The effect of changes in pressure and temperature on electrical properties of rocks. There has been increasing interest in the electrical properties of rocks at depth within the Earth and the Moon. The reason for this interest has been consideration of the use of electrical properties in studying the interior of the Earth and its satellite, particularly to depths of tens or hundreds of kilometers. At such depths pressures and temperatures are very great, and laboratory studies in which these pressures and temperatures are duplicated have been used to predict what the electrical properties at depth actually are. More direct measurements of the electrical properties deep within the Earth have been made by using surface-based electrical surveys of various sorts. An important side aspect of the study of electrical properties has been the observation that, when pressures near the crushing strength are applied to a rock, marked changes in electrical properties occur, probably caused by the development of incipient fractures. Such changes in resistivity might be used in predicting earthquakes, if they can be measured in the ground. See also: Geoelectricity; Geophysical exploration
Electrical zones in the Earth
Attempts to measure the electrical properties of rocks to depths of tens or hundreds of kilometers in the Earth indicate that the Earth's crust is zoned electrically. The surface zone, with which scientists are most familiar, consists of a sequence of sedimentary rocks, along with fractured crystalline and metamorphic rocks, all of which are moderately good conductors of electricity because they contain relatively large amounts of water in pore spaces and other voids. This zone, which may range in thickness from a kilometer to several tens of kilometers, has conductivities varying from about ½ ohm-m in recent sediments to 1000 ohm-m or more in weathered crystalline rock.
The basement rocks beneath this surface zone are crystalline, igneous, or metamorphic rocks which are much more dense, having little pore space in which water may collect. Since most rock-forming minerals are good insulators at normal temperatures, conduction of electricity in such rocks is determined almost entirely by the water in them. As a result, this part of the Earth's crust is electrically resistant. There are numerous experimental difficulties involved in trying to measure the electrical properties of this part of the Earth's crust, but it appears that the resistivity lies in the range 10,000–1,000,000 ohm-m.
At rather moderate depths beneath the surface of the second zone, resistivity begins to decrease with depth. This decrease is considered to be the result of higher temperatures, which almost certainly are present at great depths. High temperatures lead to partial ionization of the molecular structure of minerals composing a rock, and the ions render even the insulating minerals conductive. As temperature increases with depth, this effect becomes more pronounced and resistivity decreases markedly. The maximum resistivity in the Earth's crust probably occurs at depths of only 3–6 mi (5–10 km) below the surface of the crystalline basement. At depths of 18–24 mi (30–40 km), corresponding to the top of the mantle, resistivity is only about a few hundred ohm-meters. Beyond this depth, resistivity seems to decrease slightly with increasing depth, but again drops sharply at depths around 420 mi (700 km). At greater depths the resistivity is less than 1 ohm-m. A profile of resistivity as it varies downward into the Earth is shown in Fig. 1.
At relatively shallow depths information is gained by lowering equipment into bore holes to measure the electrical properties of rocks directly. Another approach is the use of low-frequency currents introduced into the Earth at the surface. These methods are restricted to use in studying resistivities at depths less than 6 mi (10 km). Information from greater depths is obtained by studying slow variations in the Earth's magnetic field and the currents induced in the Earth by these variations. Fluctuations in the magnetic field with periods measured in months penetrate to distances up to 600 mi (1000 km), and they may be used in estimating conductivity to these depths.
Laboratory investigations have also been pursued to find how the electrical properties of rocks depend on the pressure and temperature deep in the Earth. Inasmuch as the temperature at depths of 24–36 mi (40–60 km) is probably in the range 1470–2200°F (800–1200°C) and the pressure is in the range 140,000–210,000 lb/in.2 (10,000–15,000 kg/cm2), these conditions are difficult to duplicate in the laboratory. Both electrical conductivity and dielectric constant have been measured by several researchers at pressures to 700,000 lb/in.2 (50,000 kg/cm2) and temperatures to 2200°F (1200°C), using dry rock samples. The most obvious conclusion from such studies is that for this range of conditions the effect of temperature on electrical properties is much more profound than that of pressure. An increase in temperature from 68 to 1830°F (20 to 1000°C) will reduce the resistivity of a dry rock by a factor of 106. On the other hand, an increase in pressure from 0 to 700,000 lb/in.2 (50,000 kg/cm2) changes the resistivity of a dry rock by less than 100%. Typical behavior for the resistivity of dry rocks as temperature is raised is shown by curves in Fig. 2. At temperatures up to about half the melting point, conduction increases relatively slowly with increasing temperature, and it is thought that conduction is due to impurities, which contribute ions under weak thermal excitation. At temperatures above half the melting point, conduction increases rapidly with increasing temperature, with conduction being caused by ions torn from normal lattice positions in the crystals by violent thermal agitation. At the melting point, conduction increases abruptly as a rock melts and becomes highly ionized.
