Fine detection of shallow coal mined-out areas by multichannel analysis of surface waves

In order to finely detect the distribution range of shallow coal mined-out areas, a low-frequency focused factor is introduced into the phase shift method, and a low-frequency focused phase shift method is proposed, which has the advantage of extracting low-frequency dispersion energy on a short receivor array.The synthetic test results show that the low-frequency focused phase shift method significantly improves the focus of the dispersion energy at the low frequency, extends the range of frequency bands that can be picked up, and increases the detection depth.Meanwhile, the method shortens the spread length required to calculate the dispersion energy, ensuring that the MASW method has a high lateral resolution.The real data in the shallow coal mined-out area shows that using the low-frequency focused phase shift method in the dispersion energy extraction, the MASW method obtains high-precision horizontal slices of the shear wave velocity and identifies the extent of the mined-out area, the location of the security pillar and its geometry.It confirms the effectiveness of the low-frequency focused phase shift method to improve the lateral resolution of MASW and the feasibility to detect the shallow coal mined-out area finely.