EM Methods

Measurements from controlled source electromagnetics (CSEM) and magnetotellurics (MT) can provide valuable information of the target area.

CSEM

CSEM uses an active source that transmits a low frequency EM field into the subsurface. In the marine environment, the high powered source is a horizontal electric dipole towed approximately 30 m above the sea floor. On land, the active source is usually a large grounded dipole powered by a strong generator. In both cases, the source transmits a carefully-designed waveform intended to produce the best signal/noise for the target geological conditions.

The EM fields propagate through the earth and are measured by high-sensitivity receivers placed on the sea floor (marine) or on the ground (land). The measured data contain valuable information about the subsurface resistivity, which reflects changes in lithology, porosity, and fluid content.

The data are interpreted by inversion, usually with the objective of characterizing more resistive units which might be associated with the presence of hydrocarbons. This helps to identify potential reservoir fluid contents ahead of the drill bit.

Typically, a potential reservoir is identified with seismic data. CSEM is then used to analyze its resistivity, taking advantage of the resistivity contrast between oil or gas-saturated rocks and those with significant water content.

MT

The MT method uses passive low-frequency, plane-wave energy, generated by sources such as solar wind and lightning. These fields generate secondary EM fields in the earth which are measured with extremely sensitive receivers to reveal information relating to the resistivity distribution as a function of depth and lateral position. This is obtained through inversion and modeling of the field data.

MT is able to determine average formation resistivity to depths greater than 10 km, with a precision of typically 5% to 10% of depth. For this reason MT is usually considered to be a broad-brush reconnaissance tool for identifying gross rock types and thicknesses at large scales. MT is particularly useful in more difficult seismic areas such as sub-salt, sub-basalt, sub-carbonate, and overthrust regions, as well as in geothermal and mineral exploration. MT, like CSEM, is applied both on land and offshore.