Abstract: A method of determining information inside a borehole is provided. The method includes the use of a ferrofluid seal to seal a rotating shaft used in the borehole. In preferred embodiments, the method comprises: selecting at least one sensor to be lowered into the borehole; coupling the sensor to the end of a rotating shaft; running the rotating shaft through a housing including a plurality of bearings and an oil reservoir, sealing a downhole end of the housing from an exterior of the housing with a ferrofluid seal; and forcing the sensor, rotating shaft, housing, bearings, oil reservoir, and ferrofluid seal into the borehole to make a measurement.

Abstract: Among other things, there are disclosed embodiments of a well-logging tool that inspects features of the down-hole environment using ultrasonic signals with a frequency in the range of 3-5 MHz. The ultrasonic signals are encoded, and their time-of-flight and amplitude provide information on features of interior surface of the casing on the order of 1 mm, and on the quality of the cement bond behind the casing.

Abstract: Among other things, there are disclosed embodiments of a well-logging tool that inspects features of the down-hole environment using ultrasonic signals with a frequency in the range of 3-5 MHz. The ultrasonic signals are encoded, and their time-of-flight and amplitude provide information on features of interior surface of the casing on the order of 1 mm, and on the quality of the cement bond behind the casing.

Abstract: Among other things, there are disclosed embodiments of devices, suitable for incorporation into well-logging tools, for the in situ measurement of the acoustic impedance of a fluid in the well bore, such as drilling mud. The device includes a sonic emitter that emits sonic pulses into a solid probe. The pulses are partially transmitted and partially reflected at opposite surfaces, and the decay of the amplitude of successive sonic pulses is used to determine the acoustic impedance of the fluid.