VLF

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§

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 tem-poral 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 loca-tion is rather complex, and modeling should be carried out before specifying VLF stations for a survey area.

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

Spatial and temporal variations have been studied using field monitoring of the transmitted signal. The results of field experiments 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, par-ticularly 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.

geological mapping in nonconductive environments. As ratio measurements are sensitive to orientation of magnetic sen-sors, 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 com-pensate 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 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:Departement de 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

15,- -r

1

Sunset Sunset Interruption --l>-NAA(Cutler) NSS (Annapo 1is) Sunrise Sunrise 4 8 12 16 20 24

Eastern daylight-saving time (hours)

4 8 12 16 20 24

Eastern daylight-saving time (hours)

E <, > .§ >-..., 10 ... UI C

..

...,

c ... u ....

..

... ₅ .... u ... c,

...,

U Ql .... lJ.J 0 0 b) 15 E <, > .§ >-...._{... 10} UI C Ql .... C ... U .... Ql ... ₅ .... u ... c, ..., u

..

.... lJ.J a)

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 pertur-bation can be estimated from world maps of signal-to-noise ratios prepared for selected transmitter locations and sea-sons (Watt, 1967,Hauser and Rhoads, 1974).

Temporal variations in the primary field produce a low-frequency 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 - fluctuations and interruption of the transmitter

power,

- sunrise and sunset fading, - slow drift during the day, and - rapid fluctuations during the night.

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.

TEMPORAL VARIATIONS

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, Que-bec. This facility, the Spectrum Control Centre, is main-tained by the Department of Communications of the Cana-dian Federal Government.

A rhombic electric antenna was used as a receiver. Itwas 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 a24hour period are presented.

Figure 1 displays examples ofVLF diurnal variations over a24hour period. The following types of temporal variations have been identified:

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, signifi-cant 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 be-tween the ground and reflected sky waves.

In this paper, we review the temporal and spatial varia-tions 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 experi-ment on correlation of temporal variations with distants receivers. This experiment shows limitations in the use of a base station for correction of temporal variations.

VLF Primary Field Variation 99 variations in the character of signals originating from

dif-ferent 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 (Figure1).The assumption is normally made that data obtained at base stations can be used to correct field mea-surements. 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 devel-oped for radio propagation can be used in such studies.

NAA (Cutler) POWER: 1 MW FREQUENCY: 24 kHz

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 com-puted 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 trans-mitters 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

EXPONENTIAL IONOSPHERE FROM WAIT AND SPIES (1964)

TX-RX~ 1;>.,.

NSS (Annapolis) POWER: 400 kW FREQUENCY: 21.4 kHz

GROUND CONDUCTIVITY : 2 mS/m

GROUND RELATIVE DIELECTRIC PERMITTIVITY: 20

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

b)

SPATIAL VARIATIONS EXPERIMENTS

Local variations in the primary field strength will obvi-ously affect the quality ofVLF surveys. The problem can be studied theoretically or experimentally. Correlations between VLF signal-phase variations with distance have been studied by Pressey et al. (1961) and Sobczak and Taylor (1970) among others. However, as the purpose of their investigation was a better design of navigation systems, their conclusions which focused on phase variations are not applicable to amplitude measurements. To study the dependence of amplitude varia-tions on distance, we opted for field observavaria-tions followed by correlation with theoretically obtained results.

In December 1989 and May 1990, an experiment was set up south of Montreal, Quebec, in which two receivers mea-sured variations in the primary VLF field during several half-day periods. The location of the study area and VLF transmitters used in the experiment is shown in Figure 4a. Also indicated are the distances between the area and the transmit-ters. Commercial VLF instruments of two types were used to record the magnetic field intensity continuously at locations depicted in Figure 4b. In December 1989, two Scintrex VLF-3 units, which measure the horizontal magnetic field intensity, were used. In May 1990, the VLF total field strength was measured with two EDA Omniplus receivers. Measurements were recorded every lOs for periods up to 6 hours long. With each instrument, several receiver separations were used and measurements were carried out over a period of several days. The radio propagation model described above has been used to predict spatial variations in the area. The total field and the contribution of the ground wave and the first sky hop are displayed for transmitter distances relevant to the survey area. For the NAA transmitter (Figure 5a), the ground wave contribution is more important than that of the first sky hop. At this location, a decrease of 30 percent in the NAA primary total-field intensity has been predicted for a distance increase of 100 km (from 400 to 500 km). For the more distant NSS transmitter (Figure 5b), the first sky hop con-tribution is more important, and the total field remains relatively constant over the distance depicted.

