The detection of fractures in the wall of a test drilling is currently accomplished using at least one of the following methods: (1) the detection of losses of mud; (2) the interpretation and correlation of porosity measurements, and, in particular, the correlation of porosity measurements provided by low-frequency (20 kHz) sonic logging and neutron logging; (3)the interpretation of anomalies in signals provided by sonic logging or microsonic (300 kHz) logging.
The last of the above-mentioned methods of logging involves transmitting a sonic wave through strata of variable thickness (from 15 to 90 cm), and the variation in the amplitudes of the received signal is dependent not only upon the presence of any fractures, but also on many other parameters, such as porosity, longitudinal and transverse speeds, input and return angles of incidence, intercalations, and the like, in such a way that the interpretation of these types of logging is complex and provides only indirect indications.
British Pat. No. 1,031,200 has already proposed a method by which ultrasonic pulses are sent into a liquid-filled well in a direction which is essentially perpendicular to the axis of the well and the reflected pulses are received along the transmission direction. This patent indicates that the amplitudes and shapes of the reflected waves may provide in particular indications of the existence and locations of cracks in the wall of the well. This patent does not give, however, any precise indication of how these cracks could be detected and describes no means of obtaining sufficient resolution in the detection of the cracks.
U.S. Pat. No. 3,175,639 has also described a well-logging method which also includes the transmission of ultrasonic pulses in a direction which is essentially perpendicular to the axis of a wall and the reception of the reflected ultrasonic pulses along the transmission direction, utilizing an ultrasonic transducer which is applied in direct contact to the cake of mud which covers the wall of the well or which is separated from this case by a known thickness of mud s. The purpose of this method, however, is to determine the porosity and permeability of the geological formation surrounding a test well and concerns itself with the interface between the cake of mud which covers the well wall and the formations which are flooded with water in the vicinity of the well wall. This method, which utilizes a transmission frequency on the order of 10 kHz, is based on determining the acoustic impedance of the water-flooded zone and is in no way suited for precise detection of cracks in the wall of the well.
U.S. Pat. Nos. 3,511,334 and 3,369,626 disclose a logging tool having a rotating component which transmits and receives acoustic signals with a frequency on the order of 1-2 MHz perpendicular to the interface being studied. The reflected signals modulate the intensity of the beam of an oscilloscope, and the absence of a reflected signal in response to a transmitted acoustic signal is interpreted to mean that there is an anomaly in the wall of the well. Such a logging tool has the drawback that in practice it is very sensitive to variations in the diameter of a test drilling because it utilizes a rotating transmitter set which is positioned on the axis of the test drilling, i.e., a transmitter which is spaced relatively far from the wall. The time between transmission and reception thus depends on the variations in the diameter of the test drilling and is affected by mistakes in centering the transmitter-receiver component. In addition, the attachment of this component to a vibrating metal part of the probe body causes significant background noise which disrupts the measurements. Finally, the active surface of the transducer facing the wall of the test drilling is covered by an epoxy resin which creates a break in acoustic impedance and consequently causes multiple echoes upon reception. The existence of this epoxy layer, the acoustic impedance of which is not related to that of the fluid contained in the well (drilling mud), also reduces the acoustic intensity which is transmitted toward the wall of the well.
From French Patent Application No. 2,486,997, there is also known a dip method which utilizes variations in echo amplitudes and correlations of these amplitudes.
Finally, the prior state of the art can be illustrated by U.S. Pat. Nos. 3,356,177 and 4,168,583 which employ a transmitter and receiver which are separated from one another, and U.S. Pat. No. 3,688,569 which employs a component which transmits signals along a direction which is not perpendicular to the surface to be examined.