To enhance the robustness of the 3D shear wave inversion results, the checkerboard resolution tests are performed to assess the lateral resolution and the procedure is similar to what Fang et al (2015) described. The 3D reference velocity model is constructed based on the CPS model in Fig. 4b. After several comparative tests, the study region is meshed with 25 by 25 grid points with an interval of 0.01°in latitude and longitude. And 18 grid nodes are set along the depth direction from the ground surface to 13 km underground, according to the interface depth of the velocity structure.
Theoretically, the recovery ability of checkerboards depends primarily on the surface-wave ray-path coverage. Thus, we first check the surface-wave ray-path coverage at different periods and then conduct the checkerboard resolution test. Figure 7 shows the path coverage of group velocity measurements at the 6 selected periods. The result shows that the surface-wave path coverage density of the study region is relatively high, indicating the data set can well resolve the structure of the study region. The ray-path number decreases with the period's increase, such as the ray-path number at 4 s period is only about 27% of that at 1.5 s period.
Figure 8 shows the recovery results of the lateral checkerboard at different depths. The recovery results of the lateral checkerboard show that the recovery of the detection board is affected by the ray-path coverage density(Fig. 8). The checkerboard test is well recovered in the area with dense ray-path coverage and the recovery is poorly in the area with poor ray-path coverage. Overall, the Anyuan mining area and its adjacent region are well recovered with an anomaly size is approximately 0.02°×0.02°.
During the inversion, we used the same velocity model and the parameters as that in the checkerboard resolution test. The inversion process is relatively stable and the root-mean-square (RMS) value of surface-wave travel-time residuals decreases with the increase of iterations times as Fig. 9 shows. The RMS value of travel-time residuals decreases quickly at the first three iterations, and then it decreases slowly and finally converges. After 10 times iterations, the average travel time residuals decreased from 0.454s to 0.354s, which presents that the final velocity model fits the observed data better.
The coal-bearing strata of the study region belong to the Anyuan Formation of the Upper Triassic, which can be divided into Zijiacheng member, Sanjiacheng member, and Sanqiutian member from bottom to top (Fig. 10). The lithofacies assemblages of the Anyuan Formation are mainly gray to light gray, light purple sandstone, siltstone, sandy shale, and carbonaceous shale (Li et al., 2016). And the sedimentary are mainly Marine-continental transitional facies and the coal seams are characterized by multiple layers, complex structures, and uneven distribution (Liu et al., 2021).
Figure 11 shows the shear wave velocity structure maps at different depths from the direct surface inversion of group velocity dispersion data. The patterns show that the shear wave velocity structure varies greatly at the same depth, which reveals that the shallow crustal structure of the study region has strong lateral inhomogeneous. At the depth of 0.4 km, underground, the shear wave velocity structure in the basin areas and coal mining areas shows low-velocity anomalies, while the uplifted mountain areas without coal mine distribution exhibit high-velocity anomalies. Generally speaking, the low-velocity anomalies of shear waves in the basin areas could be attributed to the sedimentary layer in the ground surface and the high-velocity anomalies of shear waves in the uplifted mountain areas could be interpreted as sedimentary layers missing or bedrock exposing. However, the Anyuan mining area is located in the uplifted mountainous, but its shear wave velocity structure exhibits low-velocity anomalies. This abnormal phenomenon could be related to the sedimentary environment of coal seam and coal mining activities. The Sedimentary environment of the Anyuan mining area and its adjacent regions are Marine and continental facies interaction with sedimentary basins, containing large amounts of sand and silt (Xi et al.,1999; Li et al., 2003).In addition, the mining history of the Anyuan coal mine have over one hundred years, so a lot of coalmine gob area and coalbed methane would be produced in the mining process (Li et al.,2003; Qiu et al.,2019; Xi et al., 2020). All of these factors could cause the shear wave velocity structure of the Anyuan mining area present low-velocity anomalies.
