for the cement slurry into hollow area under earth mantle 灌入地幔之下空洞的水泥浆

as you have oil digging facilities, just reverse, and pump the cement slurry (density 50%) into the hollow. then measure the depth between the water surface and mantle. underground river is good for the soil, and will change desert to soil.
it is HUGE investment, which requires more than a lot LARGE amount of the cement slurry into the hollow area.
因为各国都有石油开采设备，只需要逆向修改设计，把水泥浆（密度50%）灌入地心空洞。测量水面到地幔层的高度。地下河对会改善土壤，可以将沙漠改善为良田。
这是庞大的投资，很庞大的地下空洞，需要灌入大量的水泥浆。