MULTI-TRANSIENT ELECTROMAGNETICS MTEM

The MTEM (Multi-Transient Electro-Magnetics) technique produces resistivity profiles over prospective reservoirs to identify hydrocarbons before beginning costly drilling programs. MTEM combines seismic and electro-magnetic survey techniques to generate data that are processed to produce absolute resistivities as a function of depth.

The principle behind MTEM technology is straightforward: by rapidly reversing the voltage between two electrodes, a transient is directed into the earth and the earth’s impulse response is recorded between receiver electrodes. Proprietary processing produces a resistivity cross section of the earth, with high resistors indicating hydrocarbons.

Data can be recorded in either onshore or offshore environments. Onshore includes a series of source and receiver electrodes that are pushed into the ground at surveyed positions. The source is connected to two source electrodes, while pairs of receiver electrodes are connected to MTEM acquisition units. The source is fired and the voltage across the pairs of receiver electrodes is recorded. A key feature of the patented technique is that the source and receiver performance for every shot is measured and that recording is used as the first step in processing.

Once data are recorded, the voltage is reversed, producing the next MTEM transient and acquiring another shot. Proprietary software calculates the signal to noise ratio while data is collected, enabling the operator to know in real time when sufficient data for that area has been collected. The source and receiver electrodes are then moved down the line to build up the subsurface picture.

Reciprocity allows the technique to be used in obstructed areas to overcome natural or manmade obstructions (e.g. rivers). Real-time appraisal technique provides a first look at the reservoir in the field within 24 hours of data collection. After noise removal, deconvolution, stacking, and conversion to log-time, MTEM data look much like seismic data. By picking the arrival from the subsurface, the travel time for each sourcereceiver pair is determined in a similar manner to velocity picking in seismic processing.

Mapping produces a plot of resistivity against depth to confirm the presence or absence of resistors such as petroleum. State of the art inversion methods using travel times and absolute amplitudes can be used to produce more advanced products. MTEM can thus be a direct hydrocarbon indicator, in contrast to seismic, which primarily detects lithology.