Subsurface Radio Propagation Experiments (1968)

7/30/2019 Subsurface Radio Propagation Experiments (1968)

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Subsuriace Radio Propagation Experiments 1Carson K. H. Tsao and J. T. deBettencourt

Raytheon Company, Norwood, Mass . 02062, U.S.A.

~ r o p a g a t i o n experiments were conducted on Cape Cod, the Adirondack region of New York State,and m Cheyenne Mountain in Colorado to determine the feasibility of communication b e t w e ~ n verticallinear antennas located in drill holes in the rock below overburden, and similar antennas located onthe surface of the ground.The bulk conductivity of the propagation media, i.e., the average conductivity along the path hetween the antennas, must be determined by measurements of propagation loss and transmission delay,b ~ c a u s e of the lack of homogeneity in the rock media. High values of bulk conductivity limited e ~ p P - r i ments I? the VI;F reJZion on Cape Cod and the Adirondacks, which involved ranges of 1.6 km and 5 krn.respectively. D1stances up to 48 km in the LF region were achieved by upandover transmissions frominside Cheyenne Mountain.Analysis of the experimental results and theoretical considerations indicate that the usefulnessof the throu!lhtherock mechanism is severely limited by the bulk conductivity. For shallow antennadepths, particularly if the overburden is thin or absent, substantial transmission ranges can be achievedby using up-over-down propagation.

1. Introduction;There has been considerable interest recently inpossibility of using subsurface rock strata for. dened radio communications. With both trans)ting and receiving antennas situated in th e bedrock

. 1 ~ w the .surface of the earth, propagation can behteved dtrectly through the rock medium betweene antennas. Through-the-rock propagation has been,e principal concern of a series of investigations onape Cod, and in the Mineville an d Black River areas_ ted in northern New York State. The "up-and-over"

~ c h a n i s m was also investigated in the Cheyenne~ m n t a i n area in Colorado, between an antenna buried,eply in the rock an d an antenna located on th e suri!e of the ground. This paper reviews the various~ h ~ i q u e s and summarizes the experimental resultsJamed in the course of these investigations .any programs of theoretical and experimentalestigations in this field have been conducted ine past. References cited here are but a small portionj or addition to, the bibliography to be found else,ere. To cite bu t a few publications, Hansen (1963).es an excellent survey that considers up-over-and;wn propagation. Ames, et al. (1963) consider the,tenttal of communicating via direct through-the-rock.. mission. Biggs and Swarm (1963) treat the fieldsrur from a buried antenna to antennas on the surfaceelevated in the air, neglecting the influence of thel'hi, work wu s p n n by thr Air Furcor Cambrid,:eo Res.tarch J aburatutie" undC"r_net No. AF19162S)-2362 and. in 1ntrt. hy 1heo Raylheouo Cumpan;. Nurwnod MasF-.

ionosphere; this can be designated the up-and-overmechanism. Wait (1963) develops the idea of a waveguided in a low-loss horizontal layer in th e earth'scrust. A thorough review of progress on the subjectis presented by deBettencourt (1966)2. Experimental Arrangements

In conducting through-the-rock propagation experiments, access to the rock is made by drilling verticalholes through the overburden an d into the rock.Vertical linear antennas are placed in the holes belowthe conducting overburden as shown in figure l . Sincethe overburden generally is more conducting than therock below, the overburden serves as a reflectingground plane, allowing monopoles to be driven againstit. The antennas are connected by transmission linesto the transmitter and receiver located on the surfaceof the ground. The properties of antennas in drill holesare discussed in detail by deBettencourt and Sutcliffe(1962).For these experiments a 100-W audio amplifier wasused as a transmitter, an d either a low frequencycommunication receiver or a narrowband wave analyzer was used as a receiver. The magnitudes an dphase angles of mutual impedances between th etransmitting and receiving antennas were obtainedthrough measurements. From these, the bulk conductivity, i.e., the average conductivity, of the mediumbetween the antennas was deduced. Th e length of th eantennas in the drill holes varied from the 150-m

