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Radio-Wave Propagation in the Earth's Crust

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Radio-Wave Propagation in the Earth's Crust JOURNAL OF RESEARCH of the National Bureau of Standards-D. Radio Propagation Vol. 6SD. No.2, March- April 1961 Radio-Wave Propagation in the Earth1s Crusf'2 Harold A. Wheeler (F ebruary 26, 1960) There is a reasonable basis for postulating t he existence of a useful waveguide deep in the earth's crust, of t he order of 2 t o 20 km below the surface. Its di electric is basem ent I'Oc], of very low conductivity. Its upper boundary is formed by the conductive layers ncar the surface. Its lower boundary is formed by a high-temperature conductive layer far below the surface, termed the "thermal ionosphere" by analogy to the well-known "radiation ionosphere" far above the surface. The electrical conductivity of the basem.ent rock has not been explored. An example based on reasonable estimates indicates that transmission at 1.5 kc/s might be possible for a distance of the order of 1500 km. This waveguide is located under land and sea over t he entire surface of the earth. It may be useful for radio transmission from t he shore to a submarine on the fl oor of t he ocean. The sending antenna might be a long conductor in a drill hole deep in the basement rock ; the receivi ng antenna might be a vertical loop in t he water . In Lhe earth's crust, there appears to be a deep S waveguicleLhat has not yet been explored. This waveguide extends under all the surface area, so --::-~_:.-:J?C:__ ~ it suggests the possibility of wave propagation under I --_ R OCEAN the ocean floor. This might enable communication --- - _ 0 from land to a submarine lo cated on or neal' the -- -.;;;- ------- ocean floor. If below a certain depLh , j t happens --- ---- - WAVEGUIDE that the excess radio noise from electric storms would become weaker than thermal noise, and no otber THERMAL IONOSPHERE so urce of appreciable radio noise is recognized. FIGURE 1. Communicati on tlwough the deep waveguide under This waveguide comprises basemen t rock as a the ocean. dielectrie between upper and lower conductive boundaries. The upper boundary is formed of the guide into the ocean ju t above, and is sampled by well-known geological straLa lo cated between the an antenna at the receiver. surface and the basement rock, with conduetivity Figure 2 shows an arrangement for the sender provided by electrolytic solutions and semiconductive antenna. It is a long conductor (pipe) sunk into a minerals. The lower boundary is provided by high­ drill hole flIl ed with oil insulaLion. The example temperature conductivity in the basement rock. shown has conducting material down to a depth of In concept, Lhe lower boundary is similar to Lhe about 1 km. Thl'ough this layer of earth, there is usual ionosphere, bein g formed by gradually increas­ an outer pipe which forms the outcr conductor of a ing conductivity. In the usual ionosphere [Wait, coaxial tran mission line. This pipe is in contact 1957], caused by extratenestl'ial radiation, tbe con­ with a conductin g surface slleh as water or radial ductivity increases with height. In the present wires in the ground. Below this layer of earth, the case, however, the conduetivity increases with deptb inner conductor extends further about 2 km into and is caused by the increasing temperature in the the basement-rock dielectric. This extension radiates dielectric material. Therefore it may be designated into the waveguide in the usual manner. as the "inverted ionosphere" or "thermal iono­ sphere." Figure 1 shows how this waveguide may be used S for eommunication from a shore sending station (8) WATER to an underwater receiving station (R). The latter may be a submarine on the bottom of the ocean. The sender launches a vertically polarized tl'ansverse­ electro-magnetic (TEM) wave by means of a vertical ~"~"'~ wire projecting into the basement rock. The wave WAV EGUIDE : ~ ( T EM MODE ) : is propagated in the deep waveguide between the :I 1 I I 2 km surface conductor and the thermal ionosphere. I I Some power from the wave leaks out of the wave- I, I I I Contribution from '~'hee l e r Laboratories, Great Neck, N.Y., and Develop­ I L_ -' mental Eng in eerin~ Co rp ., Leesburg, Va. , Paper presented at Conference on tbe Propagation of ELF Radio Waves, Boulder, Colo ., January 27, 1960. FIGU RE 2. Sender antenna in the dee p waveguide. 