Thermal anomalies as informative signs of underground nuclear explosions

Investigation of residual effects from peaceful explosions is a laborious and expensive task, requiring the creation of special missions with the appropri­ate hardware and monitoring equipment including vehicles, staffed by highly qualified scientific and technical personnel. For example, to study the thermal fields, among other things, requires manned aircraft. It is consider­ably more convenient to study the geophysical implications and methods of their control at test ranges where a developed technological infrastruc­ture and trained personnel with the necessary qualifications exist to ensure that the results of these studies for relevant peaceful uses of nuclear explo­sions are adequate. Therefore, a significant part of the material in this section is based on the results of experiments conducted at the Semipalat — insk nuclear proving ground.

The majority of the surveyed explosions took place at the Degelen moun­tain range, located near the Kalba-Chingiz deep fault. This complex, mostly granite, volcanic and volcanic-sedimentary rocks, forms a large structure with a diameter of about 30 km. Intrusive rocks are interspersed in the form of individual granite-like bodies of relatively small size. A smaller part of the surveyed explosions were in the area of the test site Balapan located close to the eastern border of the landfill. Geologically, much of it is placed in the Zaisan folded region. A latitudinal piece of the Kalba-Chingiz deep fault, which separates this area from Chingiz-Tarbagatai, runs almost along the southern border of the latter. The depth of the water table is 200-400 m. The entire area is characterized by a homogeneous filler surface, folded eluvial sands of 4-6 m, or dense clays (Busygin and Andreev 2004).

Climatic conditions at Semipalatinsk are sharply continental with an average temperature of about +1°C. Summer is hot and dry with tempera­tures up to +40°C. Autumn and spring are cloudy and cold with average temperatures not higher than +7°C. The exception is May, when it is warm and clear. Winter is cold with little snow and with temperatures as low as -40°C. These geological and climatic characteristics of the area determine the conditions of conservation of thermal lesions in the rocks, the formation of thermal anomalies on the ground surface, and the possibility of their detection.

The first results of the thermal regime created by the underground explo­sions (UGE) on the ground surface were obtained in the late 1980s and were published in a series of papers by Busygin et al. (1999) and Busygin and Andreev (2004). First ring-shaped forms were discovered covering the cleavage zone of the UGE as they were luminous in the infrared spectrum. The physics of these phenomena remains unclear. The formulation and solution of rigorous mathematical tasks was required to describe the proc­esses of heat transfer and gas flow. However, a comprehensive package of initial data and a set of direct measurements of temperature and air flow in the cavity and the Earth ’s surface, made in a wide range of temporary, geometric, and meteorological conditions, was also required.

Review of materials on the sprung hole of a UGE shows that for many years they have a high internal temperature, slowly decreasing over time (Israel, 1974’ Taylar, 1973). Results for the domestic UGE show that the average air temperature in the boiler cavities of the explosion conducted more than 10 years before, is 30-50°C, i. e., the boiler cavities of UGEs are long-term sources of heat.

It follows from Section 27.2 that the boiler cavity after the UGE is not absolutely airtight. The presence of anthropogenic influences, fracture zones, column collapses and other tectonic features makes the contents of the boiler cavity available for air transport and, consequently, for the removal of heat and gases present in the cavity to come to the surface. To control the intensity and configuration of thermal anomalies on the ground surface, the method of heat shot is employed from onboard aircraft, using the ‘Volcano’ thermal imaging equipment which is modified with a unit controlling the film transport rate, which requires a flight height range of 200-3,500 m above the surface. The method of optical-and-mechanical scan­ning was used in the direction perpendicular to the direction of travel of the thermal imager in the aircraft. The flights carried out tasks over the examined area, and the height of the flight was supposed to provide the required coverage.

The optical part of the recording apparatus was a cooled infrared radi­ometer with a sensitivity of 8-14 microns. The sensitive nature of the equip­ment required that it be placed in a hanging gondola on the outer side of the fuselage of the carrier, which eliminated the effects of the aircraft glass windows. Along with the heat-sensing aerial photography conducted in the visible spectrum which allowed detailed information about the surrounding landscape to be obtained, there was a need to decrypt the thermal images and a need to accurately reference the area of the thermal objects. In this way (Busygin and Andreev, 2004), more than 50 UGE were examined during the period from 1 to 26 after the date of the initial measurement.

Almost all of the surveys performed on the ground surface in the epicentral area were observed to be ring-shaped or curved thermal structures, cover­ing the cleavage zone of the explosion. The typical form of these structures is shown in Fig. 27.1.