Figure 6 displays diurnal variations observed with a Scin-trex VLF-3 receiver on December 5,1989, at location B. The instrument has a resolution of one unit, which corresponds to 156 nA/m at 21.4 kHz (NSS) and 139 nAim at 24 kHz (NAA). The signal varies relatively slowly during the 6.5 hour interval around a mean of 25/LA/m.

Spectral analysis has been used to characterize the fre-quency content of the data. For short series, Thomson (1982) They also decrease with frequency. For example, the iono-spheric model selected in this paper does not produce transverse magnetic propagation. However, variations of the wavetilt observed at sunrise and sunset by Thiel and Chant (1982) are attributed to mode conversions.

Near the minima, horizontal magnetic amplitudes vary strongly with distance. As daytime variations of ionization affect the intensity and phase of the sky-wave contributions, they also affect the minima locations. Because of the strong variations of the fields near the minima, studies of the temporal and spatial variations of the primary field at these locations are important.

...

---- Total field --- Ground wave

.. First sky hop ---- Total field --- Ground wave

First sky hop

--.

NSS NAA 500 750 1000 1250 1500 1750 2000 Tx-Ax Distance ~m) 500 750 1000 1250 1500 1750 2000 Tx-Rx distance (km) 250 250 .~ 50 ... Ql C 01 (Q E 25 >-... .;;; 100 c Ql ... C .,.., "tJ 75 ... Ql .,.., ... .~ 50 ... Ql C 01 (Q E 25 ... (Q ... c o .::: 0+-_--.-_ _,.-"-_--.-_ _,.-_--.-_ _,.-_--.-'-=-"=+ ~ 0 I a)

FIG. 3. Predicted primary field variations for transmitters (a) NAA and (b) NSS along a south-north propagation path. Amplitudes of the ground wave, the first sky-wave contri-bution, and the complex sum of all contributions (ground wave plus four sky hops) are presented.

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 mag-netic polarization of waves that are reflected by the iono-sphere. Bracewell et al. (1951) observed that minima in conversion coefficients occur during the day and in summer.

"tJ 75 ... Ql .,.., ... c-, ... .~100 c Ql ... C .,.., ... (Q ... c o .~ 0+----.---,.----.---,.----.---,.--...

..::..::..:=:r-~ 0 I E '- 1 2 5 . , - - - ,

1

E <,125,- --.,.

1

VLF Primary Field Variation 101 a) o 150 km ! ! NSS (Annapolis) 75° b) o Sle-Brigide Ste-Sabine 5 km ! D 0 Ferndon

*

~

Farnham NAA (Cutler) - -...~ 455 km

c

*

o o ! B

*

0 St-Alexandre / NSS (Annapolis) 750 km Iberville Mont@ St-Gregoire

FIG. 4. (a) Location of the survey area relative to NAA and NSS transmitters and (b) detailed map showing the location of the receiver sites (A, B, C, D).

and Walden (1990) recommended multitaper spectral analy-sis because of higher resolution and low leakage of the spheroidal windows. Figure 7 depicts the spectrum of the series of Figure 6, that was estimated with a weighted average of eigenspectrums computed with 41T prolate eigen-tapers. The energy maximum is concentrated near DC, below 10 cycles/hour, which corresponds to a period of 6 minutes. At higher frequencies, the signal is characterized by a constant noise level at -47 dB, which reflects the instrument error of 140 nAlm at 24 kHz.

In all surveys, coherence of the transfer function relating signals recorded at separated receivers is at low frequency, where the energy is concentrated. However, in this fre-quency band, the gain and phase of the transfer function varies with frequency. Figure 8 presents results that were recorded on December 9, 1990.Two receivers located 18 km

apart (at location A and C of Figure 4b) recorded the intensity of the horizontal magnetic field. Most events can be visually correlated on the two traces. However, the low-frequency drift is different at the two receivers. In Figure 9 coherence, gain, and phase of the transfer function relating signal D to signal A depicted in Figure 8 are compared with estimates for signals received with zero separation. The coherence deteriorates for distant receivers. Also the esti-mated gain and phase become noisier.

