The purpose of this paper is to investigate the feasibility of using seismic velocity measurements to estimate stress underground. An in-seam seismic survey was conducted in the Dutch Creek mine in the Coal Basin of Colorado. Seismic waves were transmitted under a gob produced by an advancing longwall panel. Stress levels were evaluated from seismic velocities in both the high velocity wall rock and in a coal seam located under the gob. To measure stress in the wall, velocity was determined from the first arriving refracted waves. To measure stress in the coal, velocity was determined from SH channel waves. Stress levels were evaluated from velocities using empirical relationships defined from ultrasonic analysis on core samples.
UNDERGROUND SEISMIC SURVEY
The Coal Basin coal interval is about 26 ft thick consisting of 11 ft of coal at the bottom (A bed) and another 10 ft of coal at the top (B bed). Separating the two coal seams is carbonaceous siltstone approximately 5 ft thick. The roof rock above the coal interval consists mostly of siltstone. The floor rock is a massive medium grained sandstone, known as the Rollins Sandstone. Longwall mining operations in the B bed are under approximately 2600 ft of overburden. A detailed description of the geology of the Coal Basin coal interval is given by Collins (1977).
The focus of this study was a seismic survey conducted around the gob produced by advancing Longwall 101 (Reeves 1984). To perform the survey, access to the gob was gained by the headgate and tailgate roadways that run from the main entry into the mine to the longwall face. The gate roads are separated from the caved zone or gob by concrete packwalls fabricated during mining to support the roof. Figure 1 shows the geometry of the source and receiver locations for the underground seismic survey. The thirty-six separate ray paths analyzed by the study are shown. Twelve vertical receiver holes (numbered 1 to 12) were drilled into the headgate roadway and thirty-six source holes (numbered 1 to 36) were drilled into the tailgate. The source to receiver offsets for each ray path were measured to approximately ñ1.8 ft. The offsets range between 508 to 533 ft.