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l sachusetts. on Cape Cod used two drill holes 300 rn! deep. separated by 1.6 km (Tsao. 1963). Transmissionswere made at frequencies below 7.5 kHz. Later, meas

urements were made in the Mineville area in NewYork State, where two existing deeper drill holes,separated by 1.6 krn, were used as antenna sites fortransmissions at frequencies up to 50 kHz (Tsao,1964a). Subsequently, based on results obt ained in theMineville area, and also from supporting data from asurface resistivity survey, two drill holes were madeavailable to establish a 5-km path in the Black Riverarea in New York State, where tntnsmissions weremade at frequencies below 100 kHz (Tsao, 1964b).Later, experiments were conducted in the Che yenne1 Mountain area of Colorado to investigate the up-andover transmission (Tsao, 1965). As shown in figure 2,a transmitting antenna was placed in a slanted drillhole such that the antenna was parallel to and about

210 m below the eastern slope of Cheyenne Mountain.Access to the drill hole was made at Turnaround, apoint along a tunnel road leading into the NORADcomplex. The antenna orientation was selected tofavor up-and-over transmission to surface pointslocated east of the transmitting antenna.

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FIGURE 1. Schematic configuration for through-the -rockpropagation experiments.50

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FIGURE 3. Mutual impedance vs. frequency, I -mile path,Cape Cod, Mass.

i1ubsurface I. Experiments ond ofesultsTronsmissions3.1, 1.6-km Transmissions on Cape Cod

On Cape Cod, transmissions were made at frequencies below 7.5 kHz over the 1.6-km path betweenantennas inserted in the 305 m deep drill holes (Tsao,1963). Figure 3 shows the magnitude of the mutualimpedance, i.e ., the magnitude of ratio of the voltageinduced in the receiving antenna to the current at theinput terminal of the transmitting antenna. It is seenthat the magnitude of the mutual impedance is constantat the lower end of the frequency scale. This is becausethe electrical distance between the antennas is short.and only the quasi-electric field is important. (In t h i ~case, the mutual impedance does not have an inductiv(component and can be referred to as the mutua'resistance.) As the frequency increases, the electricadistance increases. Hence, th e magnitude of thtmutual impedance decreases rapidly, due to the increased importance of the exponential damping in th(far field zone.The measured phase angle of mutual impedancea function of frequency for the Cape Cod pathshown in figure 4. Th e square root dependency olphase a n ~ l e to frequency indicates that the rod

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. ~ ~ ~ F i1\tedium along the path ha s a large loss tangent. Meas- In the Black River experiment there is strong I!i' rnent of the phase angle of the mutual impedance evidence that at frequencies above 50 kHz, signalsI :quires the use of common reference phases at both were received due mainly to propagation through thei.: e transmitter the receiver. On C ~ a p e Cod_, t?is air, i.e., the up-over-down or up-and-over mechanism.:. ference was denved from a LORAN C transmissiOn This is shown in figure 6 by comparing th e measuredJiginating on Nantucket Island (Tsao and deBetten field intensities with the estimated strengths of sig-1urt, 1967). nals in air and in the earth, as shown by the solid andi ':The bulk conductivity of the rock for the 1.6-km . the dashed curves, respectively. There is equallyi th on Cape Cod was deduced from the amplitude strong evidence that at frequencies below 50 kHz, thepd phase data independently, an d was found to be signals received in the hole were primarily due to the'lpproximately I X 10- a mhos/m. direct or through-the-rock propagation. The bulk conductivity in this area was found to be about 8 X 10-5

f 3.2 . 1.6-km Tronsmissi.ons ot Mineville, New York mhos/m.; Measurements of amplitude and phase of the mutual 3.4. Tronsmission From Inside Cheyenne Mountoin

( :. pedance were also ma.de at Mineville between an. nnas in deeper drill holes 1.6-km apart. At Mineville,

....easurements were made at frequencies up to 50Hz (Tsao, 1964a). From these measurements , thettenuation and phase conSt!lnts of the propagation_ dium were deduced as shown in figure 5. At thewer frequency, the attenuation and the phaseonstants are equal an d proportional to the square:oot of the frequency due to the large loss tangent.) frequencies near 50 kHz, as the loss tangent he

.' mes small the phase constant becomes greater. an the attenuation constant. Using a curve-fitting ethod, these data provided a bulk conductivity of,5 X 10-4 mhos/m and a relative dielectric constantf 48. The corresponding theoretical attenuation andhase constants are shown by the solid curved lines figure 5.; 3.3. 5-km Tronsmissions ot Block River, New York

tTransmission tests over a 5-km path were made..etween drill holes in the Black River area (Tsao,a964b). Only the magnitudes of the field intensitiesr ere measured . Field intensities measured in air andIll the hole at the receiving site (Wadhams, N.Y.) arerown in figure 6.It1ti.!.

