Abstract An ultrasonic tool has been developed for MWD applications that simultaneously performs realtime bore-hole caliper measurements and realtime gas influx detection. The caliper determines the ovalness of the hole with a high degree of accuracy and excellent vertical resolution. These measurements are transmitted to the surface and also used to compensate LWD measurements. Furthermore, with early and accurate real-time caliper measurements, bore-hole instability can be detected. Real-time gas influx detection insures early and reliable gas kick detection and should provide a considerable improvement in drilling rig safety. The new tool is particularly useful for avoiding problems associated with shallow gas formation where response time for kick detection is critical.