SUDDEN IONOSPHERIC DISTURBANCES

In addition to temporal variations identified in Figure 1, there are sudden ionospheric disturbances, particularly those associated with solar flares, that affect VLF measure-ments. Figure 10 presents measurements recorded on May 10, 1990, with two EDA receivers 27 km apart. At about

0-,-- ----.

FIG. 6. NAA variations recorded at site B on December 5, 1989, with a Scintrex VLF-3 receiver, with readings once every 10 s.

NAA

10 11 12 13 14 15 16

Eastern standard time (hours)

500 E <, ₃₀ <t 2- >-... 25 ..., Ul NAA c:QJ ... c: 20 - - Total field ..., ~ --- Ground wave r l QJ 15

First sky hop ..., u ..., ... ₁₀ QJ c: Cl

'"

E r l

.,

5 ... c: 0 N ₀ ..., L ₉ 0 I 425 450 475 Tx-Rx distance (km)

a)

E

:t

100 2-c-, ... ..., Ul c: 75 QJ ... c: ..., ~ r l QJ ..., 50 .... u ..., ... QJ c: Cl 25

'"

E r l

'"

... c: 0 N 0 ..., L 400 0 I b) E <, ₃₀ <t 2-noise level 40 60 80 100 120 140 160 180 Frequency (cycles/hour) 20 -60+-_-,-_--,,--_,-_-,--_--,_ _,-_-,--_--,_ _

+

o >-~ -20 Ul c: QJ ~ E-30 ::l L ... u ~-40 Ul L QJ 3:-50 o a. <D-10 Eo c-,

---

l'lSS ... 25 ..., Ul c: ... QJ ... c: 20 ..., ~ r l QJ ..., 15 .... u ..., ... 10 QJ - - Total field c: Cl

'"

--- Ground wave E

r l 5 _{First sky hop}

'"

... c: 0 N ₀ ..., L 700 725 750 775 800 0 I _{Tx-Rx distance (km)}

FIG. 5. Predicted primary field intensity over the survey area, for (a) NAA signal along an east-west line, and (b) NSS signal along a south-north line.

FIG. 7. Spectrum of field data depicted in Figure 6. The spectrum was computed with 41T eigentapers. The instru-ment noise level is also indicated.

VLF Primary Field Variation 103 2 . 0 , - - _ - - - , b) 15.0 12.5 Separation Separation o 0 km • 1B km o 0km • 1B km 5.0 7.5 10.0 Frequency (cycles/hour) 2.5 0.2 0.0+-_ _- ,_ _--,,--_ _.---_ _---,_ _--,,--_ _

+

0.0 1.5

~

1. 0

JPlIIlIlll!!,~

. .

t::-:~~--;;!~Z'li!iii~rIif~~~lt?"'\l~__t

C!l Q)0.6 iJ c Q) L Q) s: 00.4 u 0.5 a) O.B

sudden ionospheric disturbances, the two parameters change with frequency (or time) and with the receiver locations. In such situations, it is impossible to use the base-station data to correct for the variations observed at the moving receiver. As in the case of magnetic storms and their treatment in magnetic field surveys, VLF data acquired during solar flares should not be used for interpretation or map compilation.

11:30, a significant decrease in the NAA signal was observed at both receivers (A and D), which was followed by a temporary return to the original value and an even sharper drop at 15:30. Figure 11 compares the records of signals received from the Cutler and Annapolis transmitters. At the time of the drop in the NAA signal at 11 :30, the NSS signal increased by 80 percent. Unfortunately, no NSS data were available for the latter event because the Annapolis station was off the air. These two events can be correlated to solar flares of M3.9 and X3.4 X-Ray Class, respectively (National Oceanic and Atmospheric Administration, 1990).

As mentioned in the Introduction, the effects of solar flares on radio propagation were first observed by Dellinger (1937). Davies (1966) explained the mechanism in the follow-ing way: X-ray emissions associated with solar flares in-crease the ionization of the D-layer. The effective height of reflection of the VLF waves decreases for several kilometers over a short period of time, followed by a slower recovery. Variations in the reflection height modify, accordingly, the intensity and phase of the sky hop contributions to the total field intensities. As solar flares become more frequent during years close to the peak of the l l-year sunspot cycle, sudden ionospheric disturbances are expected to increase and reach a maximum in 1992.