210 T R A N S M I ~ T I N G ' A N T ~ O M E ; E ' ; ~ ~ J, PVCINSOCINHOLENO.I il [r:RECEIVING ANT. 30 0 METER RGB : /3 t:

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Following the experiments. described above, propagation tests were made in the Cheyenne Mountain areato determine whether transmissions could be madefrom inside Cheyenne Mountain to receivers on thesurface east of the mountain (Tsao, 1965). Th e throughthe-rock transmission was not attempted in this area .An antenna buried 210 m below the surface of Cheyenne Mountain (fig. 2) was used for transmission;measurements of field intensity were made on thesurface of the ground. Th e trans mission path wasonly partially through rockMeasurements of surface field intensities due totransmission from inside Che yenne Mountain weremade along several 50-km radials from the transmitter, at a frequency of 70 kH z. The measured fieldintensities, normalized to l A of input current to thetransmitting antennas , are shown in figure 7. Th efield intensity along each radial varied inversely withdistance. The inverse distance dependency is characteristic of groundwave propagation for the fre quencyand distances involved, because the ground attenuation is negligible. The bulk conductivity of the rocksurrounding the buried antenna was estimated to beabout 3.5 X I0 - 4 mhos/m.

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.1eststtvlly data for correlation with the bulk condudtivity data obtained from the more involved transmis: Ision tests described above. 1::w... 40w:1.........J0 ~30a:::::Ewz0 20..,>0"'""' 10I>-!:::"'.., n...0..J.., -10

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FI GURE 7. StLrfa ce field intensity at 70 kHz due to trans-missions from inside th e mountain.

3.5. Comparison of Conductivity Values DeducedFrom Transmission Tests and Surface ResistivityMeasurements

The conductivity and layer thickness of a horizontally stratified region of the earth' s crust can bededuced from measurements derived from the surfaceresistivity method. In this method, the apparentresistivity (conductivity) of the earth located belowhorizontally polarized transmitting and receivingdipoles located on the earth 's surface is deduced frommeasurements of mutual impedance, i.e., the ratio ofvoltage induced in the receiving dipole to the current

Surface resistivity measurements on Cape Codyielded rock layer conductivities of 1.7 X 10-4 til2.7 X 10-4 mhos/m , depending on the variation h1th e estimates of the thickness and resistivity ofoverburden . The bulk conductivity was found to havba much higher value of aoout 1 X 10-3 mhos/m , ~deduced from the rock strata radio propagation tests.At Mineville in northern New York State, the dat'pfrom surface resistivity measurements showed seriouEscatter and were considered unusable for s i m p i ~interpretation. This difficulty was attributed togeological inhomogeneity in the area. Neverthelessradio transmission tests provided a measurable bullconductivity of 2.5 X 10- 4 mhos/m .At Black River, also in northern New York Statethe gPology was considered more homogeneous tharat Cape Cod or Mineville. Surface resistivity measurements at Black River yielded a rock conductivity o5 X 10 - 5 mhos/m. This agrees well with the bullconductivity of 8 X 10 - " mhos/m , determined fronthe through-the-rock radio transmission measurementsApparently, surface resistivity measurements artprimarily useful in regions that consis t of simplthomogeneous layers. Analysis of data is quite difficu!if there is severe lateral inhomogeneity in addition t1horizontal layering in the geology, since interpretatio1of the data depends on evaluation of the variation oapparent resistivity with dipole-to-dipole separationThere is , however, an important advantage in usinthis method; because of its simplicity and with nrequiremen t to drill deep holes in the rock, it coulbe used as a tool for obtaining preliminary informatioon the rock strata conductivity data. However, bulconductivity of a through-the-rock path should evertually be determined direc tly through subsurfacpropagation measurements.

