WELLOG                                         TRANSIENT EM

 

 

 

Transient EM (TEM) provides exploration companies with a valuable tool for defining conductive mineralized zones. Because resistivity (the inverse of conductivity) can be measured, the method may also be applied to location of hydrocarbon contaminant plumes.

 

In most applications, however, the method is applied to locating buried conductive masses. Examples of buried conductive masses can vary from metal contaminants below the surface, valuable metal resources including gold placer and lode deposits, buried underground storage tanks and well casings. Larger scale applications are directed toward location of massive sulfide deposits.

 

A typical TEM system uses a transmitter that transmits short electromagnetic pulses through a loop or coil. The size of the loop may vary from less than one square meter to a square kilometer depending on the application. After termination of the transmitter pulse, a receiver is employed to measure the residual, secondary transient electromagnetic field. In areas having low conductivity, and essentially no metal content or mineralization, the secondary electromagnetic field dissipates rapidly. On the other hand, areas having more conductive mass and particularly areas having high mineralization and metal content cause the secondary field to remain for a longer duration. The secondary field is related to the conductivity of the conductors within the electromagnetic field and their geometric position i.e. horizontal, vertical or dipping conductor.

 

Modern TEM systems use receivers having three axis loop sensors.

 

A computer having a high speed digital acquisition system is used for measuring the received secondary signal. The secondary signal is measured at prescribed time intervals beginning at termination of the transmitter pulse and continuing until the secondary field no longer remains. A typical 2 millisecond secondary waveform is sampled 32 times in order to provide sufficient information relevant to the decay profile.

 

The complete receiver system uses three receiver coils. The coils are mounted perpendicular to each other with a common intersection of their axis (orthogonal). Each coil provides a signal that is amplified by a programmable low noise amplifier to an appropriate level. The amplifier output voltages are applied to the input of an analog to digital converter for acquisition and transmitted over a serial data link to a laptop pc for storage in memory.

 

TEM is used in handheld “metal detectors” for portable metal detection applications. In other applications large transmitting loops are mounted on fixed wing aircraft and helicopters.

 

WELLOG has developed TEM systems for investigation of the shallow subsurface. Our systems are designed to offer the best compromise in portability and depth of investigation. Systems range from handheld to roll along to large horizontal loop systems.

 

 Revised 11-07-2016     © WELLOG 2007-2016    All Rights Reserved