There is disclosed in one implementation a method of or for use in or for detecting, measuring and/or determining at least one variable or characteristic in a space, such as a well, container or vessel. In one implementation the method comprises: transmitting a first electromagnetic signal from a first position to a feature within the space; receiving a second electromagnetic signal at a second position after reflection of the transmitted first electromagnetic signal from the feature; transmitting a third electromagnetic signal from a third position to a calibration feature within the space; receiving a fourth electromagnetic signal at a fourth position after reflection of the transmitted third electromagnetic signal from the calibration feature. The method further comprises: subsequently transmitting a further first electromagnetic signal from the first portion to the feature; receiving a further second electromagnetic signal at the second position after reflection of the transmitted further first electromagnetic signal from the feature; transmitting a further third electromagnetic signal from the third position to the calibration feature; receiving a further fourth electromagnetic signal at the fourth position after reflection of the transmitted further third electromagnetic signal from the calibration feature. In so doing one can determining (the) at least one variable or characteristics from a difference or variation in time between the transmission of the first electromagnetic signal and reception of the second electromagnetic signal and the transmission of the further first electromagnetic signal and receipt of the further second electromagnetic signal and a difference or variation in time between the transmission of the third electromagnetic signal and receipt of the fourth electromagnetic signal and the transmission of the further third electromagnetic signal and receipt of the further fourth electromagnetic signal.

Certain aspects of the present disclosure relate to calibrating a formation signal for a mixed set measurement device. A first air-hang response of a first measurement device for a tubular string associated with a drilling operation can be measured. A second air-hang response of a second measurement device for the tubular string associated with the drilling operation can be measured. The first air-hang response and the second air-hang response can be used to determine a mixed air-hang response of the mixed set measurement device. The mixed air-hang response can be used to calibrate a formation signal for the mixed set measurement device.

A method of calculating the temperature of a geological structure is disclosed, wherein there is provided a magnetic parameter of the geological structure. The method includes inverting the magnetic parameter to estimate the temperature of the geological structure.

A method of calculating the temperature of a geological structure is disclosed, wherein there is provided a magnetic parameter of the geological structure. The method includes inverting the magnetic parameter to estimate the temperature of the geological structure.

Electromagnetic logging tools to inspect nested pipes using hybrid frequency- and time-domain logging techniques are disclosed. The disclosed logging system and processing workflows combine frequency-domain and time-domain techniques to overcome the disadvantages of using frequency-domain or time domain tools in isolation.

An apparatus for ground penetrating radar includes an antenna disposed within a sleeve. The sleeve includes a radar-absorbing material for attenuating the amplitude of incident radar waves. The sleeve has an aperture for permitting radar waves ω pass into and out of the sleeve. A method for surveying a formation using ground penetrating radar includes rotating the antenna and sleeve while keeping the antenna and sleeve longitudinally stationary, and recording data including the amplitude of waves received by the antenna and the position of the aperture when such waves are received.

Systems, methods, and computer-readable media for in-situ calibration of magnetic field measurements. In some examples, a method can involve generating a magnetic field via a magnetic field source that is coupled to a downhole tool. The magnetic field source can be located within a fixed distance from one or more sensors coupled to the downhole tool. The method can also involve obtaining respective field measurements of the known magnetic field from the one or more sensors, and comparing the respective field measurements from the one or more sensors with respective reference measurements previously obtained from the one or more sensors to yield respective comparisons. The method can then involve determining, based on the respective comparisons, a respective sensitivity drift for each of the one or more sensors.

Systems, methods, and computer-readable media for in-situ calibration of magnetic field measurements. In some examples, a method can involve generating a magnetic field via a magnetic field source that is coupled to a downhole tool. The magnetic field source can be located within a fixed distance from one or more sensors coupled to the downhole tool. The method can also involve obtaining respective field measurements of the known magnetic field from the one or more sensors, and comparing the respective field measurements from the one or more sensors with respective reference measurements previously obtained from the one or more sensors to yield respective comparisons. The method can then involve determining, based on the respective comparisons, a respective sensitivity drift for each of the one or more sensors.

Detecting discontinuities in a well casing according to some aspects includes actively monitoring the electric current response in an antenna or in antennas during the insertion of a tubing string. The method does not require permanent magnets, or any hardware elements beyond those that are already present in a typical behind-casing measurement system. A system according to some aspects includes an antenna that is mounted in or on a tubing string and a voltage source that can be connected to the antenna. A processing device is connected to the antenna and the voltage source. The processing device applies an AC voltage to the antenna and monitors a current generated in the antenna while the AC voltage is being applied.

A method includes generating a ranging model of a drilling wellbore to be drilled and generating a predicted signal along measured depths of the drilling wellbore based on the ranging model. The method includes performing the following operations until the drilling wellbore has been drilled to a defined depth. The following operations include drilling, with a drill string, the drilling wellbore to an increment of the defined depth and detecting, by a sensor positioned on the drill string, an electromagnetic field emanating from a target wellbore. The following operations include determining ranging measurements to the target wellbore at the increment based on the electromagnetic field and calibrating the predicted signal based on the ranging measurements. The following operations include determining ranging accuracy for all deeper depths in the wellbore and making drilling decisions or adjusting drilling operations based on the predicted ranging accuracy for deeper depths.