Abstract: In a rotary drilling system using a drill string, and a mud circulating system, in which mud is passed from a pump to a standpipe and drill string, to and through the bit, into the annulus of the borehole, and to the surface, a method of detecting the entry of gas into the mud in the annulus at or near the bit comprising creating and measuring in the high pressure mud conduit a selected type of pressure disturbance, inserting a pressure sensor in the casing near the wellhead to detect the presence of this pressure disturbance after traveling down the mud column in the drill pipe and up through the annulus, and recording these two pressures. Comparison is made of the two pressure traces, from which can be determined the time lag between the initiation, at one sensor, and the reception of the characteristic pressure variations at the other sensor, and also the attenuation of the characteristic pressure variations due to the travel of the elastic waves through the mud column.