A circular electrical dipole creates a field consisting only of a TM mode. This results in enhanced sensitivity towards thin resistive layers. As the B-field components are zero over layered cases their measurement results in an exceptional sensitivity towards 3D structures. In field experiments over 10 hydrocarbon reservoirs unusually strong vertical magnetic field components have been observed that coincide with the known extensions of the reservoirs.
Within the last decade Electromagnetic (EM) methods have drawn increasing attention within the oil & gas industry. This is primarily due to the introduction of marine CSEM as a tool to detect and delineate thin resistors not seen with more traditional methods like Magnetotellurics (MT). While early euphoria and claims based on the direct hydrocarbon indication capability of the method were found to be not sustainable CSEM’s ability to detect thin resistors and the usefulness of EM methods in oil &gas exploration has been proven in many cases. (Hesthammer et al, 2010). One key distinguishing factor between CSEM and other forms of EM methods is that the horizontal line source used in CSEM creates a field that comprises of TE and TM mode. Other methods like MT or TEM using a loop source basically create a horizontal eddy current pattern and mainly sense the horizontal resistivity of the subsurface. In a layered Earth such methods only create a TE mode. A thin resistive layer embedded in a conductive background is seen by horizontal currents as a parallel circuit of the resistor and the surrounding conductors in which the resistor contributes little to nothing to the overall resistivity. This feature of TE mode based EM methods is typically described as detection of the transverse conductance rather than the individual conductivities of a particular layer. TE mode methods have been proven to be useful in basin characterization and background characterization for salt bodies. They however cannot provide resistivity information of the reservoir itself. The field of the horizontal electrical dipole used as source by CSEM methods contains in the case of a layered earth TE and TM mode and the TM mode creates a vertical current flow. Contrary to the horizontal currents vertical currents see the sequence of 1D layers as a series of resistors and by that even a thin resistive layer may cause a measurable effect. As the TM mode is superior in delineating thin resistive targets it is desirable to measure the TM mode in the absence of any additional TE mode. The idea of using vertical bipole sources is not new. The basic principles have been described in the various publications on the MOSES experiments in the early 1980th see Chave. et al. 1991 Or in an even earlier Soviet Union patent from Nazarenko (1961). Analytical and numerical studies for the land case have been published by Goldman (1990) and Pellerin and Hohmann (1995). While the earlier studies focus on low frequencies or DC more recent work uses transient vertical sources in the marine environment (Barsukov et al., 2005).