Cooperative Institute for Research in Environmental Sciences

A winning proposal for the Innovative Research Program, 2009:

Passive Radio Imaging for Applications in Water Resource Management, Glaciology, and Space Weather Monitoring

Investigators: Nick Zabotin and Oleg Godin, CIRES, University of Colorado

Objective
To develop a new, passive method of measurements of the water table depth, the ice thickness, and the altitudes of ionospheric layers through observation of autocorrelation of polarization components of the ambient radio noise with modern digital HF receiving systems.

Background and Importance
An approach to remote sensing of the environment called passive imaging or noise interferometry has become increasingly popular after its success was demonstrated experimentally a few years ago in helioseismology and in ultrasonics. Its applications have become very popular in regional seismology, geological prospecting, and ocean acoustics. The basic idea is very simple: instead of a dedicated probing signal, to use correlation reception of ambient noise for interrogating the environment. It is not unlike the way daylight is utilized by the eyes. The approach differs from conventional radiometry by coherent processing of diffuse wave fields. It has been shown theoretically and verified experimentally that the twopoint correlation function of a diffuse noise reproduces the shape of the Green’s function, which describes propagation of a deterministic probing signal between the two points. Moreover, there exist applications that require no controlled source of waves and only one receiver (the passive fathometer technique, for example).

We anticipate a possibility of expanding the passive imaging technique to the area of HF radio wave propagation, where such applications as monitoring the water table level, measuring ice thickness or determining an altitude of ionospheric layers, are feasible. Implementing passive techniques in this field would have positive environmental and societal impact: the radio spectrum is densely occupied by various vital applications, and using new active sources of radio signals is often prohibited. Passive techniques allow using frequencies that may be unavailable for active methods. Low power consumption of passive sensors increases the time of their autonomous operation, while their low cost allow for a large number of networked sensors and an improved spatial resolution of tomographic inversions. Of course, frequencies and necessary data acquisition rates are many orders of magnitude higher for radio than for seismic waves. Passive imaging with HF radio waves would not be possible if not for the digital receivers that became available only recently.

fig. 1 Radio noise in the HF band is present constantly. It is caused by multiple and globally-distributed sources of natural (lightning) and of technogenic (industrial noise and distant radio stations) origin. On the other hand, now we have numerous instruments incorporating modern fully digital HF receivers highly appropriate for measuring and studying the noise properties. Among them, first and foremost, there are four VIPIR systems just installed at Jicamarca Radio Observatory, Peru, at NASA’s Wallops Flight Facility, Virginia, at Tomsk State University, Russia, and in Boulder, Colorado. The photographs on the right show receiving antenna array and 8-channel fully digital HF radar installed at Wallops. The eight dipole antennas form two orthogonal lines, ~100 m in length, and allow accurate measurements of polarization, direction of arrival, and amplitude of the received signal. The primary purpose of the VIPIR systems is active radio sounding of the ionosphere. In this project, we will be using them in an innovative way.

Research Plan
The experimental setup allows testing of at least two diagnostic schemes.

1) In many situations of practical interest, the incoming signal is a result of interference of a direct signal and a signal reflected from underlying surface(s) where a jump of the dielectric permittivity occurs (e.g., the water table level or the upper and lower boundaries of an ice layer; see a sketch below). In this situation, theory predicts a pronounced dependence of the noise auto- and crosscorrelation on wave polarization and thickness of the layer. This scheme may be used to retrieve the layer thickness from measurements of the correlation of polarization components of the signal.

fig. 1

fig. 12) Radio noise propagating in the vertical direction is usually relatively weak. However, its presence is still noticeable because of roughness of both the ground surface and the ionosphere boundary. Auto-correlation function of the amplitude of the electromagnetic noise field measured at a single location must show double peaks corresponding to the two-way propagation time between the ground and the ionosphere (see a sketch on the right). This property may be used to determine the altitude of the ionosphere, a quantity that strongly depends on the space weather conditions.

We plan a series of measurement sessions at one or two of the VIPIR locations, to be performed in the passive mode (i.e., when only ambient noise is recorded). The data will be processed statistically to reveal the theoretical relationships mentioned above. Results of the project will be published as papers at scientific meetings and in the journal Geophysical Research Letters.

Why is this innovative and important?
Measurements of this kind have never been undertaken in the HF band. Note that no radiation of a dedicated signal is assumed, all proposed measurements will be done with the use of ambient noise only. This method is energy efficient, representing real “green science." It implies no interference for existing communication and other industrial radio systems. Measurements of this kind would not be subject to any frequency use restrictions. Furthermore, only initial development requires cumbersome sensor systems like VIPIR. In case of success, compact and/or mobile systems implementing this principle can be developed. The principle itself may be patented.

Why is this interdisciplinary?
The technique described involves radiophysical methods of remote sensing and general wave physics. However, potential applications are very diverse. As it has been mentioned, inexpensive autonomous radio sensors can be developed for permanent monitoring of the water table in regions with problematic water supply. Similar devices could be used for monitoring of the polar ice mass, a critical parameter for the global warming awareness. At the same time, important information about the state of the Earth’s plasma envelope can be gained.

Expected outcome
The hypothesis will be verified that critically important environmental information can be retrieved from auto- and cross-correlation functions of HF radio noise.