189 (J", CON DUCTIVITY (mhos/m) The locatioll ot each boundary depends on the log 10 IT frequency, t he conductivit)T, and the rate of change - 12 -6 o of conductivi ty with depth. In the example to be o o outlined, these boundaries occur at temperatures in the range of 300 to 600 °C. UPPER BOUNDARY As an example of the behavior that might be u expected in this waveguide, the following numerical E 0 ~ ~ -'" values arc suggested. DIELECTRIC CONDUCTOR k'6 Frequency 1.5 kc/s Dielectric constant 6 10 200 Wavelength (in dielectric) 80 km Efl'ective boundaries of E fi eld (depths) 1- 18 km Effective boundaries of M fi eld (depths) 1- 27 km (TEM WAVE) I "- S ki n depth for E fi eld (lower boundary) 1.5km f- ::;;: "- w Skin depth for JIll field (lower boundary) 4km W f- L ength of radiator (ill waveguide) 2km 0 R eactance of radiator 1600 ELECTRI C ohms Effective length of radiator 1 km 20 400 Radiation resistance (in waveg uide) 0.4 ohms Ot her r esista ncc 20 ohms x x Radiation efficiencv 0.02 0 0 A verage power fadm' of E and JIll fields, about 0.1 0:: 0:: "- "- Napier distance (for wave attenuation) 130 kill "- "- <l <l D ecibel distance (for wave attenuation) 15 km 100- db distance 1500 km 30 600 If these values are to be experienced, communica­ tion ranges of the order of 1500 km will be possible FIGURE 3. 11ariation of conductirity with depth to form the de ep waveguide. under the surface of the earth. The assumptions for this example are based on preliminary estim ates of the best conditions that are Figure 3 shows how tll e temperature and the at all likely to be realized. The high-temperature resulting conductivity may vary with depth, espe­ conductivity needed for the lower boundary is typical cially in the basement rock at depths exceeding a of quartz and other similar minerals. The extremely few kilometers. This diagram will be used to explain low conductivity at lower temperatures is unlikel.Y, the expected behavior of the deep waveguide. At but need not be quite so low to provide a dielectric depths of about 2 to 20 km, the basement rock is that could give the indicated performance. indicated to have such low conductivity that it is a dielectric suitable for wave propagation. Above As for the properties of the basement rock, it is and below this dielectric, the conductivity is high very doubtful how low its conductivity may be. enough to serve the function of boundaries for the Its seismic properties are explored but not its waveguide. electrical conductivity. Its principal chemical com­ The upper boundary is fairly weU defined, in a ponents are known, but apparently not its small depth of the order of 1 km (perhaps down to several content of " impurities" that may determine the kilometers). Its conductivity, in the most common conductivity. It seems that core samples have been materials, ranges from a maximum of 4 mhos/m in made to only a sm.all depth (less than 1 km) in the sea water down to about 10- 4 in rather dry nonco n- basement rock, presumably because there has been ductive minerals. little prospect of valuable mineral products at a Tbe dielectric layer, shown between depths of reasonable cost. It is notable that some tests show about 2 and 20 km, may have very low conductivity, a trend toward lower conductivity (below 10 - 6) in of the order of 10- 6 to 10- 11 mho/m. The lowest the transition from the surface layers in to the base­ conductivity is observed in fu sed quartz, but prob­ men t rock. A con tinuation of this trend may enable ably is not found in nature. The present plan is such performance as is indicated in the example. useful if the conductivity is around 10- 8 or lower. Returning to the waveguide properties, the TEM The dielectric constant is about 6. mode (with vertical polarization) is t he one that has The lower boundary has some unusual properties. the greatest probabilit.v of enabling long-range (These are also characteristic of the ionosphere at communication . It is the only propagating mode frequencies below VLF.) The gradual increase of at frequencies below about 2 kc/s (i ncluding the conductivity [Van Hippel, 1954] provides an effective above example) . boundary Jor each kind of field, that for the electric This preliminary study has indicated that the field being closer than that for the magnetic field. deep waveguide is probably a definite physical In each case, there is a sort of skin depth in the phenomenon. The properties of its dielectric and boundary [Wl1eeler, 1952]. Both of these boundaries boundaries are not known quantitatively, so it is make comparable contributions to tbe total dissipa­ uncertain to what distances this waveguide may be tion factor of the waveguide, which determines the useful for communication or related purposes. Some exponential attenuation rate. rather optimistic assumptions as to these properties 190 - I lead o ne to s pe(; uhtc 0 11 di sL nllccs of Lite order of of workill g with this group is acknowledged wi Lh 1500 kill .
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