A seismic tool assembly for effecting a drill hole and enlarging a lowermost end thereof whereby an anchor can be set more securely therein. A drill bit is inserted through an opening in a drill stop member, which is mounted on a stabilizer inserted at least partially into the hole. The bit is then inserted through each of the drill stop member and the stabilizer, to attain a drilling position at the lowermost end of the hole. The lower end of the bit has a cutting portion having a cutting radius greater than that of the bit body whereby, when the bit is inserted fully into a lowermost end of the hole and rotated, the cutting portion enlarges the lowermost end, whereby flanged portions of an anchor will rest within the enlarged area, retaining the anchor in place. An upper end of the drill stop member has a narrower opening for receiving the insertion of the bit body therein and the lower end of the drill stop member has an opening greater than that of the upper end, so as to permit limited tilting movement of the bit body within the drill stop member during the drilling process to further enlarge the lowermost end of the hole.