The researchers installed inclinometers in an underground coal mine to measure horizontal shear movements in the overburden strata ahead of longwall mining. They found that shear movements developed on a single horizon near the top of the conglomerate strata, correlating closely across two sites. These movements occurred almost immediately after the longwall goaf was formed, about 425 meters ahead of the longwall face. This suggests a consistent mechanism across a large area of the mine.