The distribution of shear wave velocity structure in the depth ranges of 1.2–2.2 km exhibits similar characteristics. The Anyuan and Gaokeng mining areas still show low-velocity anomalies, but the range of the anomalies decreases with the increase of depth, and the distribution of anomalies gradually approaches the WKF and AYF zone. We also find that the shear wave velocity around the AYF zone and the piedmont basin form an obvious "Y-shaped" low-velocity anomalies zone. Overall, in the depth ranges of 0.4–1.2 km underground, the shear wave velocity of the whole Anyuan mining area presents obvious low-velocity anomalies. However, at the depth of 2.2 km underground, the shear wave velocity structure of the Anyuan mining area is bounded by the WKF zone. The shear wave velocity structure in the northern part of the mining area still presents low-velocity anomalies, while its changes to high-velocity anomalies gradually in the southern part, which is consistent with the distribution characteristic of shear wave velocity structure in high mountain areas. These phenomena may indicate the mining depth of the Anyuan mining area has reached ~ 1.2 km underground.
At the depth of 3.3 km underground, the shear wave velocity structure is well consistent with the surface and topographic features. The shear wave velocity structure in the mountainous area with higher terrain exhibit high-velocity anomalies, such as the area between LJCF and AYF zone, and the area in the south of the WKF zone, while its present low-velocity anomalies in the western basin and piedmont basin areas. It is worth noting that, although the AYF and WKF zone are located on the high mountain, the shear wave velocity structure between the faults zone still shows low-velocity anomalies. The exploration of oil and gas resources works(Li et al., 2003; Qiu et al., 2019)in the Leping depression of Jiangxi Province has proved that there are rich Coalbed methane resources beneath Anyuan mining and its adjacent region. Thus, the low-velocity anomalies of the shear wave at the depths below 2 km underground around the AYF and WKF areas may be a reflection of the presence of oil and gas between the faults zone.
Figure 12 shows the positions of the four sections and their corresponding shear wave velocity profile. AA’ profile shows the shear wave velocity of the basin area in the west of the AYF is characterized by low-velocity anomalies, and the depths of the low-velocity anomalies are negatively correlated with the surface elevation. Along the profiles, AA'and CC'the shear wave velocity structure change sharply across the AYF, and the velocity transition interface is inclined toward the northwest, which is consistent with the tendency of AYF (Tan et al., 2001; Liu et al., 2004). We also find the depths of the velocity transition interface reach ~ 3 km underground, and this may indicate the extension depth of AYF. The profiles BB'and CC' show that there is an obvious low-velocity anomaly in the northwest side of the AYF, which may be caused by the piedmont flood plain. Because the piedmont flood plain contains a large amount of sediment and detrital materials (Tan et al., 2001; Li et al., 2016), which could greatly reduce the propagation speed of the shear wave.
Along the profiles AA’, BB’, CC', and DD’, it can be seen that the shear wave velocity structure in the shallow surface layer (< 1.2km) presents low-velocity abnormal, which could relate to the sedimentary environment of the coal seam. Xi et al. (1999) and Li et al (2003) suggested that the Anyuan mining area and its adjacent region are the main composed sea-land interaction deposition and the maximum deposition thickness is up to 1.3 km. Meanwhile, the low-velocity anomalies of the shallow surface layer(<1.2km)in the Baiyuan, Anyuan, and Gaokeng mine areas are particularly significant and relate to coal mining activities. Such as coalmine gob area and coalbed methane would be produced in the process of coal mining and both of them can slow down the speed of the shear wave. It also can be seen that the low-velocity anomalies around the AYF and WKF zone are down to ~ 3 km in depth (Fig. 11 & Fig. 12) and these could be related to the fracture media, oil, and gas existing in the fault zone. The top coal mining area is often swamp facies or tidal flat facies mudstone and silty mudstone, which has a strong sealing ability for coal reservoirs (Qiu et al., 2019). Thus, the oil and gas produced during Coal mining could penetrate the faults and the gaps between the faults zone, resulting in a low-velocity anomaly.