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The monopole probe commonly used in acoustic logging can generate vibrations in its natural frequency under the excitation of the pulse signal, and excite transient electromagnetic (TEM) signals of the same frequency. The acoustic probe receives both acoustic and TEM field signals. The TEM field propagates and attenuates in conductive well fluid and formations, carrying formation resistivity information that could be used for formation evaluation. Based on the axisymmetry of the TEM field in an open-hole, theoretical calculation of the TEM field components in an open-hole model was performed. The results revealed that TEM response signals decay rapidly with time, and electric field intensity along the well axis Ez is approximately linear with formation resistivity. On this basis, we proposed a new method for measuring formation resistivity in an open-hole, which could supplement conventional logging methods. Also, it does not affect signal processing of acoustic logging and only applies the TEM signal to determine formation resistivity. The new method could accomplish a more comprehensive petroleum formation evaluation, which is of great significance to the integrated design of the well logging instrument.

Formation resistivity measurement based on a transient electromagnetic field excited by an acoustic probe in an open-hole