| Popis: |
Controlled Source Electro-Magnetic imaging (CSEM) is a recently developed technology that maps subsurface resistivity variations (Eidesmo et al., 2002). It uses a horizontal electrical dipole (HED) which emits a low frequency electromagnetic (EM) signal into the underlying seabed and downwards. As the upper sediments are effectively partial conductors, the penetration of EM fields is limited by the so-called skin-depth. In practice this means that the technique requires a low frequency EM source, typically between 0.25 – 10 Hz, to allow penetration to about 2500-3000m into the subsurface. At low frequencies, Maxwell’s equations for the electric field component reduce effectively to a diffusion process leading to strong dispersion. In the presence of an anomalous resistor the dispersion is distorted, which may be considered analogous to seismic refraction although it is physically different. Energy is constantly refracted back to the seafloor and is detected by receivers placed on the sea floor (figure 1). When the source-receiver distance (offset) is larger than the depth of reservoir burial, energy from the resistive layer will dominate the directly transmitted energy. The detection of this ‘guided and refracted’ energy is the basis of CSEM. Archie’s law indicates that the method is more sensitive to high saturation hydrocarbon pore fill and due to the low frequency nature only relatively large accumulations of high saturation will be detectable. |