To validate the existence of thermal anomalies, as long-term residual processes occurring in boiler UGE cavities, investigations were carried out in two directions. The first set of investigations was connected with the hypothesis of uneven solar heating of the soil due to the different solar exposure of mountain slopes and micro-relief. To this end, a loop of night and pre-dawn measurements in autumn and winter under cloudy conditions with zero duration of sunshine and little difference in day and night values of air temperature were performed. The results confirmed the presence of ring-shaped thermal anomalies. Indirectly, the role of solar warming from the thermal anomalies is refuted, as solar radiation during the cold season could ‘warm up’ only one side of the failure cone and warming was found in these ring-shaped patterns.

image293

(c)

27.1 Typical view of a thermal anomaly caused by an underground nuclear explosion on the surface during daytime (Busygin and Andreev, 2004): (a) and (b) explosion in gallery; (c) explosion in shaft.

The second set of investigations was conducted to test the binding of thermal anomalies on the ground surface to a picture of the local actions of UGE. The problem was solved using ground-temperature well-logging methods in the area of the thermal anomaly tied to the locality on the thermal image. Measurements of ground surface temperature were made with copper wire resistance thermocouples (temperature sensors); the standard error did not exceed 0.2-0.4°C. For the measurements of each thermal anomaly, one or two measurement lines were created. Not less than 20 sensors were placed along a cable line at a distance of about 5 m from each other (Fig. 27.2). Measurement lines were located on the ground around the diameters of circles covering a cleavage zone. The sensors are protected from direct solar radiation by special shields. The true value of the measured temperature T was calculated for each sensor separately after adjusting for the actual impedance of the line. Each cycle of measurements was carried out for three days with interval readings after 2 hours. The duration of one data point on one line does not exceed 10 minutes.

Figure 27.3 shows the typical spatial distribution of temperature for the autumn-winter period for the profile of the location of temperature sensors

image294

27.2 Scheme of the thermocouple placement on a thermal anomaly (Busygin et al., 1999): solid curve is the surface measurement line; the circles with numbers are the numbered thermocouples.

shown in Fig. 27.2 . Distances between sensors are marked as the abscissa on a proportional scale. It is evident that sensors located in a highlighted strip correspond to higher values of ground temperature compared with background values of temperature (about -9°C). The excess temperature reaches 8-10°C.

Figure 27.3 also shows that the gases exiting to the Earth’s surface have a temperature lower than the rock at the charge depth (6-8°C throughout the year). This has two causes. First, the cold-season air passing through an explosion cavity that is 20-40°C did not have sufficient time to warm up due to the high velocities of the air masses. Second, due to a lack of integrity arising from formation of a large number of deep cracks, there is deeper cooling of the rocks in the array, which significantly increases the contact area of the exhaust air from the cooled rock. To confirm the fact that the removal of heated air instead of air at the natural temperature of the boiler at the depth of the cavity was examined, a peaceful UGE was conducted in Kalmykia (Russia) in the warm season, i. e. at a background temperature of 21-23°C (Granberg et al. , 1997). Temperature thermal anomalies for it reached 28-34°C, which certainly indicates the presence of an artificial heat source from the UGE.

In parallel with the temperature well logging, estimates of the geometric dimensions of thermal anomalies were made. It was shown that a suffi­ciently broad energy spectrum at the depths of the UGE gives the maximum radius of the thermal anomalies which varies from 80 to 250 m, while the width of the thermal ring varies from 20 to 60 m. It was not possible to establish the full duration of thermal anomalies, as over a nearly ten-year period, their thermal anomalies remained virtually unchanged. For the UGE held in galleries, the largest fixed term for thermal anomalies at the time they could be observed was 25-26 years and for UGEs conducted in wells it was 16-18 years.

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N

27.3 Temperature distribution on the surface measurement profile (Busygin et al., 1999): the numbers N indicate the thermocouple numbers of the profile shown in Fig. 27.2.

It is certainly interesting to study daily and seasonal measurements of the thermal effects of UGEs at individual sites. Diurnal temperature vari­ation, obtained by simultaneous measurements on a strip heater removed from the UGE and from the undamaged section of the Earth ’s surface, averaged over 48 experiments (October-November), is shown in Fig. 27.4 (here t0c = local time). It can be seen that the thermal effect at the UGE site was observed continuously for days in the field, according to the thermal image, due to removal of heat from the air cavity (line 1). Characteristically, the temperature fluctuations during a day in the field of thermal anomalies are about 1°C, while for the damaged portion of the UGE, site surface peak-temperature reaches 4°C.

Significant differences are observed in the form of plots of temperature versus time for undisturbed and disturbed UGE sites. For undisturbed sites, the temperature dependence is very ordinary, without thermal anomalies in the afternoon heating and only minimum temperature anomalies at 7-8 a. m. All this also suggests that the observed thermal anomalies are not the result of solar heating of the Earth ’s surface and that the surface albedo changes under the influence of the UGE.