A comparison is made in Figure 12of spectrum and param-eters of the transfer function for calm and disturbed ionosphere (inthis case the transfer function relating signalD to signalA of Figure 11).The power spectrum density increases by 10dB for frequencies smaller than 5.0 cycles/hour. The coherence is near one over this frequency band. However, the gain in-creases linearly from 0.6 to approximately 2 in the frequency range (}-5 cycles/hour. Phase decreases linearly over the same frequency band, which represents an advance of signal A over signal D. This reflects the observation that the sudden ampli-tude variation started at location A before D.

Primary-field variations at a moving receiver can be pre-dicted from the base station records only if the transfer function relating the receivers is known. However, during

FIG. 8. NAA intensity recorded with two Scintrex VLF-3 receivers located 18 km apart (site A and C in Figure 4) on December 9, 1989. 15.0 15.0 12.5 12.5 5.0 7.5 10.0 Frequency (cycles/hour) 5.0 7.5 10.0

Frequence (eye les/hour)

2.5 2.5 0.0

+-__-,__--,

,-__---,__---,

+

0.0 ~-15 ttl s: a. -30+ - - - , - - - , - - - , - - - - , - - - , - - - - t 0.0

FIG. 9. Estimates of (a) coherence, (b) gain, and (c) phase of the transfer function relating signals measured at two receiv-ers 0 km apart (siteB' related to B) and 18 km apart (site C related to A). Data at sites (B, B') and sites (A, C) were recorded, respectively, on December 5 and 9, 1989.

c)

3 0 r - - - . - - - ---,.

NAA. 18 km separat ion

10 11 12 13 14 15

Eastern standard time (hours)

e

₃₀ <, ~ >-...., 25 ... UJ C Q) ...., c ₂₀ ... u .... Q) ... ₁₅ ... u ... ...., Q) ₁₀ c Cl ttl E .... 5 ttl ...., C 0 N 0 ... L 0 9 I

E 30 E 30 <, <, « _NAA « 3- 3-25 c-,

/

Location 0 ;>, 24 ... ... ... ... _~NAA 1Il 1Il C ₂₀ c Ql Ql ... ... c c 18 ... ... "C ₁₅ "C .-< .-< Ql Ql ... ... ... ... 12 u ₁₀ u ... ...

t

... ... Ql Ql Solar flare c _{Solar flare} ) c el el 6

_t

lC 5 lC E E .-< .-< _~NSS lC lC ... ... 0 0 0 0 t- t-10 11 12 13 14 15 16 10 11 12 13 14 15 16

Eastern daylight-saving time (hours) Eastern daylight-saving time (hours) FIG.10. NAA intensities measured with two EDA Omniplus

receivers located 27 km apart on May 10, 1990. Occurrences of solar flares are indicated according to solar observations.

FIG. 11.NAA and NSS intensities recorded on May 10, 1990

at location D with an EDA Omniplus receiver. Occurrence of a solar flare is indicated according to solar observations.

a) b) 5 4 o Undisturbed. 0 km • Disturbed. 27 km 2 3

Frequency (eye les/hour)

0.8 Ql0.6 (J c Ql L Ql .c 00.4 u 0.2 0.0 5 0 d) 90 4 o Undisturbed. 0 km • Disturbed. 27 km 2 3 Frequency (cycles/hour) 3.0-r-r- .,. -40 +---,---,---,---,---"'---"'t-o 20, - - - T ;>, ~ 0 1Il C Ql "C E -10 :J L ... U ~-20 1Il L Ql ~-30 o a. c) iD 10 ~ • Disturbed. 27 km • Disturbed. 27 km 2.5 o Undisturbed. 0 km 2.0 c ... 1.5 lC (') 45 !ii' Ql Ql L en Ql ~ Ql 1Il lC s: a. -45 o Undisturbed. 0 km 0.5 5 4 2 3 Frequency (cycles/hour) -90+- -,- --,,-- -r-t -t-r-

+

o 5 4 2 3 Frequency (cycles/hour) 0.0+- ---, ,-- -.,.- ---. + o

FIG. 12. Estimates of (a) spectrum, (b) coherence, (c) gain, and (d) phase of the transfer function for receiver

separation of 0 km (siteA'relative to A, quiet day) and 27 km (site D relative to A, with a disturbed ionosphere). Data were recorded, respectively, on May 5 and May 10, 1990.

