4.1. DSM Results and Regional Overall Deformation
shows the ASTER GDEM and 10 m resolution DSMs from ZY-3 stereo images in the study area using the method described in Section 3
. Table 2
presents the statistical results of multi-temporal DSMs using the method displayed in Figure 4
. After adjustment by ground points, the root mean square (RMS) of DSM elevation in the region acquired in 2015 is reduced from 5.21 m to 1.72 m. The RMS is decreased to 2.11 m from the original 11.29 m in 2014, whereas the RMS decreases to 4.04 m from the original 28.71 m in 2013.
shows the overall ground deformation using the elevation difference between ZY-3 DSMs and the ASTER GDEM. Table 3
displays statistical results of elevation changes with the minimum, maximum, mean, and standard deviation values. The table shows that the elevations from ZY-3 DSMs are higher than those from ASTER GDEM, and the mean elevation changes by 8.38 m, 6.61 m, and 3.12 m in 2013, 2014, and 2015, respectively. Due to the backfilling process while mining in this area, a deformation of more than 100 m is found, especially in the active mining area, where the slope slides steeply and the pit backfill rises on the other side.
From the results, the terrain accuracy of DSM in 2015 is significantly higher than that in 2013 and 2014.The mean RMS of DSMs in 2013–2015 is 2.62 m, and the average difference between ZY-3 DSM (2015) and ASTER GDEM is 3.12 m, so the threshold for height change is set as 3 m. DSM in 2015 is used in the analysis of deformation. Table 4
shows the statistical results of elevation changes between ZY-3 DSM in 2015 and ASTER GDEM. The results show that 71.18% of this area is deformed with the threshold, and 22.27% of the area has an elevation variation of more than 10 m. Further, 30.72% of the area shows the descending elevation phenomenon, whereas 69.28% of the area shows a rising elevation change.
4.2. Typical Region Deformation
Typical region deformation means the deformation in the typical regions in the study area. To analyze the elevation and volume changes in particular local areas, five typical regions consisting of mining pits and piling fields displayed in Figure 1
are chosen to make a detailed analysis using ZY-3 DSM in 2015 and ASTER GDEM. Table 5
presents the statistical results of deformation in the five typical regions.
The districts with obvious elevation changes are extracted using Equation (3) to obtain more detailed distribution of height increase and decrease. If the elevation of pixel (i
) increases or decreases beyond the threshold of change, it is considered an obvious elevation change. The descending threshold of obvious elevation changes δ↓
is set as −10 m, which is nearly one standard deviation of elevation changes (11.18 m in Table 3
) between 2015 ZY-3 DSM and ASTER GDEM. Considering the impact of surface construction and vegetation, the rising threshold δ↑
is set as 15 m, which is larger than δ↓.
. Figure 7
displays the distribution of seven districts with obvious elevation changes. Table 6
shows the statistical results with obvious elevation changes using ZY-3 DSM in 2015 and ASTER GDEM. Four districts of rising elevation (red polygons) and three districts of descending elevation (green polygons) have been extracted. Districts 1–4 show a rising elevation and Districts 5–7 show a descending elevation, which are all marked with numbers in Figure 7
(1) Deformation in the west open-pit mine. The average elevation change is observed to be 5.56 m in the west open-pit mine and the total ground volume change is 9.50 × 107 m3 in the range of 17.08 km2. The west open-pit mine is now in the pit closed stage; backfill work has been started with mining exploitation only in a small area. This has led to the elevation increase on average and the gradual uplift of the bottom.
Districts 4–6 in Figure 7
are all in the west open-pit mine. Elevation increase and decrease are mixed together (Figure 8
). District 4, with rising elevation, is mainly caused by backfilling. The results show that the backfill area is about 3.57 km2
, the volume of backfill is about 1.37 × 108
, and the average rising elevation is about 36.35 m. The descending area (Districts 5 and 6) is mainly in the north and south pits. District 5 is located to the north of the slope, where small-scale mining is still continuing. At the same time, some small landslides often occurred. Therefore, the results show that the elevation is reduced by 41.77 m on average, the significant descending area is about 1.15 km2
, and the descending volume is about 4.81 × 107
However, the descending area of District 6 is located to the south of the slope, where the landslide phenomenon has recently become more serious due to its long-term mining operation. The occupied area is about 1.10 km2
, the average elevation is reduced by 28.66 m, and the descending volume is about 3.14 × 107
. The results are consistent with those of previous studies [5
]. The results show that the speed of the deformation process is rapid; range and amplitude are also large. It is noteworthy that the large deformation in the southern slope is in the form of relatively small fissures at the beginning; obvious fissures were found in 2013 (Figure 2
], and by July 2014, the fissure already stretched with a length of more than 3 km, a maximum width of about 40 m, and a maximum drop of about 20 m (Figure 2
b). By 2015, the landslide deformation had become quite clear based on the DSM results.
(2) Deformation in east open-pit mine. The average elevation change is observed to be −15.40 m in the east open-pit mine. In the range of 4.68 km2
, the total ground volume change is −7.20 × 107
. The east open-pit mine is still in the normal mining stage and the amount of dumping has increased after 2015 [41
]. Due to its normal mining activities, District 7 in the east open-pit mine is the largest descending area (about 3.87 km2
). The average elevation is reduced by 31.24 m, and the volume is decreased by about 1.21 × 108
, which is larger than the total volume change (−7.20 × 107
). A possible reason for this is that one part of the waste rocks is stacked in the mining area and the other part is dumped to the piling fields outside.
A small district (District 3), with an area of about 0.86 km2
to the north of the east open-pit mine, was an underground mine area previously [9
] and the surface is reclaimed now, resulting in a rise in elevation.
(3) Deformation in three waste rocks fields. Elevations in the west waste rocks field, east waste rocks field, and Wangliang waste rocks field all increased with average values 3.33 m, 16.78 m, and 10.81 m, respectively. For the waste rocks dumped from the east open-pit mine, elevations show a significant increase in the east waste rocks field and Wangliang waste rocks field, as well as volume changes. The heights in Districts 1 and 2 in the east waste rocks field and Wangliang waste rocks field increase obviously. The area of the two rising districts is 2.90 km2 and 1.11 km2 and the volume change is 6.91 × 107 m3 and 2.40 × 107 m3, respectively. Since the dump area is not uniformly distributed, the volume change in District 1 is greater than the entire volume change in the east waste rocks field.