Fraunhofer Institute for Electronic Nano Systems
As part of the Fraunhofer lighthouse project "eHarsh", Fraunhofer ENAS has developed capacitive micromachined ultrasonic transducers (CMUT). CMUTs can be used in borehole televiewers and are capable of operating at an elevated temperature (200 °C or above) for downhole imaging.
The transducers tested in this project were single-element 2D arrays of CMUT cells fabricated at Fraunhofer ENAS using a wafer bonding process. For testing purposes, the ultrasonic transducers were mounted on a printed circuit board and integrated into an oil-filled aluminum housing with a Teflon acoustic window.
Characterizations were performed separately for transmit and receive performance in oil and in a laboratory-made drilling mud (typically a mixture of bentonite and water). In transmit mode, a calibrated hydrophone was used to measure the sound pressure produced by the transducer. The results showed that the transducer could provide measurable signals at distances greater than 5 cm, even when immersed in a laboratory made drilling mud. In receive mode, a calibrated sound wave transmitter (a piezo disc) was used and the response of the transducer was recorded, amplified by a matched receive circuit.