ATTEM: A NEW TRANSIENT ELECTROMAGNETIC INSTRUMENT SYSTERM FOR SUBSURFACE SURVEY

Abstract

A new transient electromagnetic instrument system, ATTEM (because we researched the whole process of transient electromagnetic response, we called it All-time Transient Electromagnetic) has been designed and tested at several engineering sites for subsurface survey. The ATTEM consists of two units: transmitter and receiver. The transmitter unit transmits a bipolar pulse current as excitation source, and records the turn-off current waveform through a sampling resistor connecting with transmitting bridges in series. If the configuration and position of transmitter loop are known, the primary field at receiving site can be calculated using Biot_Savart law, and then its response in turn-off period can be numerically removed from the total response recorded by receiver. The receiver recodes total response when transmitting current start to switch off. A 12-bit A/D with sampling interval of 0.1 μs records early stage of transient response and a 16-bit A/D with sampling interval of 5 μs based on the floating amplification records middle and later stages of transient response. Double A/Ds sampling in sequential stages enhances the dynamic range of recording and the resolution of subsurface survey. Both test experiment result and field experiment result indicated that the ATTEM with the algorithm for numerically removing turn-off response enhanced the capability and resolution of subsurface survey. Introduction Electromagnetic methods for subsurface survey include frequency domain electromagnetic methods and transient electromagnetic methods (TEM). The frequency domain electromagnetic methods mainly rely on the Ground-penetrating radar (GPR) as measurement apparatus. According to frequency range used, detection depth of GPR is from several centimeters to dozens of meters underground. Unfortunately, when subsurface conductivity is high, GPR cannot penetrate far enough for many applications (Campbell, 2000). Transient electromagnetic method can be applied in an area with highly conductive surface materials, but the minimum detection depth of TEM is restricted by the minimum delay time. Because TEM transmitting current turn-off has effect on early transient response (Raiche, 1984; F.Kamenetsky, 2000), the majority of commercial TEM instrumentations set the minimum delay time after the turn-off time. In that time, the effect can be neglected, but the early transient response is abandoned, which relates to the electrical properties of the subsurface geology.