Progress In Broadband Electromagnetic Sensors
I. J. Won, Dean A. Keiswetter, Tom Hall, Bill SanFilipo · 12th EEGS Symposium on the Application of Geophysics to Engineering and Environmental Problems · 1999
We have developed a suite of new broadband electromagnetic (EM) sensors for rapid, ground geophysical surveys at environmental sites. These sensors (GEM-2, GEM-3, and GEM-5 series) operate simultaneously at multiple frequencies in a bandwidth of 30 Hz to 24,000 Hz, soon to be increased to 50,000 Hz. These hand-held, digital, multi-frequency sensors, all weighing about 9 to 12 pounds, can transmit an arbitrary waveform containing multiple frequencies. We also have developed an airborne EM sensor (GEM-2A) designed using the same principles as the hand held GEM-2, but scaled up in size and built into a helicopter towed bird. Owing to the nature of its broadcast waveform and high-speed digitization, the sensor can operate either in a frequency-domain mode or in a time-domain mode. Its built-in operating software allows a surveyor to cover about one acre per hour at a line spacing of five feet. Along a survey line, the data rate is about one sweep per half-foot, resulting in about 10,000 data points or more per acre per hour. Such portability, survey speed, and high data density are important requirements for geophysical surveys at environmental sites. All of these sensors share the critical feature that they incorporate a loop configuration in which the primary field at the receiver coil (i.e., field from the transmitter if no earth or scattering body were present) is suppressed. This is accomplished either by use of a secondary measuring coil that measures the same primary (but not scattered) field which is subtracted from the main receiver coil signal, or by use of a secondary transmitting coil that creates a localized magnetic cavity around the receiver coil, Attention to detail in the antenna design and front-end electronics determines the stability of the sensor and the quality of the data, and the use of state-of-the-art digital electronics with a high level processor provides maximum flexibility. These aspects continue to evolve from the original prototypes, so that each generation improves from the previous. Multi-frequency data can be acquired in a continuous mode, with built-in sine/cosine correlation and instrument response compensation, and recorded on the fly within the non-volatile memory.