It has been observed that silicic rocks, such as granite, granodiorite, and rhyolite, are poorer conductors of electricity at all temperatures than other rock types. Rocks, such as gabbro, basalt, and dunite, which are rich in ferromagnesian minerals, are more conductive than the silicic rocks by about an order of magnitude. This is believed to result from the fact that the principal ions which contribute to conduction are the small ferromagnesian ions, which are more abundant in the dark rocks.
Typical curves for the behavior of resistivity and of dielectric constant with the application of pressure are shown in Figs. 3 and 4. The dielectric constant is found to increase by 10–20% as the pressure on a rock sample is increased to 1400–2800 lb/in.2 (100–200 kg/cm2) and then to increase more slowly as the pressure is elevated further. The increase in dielectric constant is due in part to an increase in density of the rocks under pressure.
Two forms of behavior have been recognized in studies of the behavior of resistivity at pressures up to 700,000 lb/in.2 (50,000 kg/cm2). With some rocks resistivity decreases uniformly with increasing pressure; in other rocks resistivity decreases at low applied pressures but increases at high pressures. A uniform decrease has been observed in rocks such as basalt, diabase, and amphibolite, while an increase in resistivity over part of the pressure range has been observed in microcline-rich rocks. The increase in resistivity may be associated with a decrease in the mobility of charge carriers as the crystal lattice is compressed.
The effect of pressure on the resistivity of rocks in which conduction is determined by water content rather than by the crystal matrix is quite different. Several investigators have studied the effect of having greater pressure on the crystal framework than on the pore fluid. As an overpressure is applied to the rock skeleton, the framework starts to close in on the void spaces, reducing their volume and increasing the resistivity. It has also been observed that, if the overpressure is made great enough, resistivity begins to increase again, a phenomenon which is attributed to the opening of small cracks prior to rock failure, with the movement of water into these cracks to contribute to conduction. The behavior of resistivity for a number of rock specimens as a function of frame overpressure is shown by the curves in Fig. 5. According to W. F. Brace and A. S. Orange, observations of such changes in resistivity might be used to detect minute changes in the structure of rock under stress. They observe that the buildup of strain in the Earth preceding earthquakes ought to be accompanied by changes in resistivity, and that these changes might warn of an impending earthquake. See also: High-pressure mineral synthesis; Petrology
Electrical zones in the Moon
In planning the exploration of the Moon, some thought has been given to the use of measurements of electrical properties as a means for studying the Moon's interior. Although there is little information on the electrical properties of the Moon, the knowledge gained about the Earth can be used to make an intelligent guess. The major difference is the lack of an atmosphere and moisture, which means that the surface rocks on the Moon must be quite different from those on Earth. Measurements with radar and infrared sensors have indicated that the surface of the Moon must be covered with a rock froth or soil which has a very low density—the rocks may be only about 10% solid material. Inasmuch as there is probably no moisture in this rock, it seems very likely that the electrical resistivity must be very large, perhaps 1010 ohm-m.
It is believed that temperature increases with depth inside the Moon, though perhaps not as rapidly as in the Earth. The temperature at the center of the Moon may be 3140°F (2000 K) and, as a result, the rocks in the interior can be expected to be conductive, just as they are on the Earth. The resistivity at the center of the Moon may be of the order of 1–100 ohm-m.
The change in resistivity between the highly resistant surface and the conductive interior depends very much on the composition of the Moon. Some scientists believe that the interior contains as much water as the rocks within the Earth, at depths beyond which evaporation into space has been able to take place. Others believe that the Moon had a different origin from the Earth, and thus never contained very much water. If water is present, one would expect the resistivity to drop very rapidly over the first few kilometers in depth, as water contributes to electrical conduction. If the Moon is devoid of water, the resistivity should change more gradually with depth and become low only at depths of many hundreds of kilometers, where the temperatures are high. The presence of water at moderate depths would be very important to future lunar exploration—it might be used as fuel for trips deeper into space, or even to provide a tenuous atmosphere to protect humans' installations on the Moon. Therefore it is likely that electrical surveys for water on the Moon will have a high priority in the early stages of lunar exploration. See also: Moon