Seasonal temperature variation, in contrast to the daily temperature vari­ation, was studied the least. In particular, during the warmer months there have been instances when the UGEs conducted in groups decreased by 2-3°C in the cleavage zone compared with the background temperature. To explain such phenomena, a phenomenological model for the formation and dynamics of thermal anomalies based on the principles of ‘heating effect’ was proposed. Its essence lies in the fact that the movement of air through the heated boiler cavity occurs by gas convection, and the direction of motion can be either from the portal tunnel up through tectonic faults in the epicentral area, or vice versa. From the equation for the depression

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27.4 Diurnal surface temperature variation in area of thermal anomaly (Busygin and Andreev, 2004): 1, undisturbed area; 2, heat efflux area determined on photograph.

thrust air he = A(tB — tH), where A isa coefficient for atmospheric parameters and channel exhalation of air; tH is outside air temperature; and tB is aver­aged over the profile of raising the air temperature inside the rock, it is evident that the magnitude of depression is proportional to the temperature difference outside and passing along the tectonic disturbance of air, and the direction of motion is determined by the sign of this difference. If the tem­perature tB is calculated by using the empirical formula tB = 1.1(tp — 6)/H + 6 (Busygin et al, 1999), where tp is air temperature in the boiler cavity, and H is the reduced depth of the UGE, we can obtain approximate values of the external temperature of a UGE site, for which one should observe a positive depression (he > 0). For example, for an explosion with the yield 1 kt, warhead detonation depth H = 100 m, a positive depression is observed when the outside temperature does not exceed 16°C if the air temperature in the cavity is 100°C. If the temperature in the cavity decreases to 20°C, the boundary outside temperature decreases to 7-7.5°C.

The estimates given are quite approximate until a full-scale experiment can be carried out with monitored directions of transport and air flow to the outside air temperature. It should be noted that the direct measurement of air movements is possible only in the portal tunnel. In the area of the cleavage phenomena, as mentioned above, anemometric measurements are difficult due to the complexity of micro-relief areas and the inability to visu­ally determine the position of the majority of cracks, which serve as conduits to move the air.

Air mass velocity was measured using an anemometer at a distance of 40-50 m from the tunnel portal. The direction of air mass movement is determined by the deviation of the flame or the direction of motion of smoke from burning smoke grenades (at speeds below 0.2 m/s). The meas­urements were performed at two points located at the ‘top’ and ‘bottom’ gallery. In each session, measurements of velocity were carried out at least three times for a duration of 10 s. By measuring the mean values taken for air velocity at the point of measurement, the air flow can be calculated. Results are summarized in Table 27.1 which indicate the following:

Table 27.1 Direction velocity and flow rate in gallery

Temperature of Direction of

Flow velocity at

Flow rate

external

air (°С) air flow

‘bottom’ and ‘top’ (m/s)

(m3/h)

22.4

To gantry

0.70-0.22

707

20.4

To gantry

0.75-0.20

495

17.0

To gantry

0.60-0.20

475

17.3

To gantry

0.65-<0.10

475

16.2

To gantry

0.73-0.10

466

16.2

To gantry

0.22-0.20

454

19.8

To gantry

0.70-0.10

466

21.0

To gantry

0.70-0.10

466

21.8

To gantry

0.72-0.20

705

0.5

To cavity

<0.10-0.30

314

-5.4

To cavity

<0.10-0.30

348

-6.0

To cavity

<0.10-0.20

180

-9.0

To cavity

0.10-0.30

296

-7.0

To cavity

<0.10-0.20

226

-7.4

To cavity

<0.10-0.40

226

-12.2

To cavity

<0.20-0.80

-14.0

To cavity

<0.20-0.90

Source:

3usygin et al. (1999).

The experimental results qualitatively confirm the adequacy of the pro­posed model to real processes. It should be noted that in wells, a high temperature persists for much longer than in galleries because the heat loss occurs only due to natural convection (i. e., there is no is ‘stove’ effect). According to field measurements at Semipalatinsk site, the temperatures in the wells have decreased to 42-45°C some 6 years after the explosion, while in the galleries the temperature has been observed for 1.5-2 years.

Along with air, radionuclide products are transported to the Earth’s surface. Direct measurements of the exposure dose on the profile of thermal anomalies have shown that in this case the radiation levels are 3-5 times higher than natural background levels (Fig. 27.6). Comparison of tempera­ture and gamma-radiation curves indicates a high degree of correlation of these two processes. The distribution of activity and concentration of radon behaves similarly. In the location of the thermal anomaly, the volume of radon activity is 80-100 Bq/m3. At the same time over the epicenter of the explosion, the natural background of ionizing radiation remains: 5-10 micro — R/h for gamma-rays and 30-40 Bq/m3 for radon.

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