VLF

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GEOPHYSICS, VOL. 57, NO. I (JANUARY 1992); P. 97-105, 12 FIGS.

Effect of temporal and spatial variations of the primary signal on VLF total-field surveys

Marc A. Vallee*, Michel Chouteau+, and G. J. Palacky§

Spatial and temporal variations have been studied using field monitoring of the transmitted signal. The results of fieldexperiments indicate that the nature of the received VLF fields changes significantly even over moderate distances (20-30 km) and that data cannot be reliably corrected over larger distances. This observation has a significant implication for VLF total-field surveys, particularly airborne, in which base stations have been routinely used to monitor the primary field strength and to correct the survey data. The results of primary signal monitoring are also used to demonstrate the effect of solar flares on VLF surveys. Because of the large intensity and complex electromagnetic character of solar flares, survey data recorded during such events cannot be used for map compilation and interpretation.

ABSTRACT

Most of the airborne and ground VLF instruments presently used measure the total-field response in addition to field ratios. Results of surveys using these instruments are adversely affected by spatial and temporal variations in the VLF primary field. Until now, the nature of such variations has not been studied from the point of view of geophysical surveying practice. Spatial variations are analyzed using radio propagation models. The most important result is the identification of primary field minima where surveys would be unreliable. Their dependence on the transmitter location is rather complex, and modeling should be carried out before specifying VLF stations for a survey area.

geological mapping in nonconductive environments. As ratio measurements are sensitive to orientation of magnetic sensors, which is unknown with some airborne platforms, some airborne instruments evolved that measure the total-field amplitude in addition to field ratios. Most VLF instruments presently in operation in North America are of such type, including Herz Totem, Sander VLF-EM II, and Scintrex SE-90 (Collett, 1986, Herz, 1986). Some commercial ground systems also have this option. Unfortunately, some of the problems that plagued early radio field measurements have still not been properly addressed. An important consideration when using total-field VLF data in geological mapping and exploration is how to compensate for changes in the primary field that are not related to geology. A correction becomes possible only if the origin of the primary field variations is fully understood. Although the nature of the VLF primary field has been studied in depth by radio engineers and several studies have been written on the subject (Watt, 1967), not enough attention has been paid

INTRODUCTION

Radio field intensity measurements were among the first electromagnetic (EM) methods to be considered for possible use in mineral exploration (Cloos, 1934). However, early tests indicated that field intensities measured during surveys were influenced by a number of factors not related to geology, such as fluctuation of the transmitting power, interference patterns between ground and sky waves (Hollingworth, 1926), topography (Howell, 1943), and solar flares (Dellinger, 1937). For this reason, attempts to use radio transmissions as a source of the primary EM field focused on measurements of the tilt angle or the ratio of orthogonal magnetic fields, which are insensitive to the primary field variations (Paterson and Ronka, 1971). With worldwide availability of VLF signals in the 15 to 25 kHz range, which are emitted for communications with submarines, VLF field ratio measurements have become a well-established geophysical technique used primarily for

Manuscript received by the Editor September 11, 1990; revised manuscript received June 5, 1991. *Formerly Departernent de Genie Mineral, Ecole Polytechnique; presently Centre de Technologie Noranda, 240 Boulevard Hymus, Pointe-Claire, Quebec, Canada H9R IG5. :j:Departementde Genie Mineral, Ecole Polytechnique, C.P. 6079, Succ. A, Montreal, Quebec, Canada H3C 3A7. §Geological Survey of Canada, Mineral Resources Division, 601 Booth Street, Ottawa, Ontario, Canada KIA OE8. © 1992 Society of Exploration Geophysicists. All rights reserved. 97

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Vallee et a!.

to the problem by the geophysical community carrying out and interpretating VLF surveys. From the perspective of exploration geophysics, both temporal and spatial variations of the primary VLF field are of great importance. In principle, temporal variations can be recorded with a fixed receiver (base station) and survey data subsequently corrected. This approach has become routine in airborne and ground VLF total-field surveys. However, very few practitioners realize that the use of a base station may be ineffective in many situations. As demonstrated in this study, temporal variations cannot be correlated over a large distance. The problem of spatial variations is even less understood by field geophysicists. In regional airborne VLF surveys flown by the Geological Survey of Canada, significant variations in the intensity of the primary total field have been observed. This phenomenon was believed by Dr. R. L. Grasty (1990, Pers. Comm.) to be due to interference between the ground and reflected sky waves. In this paper, we review the temporal and spatial variations affecting total-field surveys. We present a classification of temporal variations illustrated with an example. We then advocate the use of radio propagation modeling to predict the intensity of the VLF primary field and, in particular, minima in the interference patterns. Near these minima, the primary field varies rapidly with distance. Their location depends on the phase relation between ground and sky waves. Near these minima, the field intensity is also highly sensitive to the variations of the ionosphere, and temporal variations observed with distant receivers may not be well related. This study is supported by the results of an experiment on correlation of temporal variations with distants receivers. This experiment shows limitations in the use of a base station for correction of temporal variations.

