a method and apparatus of detecting fractures along the wall of a formation , such as a well formed by test - drilling , provides for the transmission of ultrasonic signals in a direction essentially perpendicular to the wall from a position spaced from the wall by a predetermined distance and the reception of the ultrasonic signals which may be reflected by the wall . the space of predetermined thickness between the transducer which generates the ultrasonic signals and the wall is filled with an intermediate material which does not form a distinct interface with the wall of the type which would be able to produce an appreciable parasitic reflection of the transmitted signals . with this arrangement , if the transducer receives a reflected ultrasonic signal during a time window which begins , following the time of transmission , after a time has elapsed which is essentially equal to the out - and - back travel time of the ultrasonic signals through the space of predetermined thickness , then it will be determined that the wall has no fractures at that point . if no signal is received during the time window , the presence of fractures or small cavities is detected .