VLF Primary Field Variation 105

CONCLUSIONS

Airborne VLF surveys have been successful in detecting local conductivity anomalies, particularly those due to clay-filled shear zones in resistive Precambrian terrains (Soon-awala and Hayles, 1986; Sinha, 1990). Such information has been of considerable value in structural geologic mapping in Canada, and was extensively used in mineral exploration. However, interpretation of VLF data has been mostly qual-itative so far. While numerous attempts have been made to interpret VLF data in a more quantitative way, the inconsis-tency in measured amplitudes always posed a serious obstacle. Some sources of measurement errors have been identified in this paper and their prediction have been explored.

Variations in VLF total-field intensities that can affect results of geophysical surveys can be divided into two categories: spatial and temporal variations. Spatial varia-tions have been investigated by modeling of the primary field intensity as a function of distance. The results clearly indicate the presence of Hollingworth interference patterns. As there is no clear remedy for the disturbance, total-field VLF surveys should not be carried out using a transmitter which, for the given survey area, is likely to be affected by this phenomenon. Before commencing field surveys in a given area, modeling should be carried out to identify the regions likely to be affected by primary field minima.

Base stations have been used routinely in VLF total-field surveys to correct for temporal variations. In a series offield experiments, the correctness of this practice has been tested. The results indicate that the parameters of the transfer function relating the signals recorded at two receivers varies with their separation and with frequency. Serious errors would be intro-duced for greater distances between the base station and the sensor. Even for a distance of 27 km, which was the greatest receiver separation in the experiment, the two signalscould not be well correlated. In airborne surveys, the aircraft is often much further from the base camp, where the primary field strength is monitored, and, in such cases, inappropriate cor-rections will distort the field data. In view of our findings, it is recommended that the existing surveying practice be reviewed. For instance, it may be desirable to set up several base stations in large survey areas.

Solar flares, whose intensity will reach an l l-year peak in 1992, cause serious disturbances in VLF signals. Data ac-quired during such events cannot be corrected and hence should not be used for map compilation or data interpreta-tion. Information on occurrence and intensity of solar flares can be obtained from research organizations, such as the National Geophysical Data Center of the National Oceanic and Atmospheric Administration, which continuously mon-itors solar and ionospheric activity.

ACKNOWLEDGMENTS

The staff of the Spectrum Control Center at St-Remi-de-Napierville, Quebec, which is operated by the Department of Communications, allowed us the use of their instrumentation. Scintrex Ltd. of Toronto, Ontario, provided their VLF-3 receivers. Sagax Geophysics Inc. and Geophysique Sigma Inc. of Montreal, Quebec, lent us EDA Omniplus instruments. Dr.

R.L.Grasty and R. Shives ofthe GeologicalSurvey of Canada discussed with us various problems of VLF surveying and

critically read the manuscript. Dr. R. Coles of the Geological Survey of Canada provided information on solar flares ob-served in May 1990. Part of the project was supported through Quebec FCAR grant EQ-2632F. GSC Contribution no. 16991.

REFERENCES

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Collett,L. S., 1986, Development of the airborne electromagnetic technique, in Palacky, G. J., Ed., Airborne resistivity mapping: Geol. Surv. of Can., Paper 86-22,9-18.

Cloos, E., 1934, Auto radio, an aid in geological mapping: Amer. J. ScL,28, 166.

Davies, K., 1966,Ionospheric radio propagation: Dover Publ. Inc. Dellinger, J. H., 1937, Sudden ionospheric disturbances: Terr. Mag.

and Atmos. Electr., 42, 49.

Hauser, J. P., and Rhoads, F. J., 1974, Coverage predictions for the Navy's fixed VLF transmitters: Nat. Res. Lab. Memo. Rep. 2884. Herz, A., 1986, Airborne EM instruments operating at VLF and higher frequencies, in Palacky , G. J., Ed., Airborne resistivity mapping: Geol. Surv. of Can., Paper 86-22,55--61.

Hollingworth, J., 1926, The propagation of radio waves: J. Inst. Electr. Eng., 64, 579-595.

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