Electromagnetic Propagation Logging
Electromagnetic propagation (EM) logging is a logging method used to measure the depth, quantity and direction of electrically conductive materials in the subsurface. The method works by sending electromagnetic waves of specific frequencies through the formations and measuring their signals as they propagate through them. The collected data usually consists of the signal attenuation, phase shift, and the signal frequency but can also include more complex information such as the waveforms shapes and amplitudes.
The electromagnetic field of the EM signals can be generated either from the surface or from e permanent or retractable logging tool. EM signals travel through the subsurface in the form of transverse electric (TE) and transverse magnetic (TM) waves. The waves can propagate with different velocities and attenuations through different formations. These differences in the wave characteristics can be used to determine the formations electrical properties and distinguish different types of formations.
The most common EM signal frequencies used in EM logging are between 0.1 MHz and 1000 MHz. Shorter wavelengths can be used to measure formation conductivity in finer detail, but the signal is often attenuated too quickly to be of useful data. The signal can also be affected by other electric and magnetic fields in the subsurface, such as those produced by oil and gas deposits.
EM logging is often used together with other EEG tools such as induction tools and electric logs in order to provide more detailed subsurface data. EM logging can be used to measure the total thickness of a formation, the geometry of its beds, and detect conductive fractures or faults in the formation. It is also a useful tool for assessing a locations potential for hydrocarbon production.
EM logging is a non-invasive technique that provides useful data on the subsurface and can help geologists to understand the structure and properties of various formations. It is often used in conjunction with other logging techniques to get a better picture of the formation and its geology. This data can then be used to assess a locations potential for hydrocarbon production and help to improve the accuracy of formations overlap evaluations.