4. Discussion and Conclusion4 .1. Separation of Transmission Modes

in the transmitting dipole. Various dipole configura- In experiments to test the transmission of signations may be used. As the dipole spacing is changed, through the subsurface rock meJium, it is always 1the apparent resistivity may vary in a manner to in- special concern to the experimenter an d othe1dicate the presence of several layers. Th e variation whether the received signal actually followed tlof apparent resistivity with the separation of the through-the-rock path instead of some other pat"dipoles provides the skilled analyst with the magnitude such as the up-over-and-down path, transmitter leaof the conductivity and the thickness of the several age, or radiation from transmission lines.

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layers. Horizontal dipole separations are comparable The effects of transmitter leakage and poor receivoto , or exceed, the range of depths desired. Past shielding may be simply evaluated by replacing tlmeasurements used direct current; re cently, alternat- antennas with dummy loads, to determine the ~ x t eing currents of a fraction of a Hertz to several Hertz of spurious radiation and pickup. In the through-thhave been employed. rock propagation experiments described above, theSurface resistivity experiments using frequencies was no evidence that the tests were so contaminateof less than l Hz were conducted on Cape Cod and in Measurement of relative field intensities in the cnorthern New York State (Cantwell and Nelson, 1963). and in the ground is the simplest of the many possibmeamemen" we'e i n t e n d e ~ ~ ~ wheth., P'opagation wa. achie" IJJ


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~ ~ ~ = 11i fhrough the rock directly, or by the up-over-and-down experiment was conducted on Cape Cod. Short dipoles Ifransmission. were used for transmitting and receiving in two drill:f In the Black River experiment, as an example, the holes separate

~ t r e n g t h s with the measured values affords a meansf determining the relative importance of the up-overd-down transmissions and the rock propagated. ignals.Aside from the measurements of relative field trengths o{ a signal in air, on the surface, and in theock below it (due to a remote , buried antenna) the ck propagated signal can be readily distinguished.om the up-over-and-down signal by the phase delay .ong its path, as measured in the experiments at . 'neville, N.Y., and on Cape Cod. The large phase! elays can only suggest transmissions through aj 'ssipative medium.I . The slopes of the at.tenuation an d phase constants,: educed from mutual Impedance measurements gen-1 rally indicate the main characteristics of the trans-ission media. Another possible observation is the'henomenon of depth gain, which is associated withropagation in a rock medium in much the same way height gain is associated with propagation in airCarolan and deBettencourt, 1963). The depth gain

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4.2. Potential Application to Subsurface CommunicationThe results of the subsurface propagation experiments indicate that through-the-rock propagation can

be used for transmission between antennas buried inrock. The useful range for this type of transmissionis limited and decreases rapidly with increased rockconductivity. Furthermore, the useful frequency rangeis very small. The range and frequency limitations arecaused by the exponential damping of the lossymedium.In addition to the transmission path through rock,propagation may also take place through the up-overand-down mechanism. Here the depths of antennas

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3 0 L - - - - - - - - - - - L - - - - - - - - - ~ ~ - - - - - - - - - - . J0 50 100 150DEPTH OF ANTENNAS BELOW OVERBURDEN IN METERSOverburden transmission loss fo r vertical electricfield at Black River drill hole. N.Y. FIGURE 9. Relative depth gain vs. depth, /-mile path,Cape Cod. I

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below the surface become important, as the exponential damping increases rapidly with antenna depthsand frequency. However. the up-over-and-down mech anism suffers refraction losses near the terminals ofthe path; these losses decrease inversely with thesquare root of frequency.The relative importance of the two transmissionmechanisms depends on the following factors: separation between antennas; antenna depths below thesurface; antenna orientations; the electrical characteristics of the earth. The through-the-rock signal ismaximized with transmitting and receiving electricdipoles erected parallel and broadside to each other.Th e up-over-and-down transmission is best with antennas erected coaxial and parallel to the surface.The advantage of the up-over-and-down transmissionwill be enhanced with increased transmission range,because of the additional exponential damping of thelonger path through the rock. However, if the rock isseparated from the air by a thick layer of highly conducting overburden, its shielding effect will greatlyincrease the total propagation loss for the up-over-anddown signal, bu t will have negligible effect on the rockpropagated signal. Furthermore, the reception in anup-over-and-down transmission will likely be limited by.atmospheric noise. Although increasing the depth ofI the receiving antenna will reduce the noise level, the

