Spatially assisted down-track median filter for GPR image post-processing
View Patent ↗A method and system for detecting the presence of subsurface objects within a medium is provided. In some embodiments, the imaging and detection system operates in a multistatic mode to collect radar return signals generated by an array of transceiver antenna pairs that is positioned across the surface and that travels down the surface. The imaging and detection system pre-processes the return signal to suppress certain undesirable effects. The imaging and detection system then generates synthetic aperture radar images from real aperture radar images generated from the pre-processed return signal. The imaging and detection system then post-processes the synthetic aperture radar images to improve detection of subsurface objects. The imaging and detection system identifies peaks in the energy levels of the post-processed image frame, which indicates the presence of a subsurface object.
1. A method performed by a computing device for filtering a target image frame in an associated sequence of image frames along a down-track direction, the target image frame having pixels with values, the method comprising:
for target pixels of the target image frame,
calculating by the computing device a median filtered value for the target pixel based on the value of corresponding pixels in each of the image frames in the associated sequence; and
determining by the computing device whether pixels in a neighborhood of the target pixel satisfy a filtering criterion based on values of the pixels in the neighborhood and their associated median filtered values; and
after determining that the filtering criterion is satisfied, replacing by the computing device the value of the target pixel with the calculated median filtered value.
2. The method of claim 1 wherein the determining includes:
calculating a difference between values of pixels of the target image frame in the neighborhood of the target pixel and their respective median-filtered values; and
computing the minimum of the difference values within the neighborhood.
3. The method of claim 2 wherein the filtering criterion satisfied when a minimum of difference values within the neighborhood is greater than zero.
4. The method of claim 1 wherein the neighborhood is a rectangular area of pixels.
5. The method of claim 1 wherein the associated sequence of image frames is generated from return signals collected on a sequence of down-track locations in the vicinity of the target down-track location.
6. The method of claim 5 wherein the return signals are acquired by receivers from ground-penetrating radar signals emitted by transmitters, the transmitters and receivers operating in multistatic mode.
7. The method of claim 6 wherein the ground-penetrating radar signals are emitted into the ground and including determining whether an explosive device is buried based on analysis of a filtered target image frame.
8. A computer-readable storage device that is not a transitory, propagating signal storing computer-executable instructions for controlling a computing device to filter a target image frame in an associated sequence of image frames along a down-track direction, the target image frame having pixels with values, the target image frame being generated from the return signals acquired by receivers from ground-penetrating radar signals emitted by transmitters, by a method comprising:
calculating a median filtered value for a target pixel of the target image based on the value of corresponding pixels in the image frames in the associated sequence;
determining whether pixels in a neighborhood of the target pixel satisfy a filtering criterion based on values of the pixels in the neighborhood and the median filtered values of the pixels in the neighborhood; and
when the filtering criterion is determined to be satisfied, replacing the value of the target pixel with the calculated median filtered value.
9. The computer-readable storage device of claim 8 wherein the determining includes:
calculating a difference between values of pixels of the target image frame in the neighborhood and median-filtered values of pixels of the target image frame in the neighborhood; and
computing a minimum of the difference values within the neighborhood.
10. The computer-readable storage device of claim 9 wherein the filtering criterion satisfied when a minimum of difference values within the neighborhood is greater than zero.
11. The computer-readable storage device of claim 8 wherein the target image frame is represents a synthetic aperture image generated from real aperture images.
12. The computer-readable storage device of claim 8 wherein the transmitters and receivers operate in multistatic mode.
13. The computer-readable storage device of claim 12 wherein the ground-penetrating radar signals are emitted into the ground and including determining whether an explosive device is buried based on analysis of a filtered target image frame.
14. A computing device for filtering a target image frame in an associated sequence of image frames along a down-track direction, the target image frame having pixels with values, the target image frame being generated from the return signals acquired by receivers from ground-penetrating radar signals emitted by transmitters, the transmitters and receivers operating in a multistatic mode, comprising:
a component that calculates a median filtered value for a pixel of the target image based on the value of corresponding pixels in the image frames in the associated sequence;
a component that determines whether pixels in a neighborhood of a target pixel satisfy a filtering criterion based on values of the pixels in the neighborhood and the median filtered values of the pixels in the neighborhood; and
a component that when replacing the value of the target pixel with a value derived from values of the pixels in the neighborhood when the criterion is satisfied.
15. The computing device of claim 14 wherein the component that calculates a difference between values of pixels of the target image frame in the neighborhood and median-filtered values of pixels of the target image frame in the neighborhood and computes a minimum of the difference values within the neighborhood.
16. The computing device of claim 15 wherein the filtering criterion satisfied when a minimum of difference values within the neighborhood is greater than zero.
17. The computing device of claim 14 wherein the replaced value for the target pixel in the target image frame is the calculated median filtered value for the target pixel.
18. The computing device of claim 14 wherein the transmitters and receivers operate in multistatic mode.
19. The computing device of claim 14 wherein the ground-penetrating radar signals are emitted into the ground and including determining whether an explosive device is buried based on analysis of a filtered target image frame.
20. The computing device of claim 14 wherein the target image frame represents a synthetic aperture image generated from real aperture images.