Experimental Studies and Verification of the Vertical Array-Differential Target Antenna Coupling (DTAC) Method for Rapid Sensing and Imaging of Subsurface Targets

Abstract

We have developed a new electromagnetic geophysical system that has significantly improved target sensing and imaging capabilities compared to conventional measurements. In a previous series of papers, we described a new measurement technology called the Differential Target Antenna Coupling (DTAC) method, which used a magnetic-field transmitter (TX) and a magneticfield receiver (RX), oriented in a horizontal array, with separation distances of the order of tens of meters, and up to tens of kilometers. In the present paper, we use the DTAC method in a verticalarray configuration, with the TX coil directly above two null-coupled RX coils. This vertical array provides a number of important advantages over the large-offset horizontal array for some applications. For example, the vertical array-DTAC method can be adapted to rapidly moving measurements, such as from a helicopter. In addition, the vertical array-DTAC method also suppresses surface clutter, such as fences, steel buildings, and parked vehicles.The DTAC method uses at least two frequencies. One frequency is used as a reference signal to establish the null direction at that frequency, and then a different frequency is used to measure the change in the null at the new frequency. We can also use an arbitrary number of frequencies with just one reference frequency for wide-bandwidth spectral measurements. We first describe the vertical array-DTAC measurement system and discuss the calibration procedure for the instrumentation. Next, we discuss the methods employed for scale-model testing of the system. We validate the vertical array-DTAC method using small-scale controlled tests and demonstrate the enhanced resolution and sensitivity of the vertical array-DTAC method. Suitable TX moments and frequency ranges can be selected for effective study of nearsurface targets (civil engineering, water resource, and environmental characterization) as well as deep targets (mining and other natural-resource exploration).

Field Tests of the Vertical-Array Differential Target Antenna Coupling (DTAC) System

“…To investigate the feasibility of such a system, Sternberg et al (2015a) studied the extension of the DTAC method to a moving, vertical TX/RX array, using a very low-power system over known tuned-coil targets. This null-field array utilizes a horizontal TX loop and an approximately orthogonal (nulled) two-axis RX that is located directly below the TX.…”

“…Sternberg et al (2015a) demonstrated some of the capabilities of the vertical-array DTAC method using very small-scale surveys and using artificial tuned-target coils. In this paper, we will expand the field testing of the DTAC method to more realistic targets.…”

Numerical Studies and Potential Applications of the Vertical Array-Differential Target Antenna Coupling (DTAC) Method for Rapid Sensing and Imaging of Subsurface Targets

“…In Sternberg et al (2015), we show experimental studies and verification for the vertical array-DTAC method. For these studies, we used small-scale subsurface targets to simulate commercial targets of interest, such as mineral, energy and water resources.…”

“…The rapidly moving measuring system requires the use of two receiver coils, which simultaneously measure the x and y fields, rather than the sequential measurement of the three field components that was used in the horizontal array-DTAC system Dvorak and Sternberg, 2013). The use of two separate receivers can be accomplished by using the simultaneous calibration procedure that is described in Sternberg et al (2015) and Sternberg and Dvorak (2002).…”

“…There are significant advantages to using a frequency-domain system (Hodges et al, 2010), or equivalently a combined on-time and off-time transient measurement (Smith, 1998;Sattel, 1998). We note that the vertical array-DTAC system has the ability to overcome limitations in previous frequency-domain and on-time measurements caused by variations in primary field pickup (see the section in this paper on Improved Robustness in the Measurements under Typical Field Conditions, as well as Sternberg et al, 2015). This makes the vertical array-DTAC system ideal to exploit the advantages of recording during the transmitter on-time.…”