- fluctuations and interruption of the transmitter power, - sunrise and sunset fading, - slow drift during the day, and - rapid fluctuations during the night. Natural VLF signal from atmospheric noise (sferics) and whistlers also causes temporal variations of the VLF field, but it mainly appears as high-frequency noise on a VLF diurnal record. The magnitude of this natural VLF perturbation can be estimated from world maps of signal-to-noise ratios prepared for selected transmitter locations and seasons (Watt, 1967, Hauser and Rhoads, 1974). Temporal variations in the primary field produce a lowfrequency drift of the VLF signal. According to their origin, these variations can be separated into two groups: (a) transmitter power variations (man-made, and hence, in principle, avoidable), (b) changes which depend on the transmitter-receiver geometry and on the physical properties of the propagating medium. At a given location, significant

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Diurnal variations that affect geophysical surveys can be recorded with a fixed receiver (base station). To identify and analyze these variations, an experiment was set up, in which intensities of electric fields were continuously measured during defined periods. In the spring of 1988, signals from VLF transmitters code-named NAA located at Cutler, Maine, and NSS at Annapolis, Maryland, were monitored at Saint-Remi-de-Napierville, 30 km south of Montreal, Quebec. This facility, the Spectrum Control Centre, is maintained by the Department of Communications of the Canadian Federal Government. A rhombic electric antenna was used as a receiver. It was connected to an HP 8568B sweep spectral analyzer, which was controlled by an HP 9122 microcomputer. An electric field intensity spectrum was swept over a second with a resolution bandwidth of 300 Hz. Intensities at 21.4 (NSS) and 24 kHz (NAA) were averaged and recorded every 10 s for periods of two to three days. Similar variations were observed each day and only examples over a 24 hour period are presented. Figure 1 displays examples ofVLF diurnal variations over a 24 hour period. The following types of temporal variations have been identified:

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TEMPORAL VARIATIONS

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FIG. 1.. Diurnal variations recorded on April 23, 1989, for transmitters NAA, Cutler and NSS, Annapolis (b). Times of sunrise, sunset, and transmitting power interruption are indicated.

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VLF Primary Field Variation

variations in the character of signals originating from different transmitters will be observed due to the fact that orientation and length of the propagation paths are different. To explain variations of the second type, a brief outline is given of VLF signal transmission. EM fields generated by an electric antenna are proportional to the square root of the transmitting power. In the VLF frequency range (15 to 25 kHz), waves propagate to the receiver location in an electric waveguide formed by the earth's surface and the ionosphere. The part of the ionosphere affecting the VLF propagation is called the D-layer located at a height of 60 to 80 km above the earth's surface. This region of the upper atmosphere is ionized by solar radiation, especially Lyman-a and soft X-rays (Davies, 1%6). Therefore, the propagation is strongly affected by the presence of the sun over the propagation path. This explains the large differences in intensities observed between night and day and the rapid changes at sunrise and sunset. Operators who would like to make the most of the day should note that sunrise and sunset periods are not suitable for VLF surveying. This has been noticed already by Thiel and Chant (1982) for wavetilt measurements. As field surveys are carried out during the day, they are affected by slow drift and rapid oscillations in transmitter power (Figure 1). The assumption is normally made that data obtained at base stations can be used to correct field measurements. Our study shows that this technique can only be applied in limited cases. SPATIAL VARIATION MODEL

In routine VLF surveys, measurements are carried out along lines in a given survey area. To predict accurately primary field variations at the mobile receiver using base station data, the patterns of change in the primary field intensity with distance must be established. Models developed for radio propagation can be used in such studies.

The field intensity of the VLF signal transmitted by an electric antenna can be calculated from a number of models. Solutions for a conductive sphere embedded in anisotropic ionosphere have been proposed since the beginning of the 20th century. An historical overview has been given by Johler and Berry (1962). Some methods were compared by Jones and Mowforth (1982). For our study, an approach based on the summation of zonal harmonics has been chosen (Johler, 1970). The contributions of the ground wave and reflected sky waves to the total field intensity are computed separately. A cartoon depicting various contributions is shown in Figure 2. The method assumes a uniformly conducting earth and a layered ionosphere. Reflection coefficients for a horizontal anisotropic ionosphere are computed using the Johler and Harper (1962) algorithm. The ionization distribution of the D-Iayeris represented by the Wait and Spies (1964) exponential model. Reflectionof a radio wave by the ionosphere is affected by the intensity and orientation of the earth's magnetic field. These parameters are obtained through the IGRF 1985 model (IAGA Division I, Working Group 1, 1986). Total horizontal magnetic field intensity has been computed along a south-north propagation path for distances from the transmitter of 200to 2000km, and parameters of the earth's magnetic field that are typical of VLF surveys in eastern North America. The results are presented for transmitters NAA and NSS in Figures 3a and 3b, respectively. In the calculation, in which four sky hops were used, the ground conductivity was assumed to be 2 mS/m and its relative dielectric permittivity 20 (ITT, 1975). In the same figure, the horizontal magnetic field intensities contributed by the ground wave and the first sky hop are separately depicted. At a distance of about 550 km, the contribution of the first sky hop exceeds that of the ground wave, which prevails near the transmitter. The contribution

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FREQUENCY: 21.4 kHz

FIG. 2. Characteristics of the VLF radio propagation model. Values of the parameters depicted were used to produce results of Figure 3.

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Vallee et al.

of other sky hops is minor at the distances depicted and hence has not been plotted. However, their effect becomes significant at greater distances from the transmitter and should not be neglected in calculations. Minima in the total field were observed and attributed by Hollingworth (1926) to interference patterns between the ground wave and the sky waves. The minima locations are relatively insensitive to the direction of propagation, but depend on the frequency of the transmitter. (Notice the slight shift in minima between NAA and NSS propagation graphs.) Only the intensity of the magnetic field perpendicular to the propagation path have been presented. However, the model selected can also predict other components of the magnetic field intensity. Intensities of these components, for a vertical electric dipole source, depend on the conversion between transverse electric polarization and transverse magnetic polarization of waves that are reflected by the ionosphere. Bracewell et al. (1951) observed that minima in conversion coefficients occur during the day and in summer.

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