Detection and Imaging of Underground Structures Using ELF/VLF Radio Waves

ELF/VLF radio waves penetrate deeply beneath the surface of the earth and interact with the geologic structure of the earth. This interaction induces secondary fields with measurable effect at and above the surface of the earth. Proper understanding of the physics of the generation and propagation of ELF/VLF waves and their interactions with earth materials will allow these waves to be used for applications such as sub-surface communications and exploration of the subsurface geological structure. The research called for under this effort is to assess the viability of exploiting the concept of electromagnetic induction to detect and image subterranean features such as tunnels, bunkers, and other potential military targets.

Geophysical surveying using natural ELF/VLF sources, such as lightning or auroral generated signals, is an established procedure. In general, however, the procedure has been developed with the interest in locating areas of highly conductive material such as metal ore deposits.

In 1994, the Senate Armed Services Committee noted the promising results of the high frequency active auroral research program (HAARP). This transmitter in Alaska, besides providing a world class research facility for ionospheric physics, could allow earth- penetrating tomography over most of the northern hemisphere. Such a capability would permit the detection and precise location of tunnels, shelters, and other underground shelters. The absence of such a capability has been noted as a serious weakness in the Department of Defense plans for precision attacks on hardened targets and for counterproliferation. In fact, the May 1994 report from the Deputy Secretary of Defense on nonproliferation and counterproliferation activities and programs recommends increased funding of $75.0 million annually for detection of underground structures. The committee recommended $5.0 million in PE 62601F to continue the HAARP project, but notes with concern that the capital cost of a full- scale HAARP facility could be as much as $90.0 million. Unless the Department of Defense is committed to include such a project in future budget requests, the recommended authorization for fiscal year 1995 will have little effect. Therefore the committee directed that none of these funds may be obligated or expended until the Secretary of Defense notifies the Committees on Armed Services of the Senate and the House of Representatives that the Department will, as part of the nonproliferation and counterproliferation program recommended in the May 1994 report, include the cost for a full- scale HAARP facility in its fiscal year 1996 budget request.

There are many significant issues that need to be resolved, however, before the concept can be evaluated for military applications such as detection of underground structures. These issues can be divided into six primary topics:

(1) the physics of, and methods for modeling the generation of ELF/VLF waves from various sources;
(2) methods for measuring ELF/VLF field components in a covert manner;
(3) methods for modeling and characterizing ELF/VLF wave propagation including the effects of surface topography and subsurface terrestrial properties;
(4) the physics of the propagation and attenuation of the secondary fields above the earth's surface;
(5) procedures and algorithms for inverting measured electromagnetic field information to obtain subterranean conductivity structure particularly aimed at identifying and characterizing man-made structures or natural voids; and
(6) algorithms for producing unambiguous detection and classification of underground voids or structures in the presence of geological noise and clutter.

The Space Effects Division of the Phillips Laboratory Geophysics Directorate is interested in receiving proposals related to the theoretical understanding and practical development and demonstration of techniques for the detection of underground structures using ELF/VLF radio waves generated by natural and man-made sources. Issues to be examined by the proposers include: methods for measuring ELF/VLF field components by covert means; methods for characterizing, and interpreting ELF/VLF wave generation and propagation, including the effects of surface topography and subsurface geophysical properties; procedures for inverting the electromagnetic field properties to obtain realistic, depth-dependent geophysical parameters with particular interest in identifying man-made structures and natural voids within the surrounding geology; and algorithm development for the unambiguous detection of underground voids or man-made structures in the presence of real-world geophysical noise and clutter.

This PRDA seeks proposals of original and innovative research dealing with the development, implementation, and demonstration of these concepts. The proposer will need to demonstrate a basic understanding of the principles of ELF/VLF wave generation, ELF/VLF guided-wave propagation, instruments and measurement procedures and problems for ELF/VLF, methods for inversion of ELF/VLF data into depth-dependent geophysical models and for detecting zones or targets of interest and for characterizing those targets. Highly original and innovative proposals dealing with one or more of these topics relevant to this progam will be considered for funding.

Several distinct methods for ELF/VLF generation are available to support these efforts. Proposers are encouraged to consider including the controlled ELF/VLF sources provided by the 960KW HF transmitter of the HAARP, presently under construction outside Gakona, Alaska and the HIPAS facility located near Fairbanks, Alaska. PL/GPS is the program manager for the HAARP facility. The Office of Naval Research controls the HIPAS facility. Both sites will be available to support the research efforts under this PRDA.

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