1 up-over-and-down signal will also be reduced by the1 same amount.1 In the subsurface experiments described in section

li3, these limitations were aggravated by the relativelyshallow depths of the an tennas and the lack of suffi. ciently thick overburden. These factors combined to' make the reception subject to serious interferenceboth from atmospheric noise and from signals whichentered through the air. Insofar as the future use ofthe through-the-rock transmission is concerned, muchdepends on the requirements for using subsur facean tennas, on the further development of geophysicaltechniques to loca te low conductivity rock s trat a deepbelow the surface of the ea rth , and on the economicsinvolved in placing an tennas at greater depths. (Itshould be noted that there is theoretical evidence fora waveguide mode of transmission in a layer of verylow conductivity bounded by more conducting layers{Wait, 1963). We have not encountered the low conductivities required for such a low-loss mechanismin our measurements.)

i1The authors acknowledge the sponsorship of thi' work by Air Force Cambridge Research Laboratorieunder contract AF19(628)-2362 and the effectivparticipation of Messrs. L. Ames, J. Frazier, ariA. O r a n ~ e of its Microwave Physics Laboratory. '!

5. References:!Ames, L. A., J. T. deBettencourt , J. W. Frazier, and A. S. Oranm:(1963), Radio communications via rock strata, IEEE Tranl'tCommun. Systems CS-11, No. 2,159-169. ~Biggs , A. W., and H. M. Swarm (1963), Radiat ion fields of an inclin '' .electric dipole immersed in a semi-infinite conducting mediutnIEEE Trans. Ant. Prop. AP-11, No.3, 306-311.Cantwell, T., and P. Nelson (1963), Deep resistivity investigatioA.in n o ~ e r n New York State, Sci. Rept. No. 4, Geoscience,Cambndge, Mass., CFSTI Doc. No. AD 294-748. ._1Carolan, J., Jr., and J. T. deBettencoun (1963), Radio waves in r o c ~near overburden-rock interface, IEEE Trans. Ant. Prop. A P - 1 1 ~No. 3, 336-338. :deBettencoun, ] . T. (1966), Review of radio propagation belowearth's surface, Progress in Radio Science, 1963-1966, Intern:Sci. Radio Union, Berkeley, Calif., 1967, Pt . 1, 697- 767.deBettencourt, J. T., and R. A. Sutcliffe (1962), Studies in deep strataradio communications, Final Rept. Raytheon Co., Norwood,CFSTI Doc. No. AD 407-840. ::Hansen, R. C. (1963}, Radiation an d reception with buried and submerged antennas and propagation , IEEE Trans. Ant. Prop.AP-11, No. 3, 207-218. ITsao, C. K. H. (1963), Investigation of electrical characteristics ofrock medium on Cape Cod, Sci. Rept. No. 1, Raytheon Co:,Norwood, Mass., contract AF 19(628)-2362, CFSTI Doc. No.A043(}-7I9. .

Tsao, C. K. H. (1964a}, Investigation of electrical characteristics ofrock medium in northern New York State, Sci. Rept. No. 3,Raytheon Co., Norwood, Mass., contract AF 19(628}-2362, CFSTI r,.Doc . No. AD 6()(}-758.Tsao, C. K. H. (l964b), Investigation of electrical characteristicsof the subsurface rock medium in Essex County, New York, Sci.Rept. No. 5, Raytheon Co., Norwood, Mass., con tract AF 19(628}-2362, CFSTI Doc. No. AD 609-683.Tsao, C. K. H. (1965), Subsurface radio transmission tests in Cheyenne Mountain Sci. Rept. No. 7, Raytheon Co., Norwood, Mass.,contract AF 19(628)-2 362 , CFSTI Doc. No. AD 615-794.Tsao, C. K. H., and J. T. deBettencourt (1967), Measurement olphase constant for rock-propagated signals,IEEE Trans. Commun.Techno!. COM-15, No.4, 592-596.Wait,]. R. (1963), The possibility of guided electromagnetic wavesin the earth's crust , IEEE Trans. Ant. Prop. AP-11, No. 3.330-335.

(Paper 3- l l- 465)

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Subsurface Radio Propagation Experiments (1968)