For mobile, landscape view is recommended.
The objective of this project was to design and fabricate a novel time-domain bistatic advanced electromagnetic induction (EMI) sensor that allowed for the physical decoupling of the transmitter (Tx) and receiver (Rx) coil(s). This bistatic, physically decoupled, portable EMI instrument with precise positioning became a valuable research instrument for several reasons. First, there was a pressing need for portable EMI instruments deployable in adverse conditions where cart-mounted systems could not function. Furthermore, a bistatic EMI instrument would deliver critical flexibility in operation and high fidelity data by allowing users to acquire sufficiently detailed data in one pass to satisfy inversion/discrimination routines. Necessary in achieving this data quality was research into a more precise scheme for positioning the receiver assembly. This project aimed to combine systematic modeling of the instrument geometry, flexible operation design, and highly precise positioning into a single, portable EMI instrument.
The multimode bistatic portable array instrument, called Pedemis (PortablE Decoupled Electromagnetic Induction System), fulfilled these objectives based on research into the optimal size, constitution, and configuration of transmitters, receivers, and supporting hardware while also incorporating several improvements over existing sensors. This new positioning method adds to the information content of the data, allowing more accurate inversions, without adding any hardware to the instrument itself. Instrument control software was also incorporated, which can deliver some feedback to the operator regarding targets in real time. The flexible operation of this instrument allows deployment in both cart-trafficable sites and more challenging sites.
The Pedemis sensor has independent Tx and Rx arrays. The coplanar 3 x 3 array of square coil transmitters is visible through a plastic cover, producing a total sensor size of about 121.5 cm x 121.5 cm. The square Rx array, with a side of about 56 cm, has a vertical non-metallic handle meant to facilitate maneuverability. In the field, poles are attached to the front and back carrying straps for two operators to carry during dynamic data collection. For static data collection, Pedemis is laid on the ground and not carried. Receivers for the Tx and Rx arrays are arranged in a 3 x 3 configuration with 20 cm center-to-center spacing. Each receiver is a triaxial receiver similar to those on the Man-Portable Vector 2 (MPV-2). The default dataset is therefore composed of 9 transmitters x 9 receivers x 3 components, or 243 time signatures recording the secondary magnetic field between 100 ms and 25 ms.
Pedemis uses the National Instrument cDAQ and a small mini-pc for controlling the Tx and Rx arrays and associated circuitry. This represents a step forward from prior portable EMI systems like the MPV-2 in terms of weight, power usage, and decreased complexity. The weight of the cDAQ chassis and modules is around 8 pounds compared to over 20 pounds for the full PXI chassis and related electronics. Instead of three lithium ion batteries and 3-hour swap intervals, only two of these batteries were used with a 5-6 hour swap interval. Also, the cDAQ chassis, with its integrated modules, is simpler and more robust than the prior full PXI chassis. The combined weight of the transmitter and receiver arrays is about 38 pounds (not including cabling or gurney poles); whereas, the overall weight of the controlling electronics plus batteries is approximately 20 pounds.
Pedemis was deployed to the Aberdeen Proving Ground (APG) Unexploded Ordnance (UXO) test site in November 2012. Using Pedemis and data processing, the best score to date on the blind grid was achieved.
Pedemis is a new, advanced EMI instrument with several deployment modes, data acquisition flexibility, and lighter and less power hungry electronics. Specifically, Pedemis has the following advantages over other advanced EMI instruments:
Results at APG demonstrated the capability of Pedemis to interrogate unknown anomalies and achieve excellent results.