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The article discusses research which assessed the behavior of downhole electromagnetic telemetry (EMT) in deep subsurface prospecting and the characteristics of uphole EMT with repeater system. Topics covered include the performance of downhole EMT during sudden water influx into adjacent formation during directional drilling, the characteristics of downhole EMT in deep horizontal well, and the behaviors of the EMT system with oil based mud (OBM) and water based mud (WBM).

Three emerging techniques for surface electromagnetic (EM) exploration with high detection accuracy and large depth of investigation, namely, WEM (wireless electromagnetic method), MTEM (multi‐channel transient electromagnetic method) and SOTEM (short‐offset grounded‐wire TEM), are introduced. In detail, we proposed a newly developed sky‐wave theory in which EM wave can propagate in full space of ionosphere‐atmosphere‐lithosphere. Correspondingly, we developed the WEM recording system to measure the sky‐wave to obtain the structures down to ∼10 km depth. We then introduced MTEM as a highly effective method for mineral exploration with target depth of 3km. Using self‐developed MTEM system, we presented the results obtained from mineral exploration study carried out in China. Lastly, we proposed the SOTEM method for a greater detection depth (1.5km) than traditional loop source TEM (500m) with a small offset and high resolution. [ABSTRACT FROM AUTHOR]

Electromagnetic telemetry is bi‐directional transmission of borehole data for effective drilling, especially in unconventional wells. In this study, numerical simulations was used to investigate the possibility of applying EM telemetry for measurement while drilling (MWD) in an ultra‐deep borehole in the Tahe region of the Tarim basin, using the drill string as an antenna excited as a gaped dipole source to quantify spatial variations of the electromagnetic field from subsurface to the surface. Results confirm that the distribution of the electric field along the drill string is sensitive to vertical variations of electric conductivity in the host rock. The simulated results show a high attenuation within the conductive formations, thereby limiting the effectiveness of EM telemetry in data transfer to the surface. In addition, the changes in thickness and resistivity of the target region affects the amplitude of the desired response. [ABSTRACT FROM AUTHOR]