or position on the earth's surface. Its advantage lies in the efficient receiver, combined with enhanced signal processing technology, which allows the remote and continuous operation of GPS stations with an accuracy of 1.5mm or less. The relative variation elevation with sub-millimeter accuracy can be measured in a large range through the array arrangement of the ground inclination monitoring network (STM), and the high-precision GPS measurement can provide the absolute elevation variable with millimeter accuracy in the study area. GPS measurement results usually provide reference data for long-term inclinometer measurement and SAR monitoring.