WELLOG                                         TRANSIENT EM

 

 

 

 

 

 

 

 

 

 

 

INTRODUCTION:

 

 

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. Large scale applications are directed toward location of massive sulfide deposits.

 

HOW IT WORKS:

 

A typical TEM system uses a transmitter that transmits short, high energy, bipolar current 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. The magnetic pulse induces current flow in conductors surrounding the coil. After termination of the transmitter pulse, induced eddy currents create a secondary magnetic field. A receiver coil is employed to measure the decay of the secondary magnetic field. In areas having low conductivity, and essentially no metal content or mineralization, the secondary magnetic field dissipates rapidly. On the other hand, areas having more conductive mass and particularly areas having high mineralization and metal content cause the secondary magnetic field to remain for a longer duration. The secondary field is proportional to the conductivity of the conductors within the expanding magnetic field. The response in the time domain is related to depth and geometric position for example horizontal, vertical or dipping conductors.

 

TEM systems may use receivers having three axis loop sensors in order to model a 3D response.

 

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 secondary waveform may be sampled 32 times in order to provide sufficient information relevant to the decay profile.

 

If a receiver system uses three receiver coils. The coils are commonly mounted perpendicular to each other (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 and transmitted over a digital serial data link to a laptop pc for storage in memory.

 

APPLICATIONS:

 

TEM is used in handheld metal detectors for portable metal detection applications. These are branded as Pulse Induction (PI) detectors. Large scale TEM survey applications employ large transmitting loops and 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 and large horizontal loop systems.

 

 Revised 12-30-2020 WELLOG 2007-2020 All Rights Reserved