Buried Sources

This conclusion is of paramount importance. It means that there is no effective mechanism for transporting molecules from a buried source to the surface when the soil is very dry. The transport process can begin anew when rainfall, or perhaps artificial introduction of water to the soil, raises the saturation to levels where diffusion can again occur. The reservoir of sorbed molecules simply awaits a mechanism. 

Once molecules reach the surface, whether from a buried source as in the foregoing discussion or by some other transport mechanism from a nonburied source, they tend to sorb onto the surface particles. Some will be in the vapor state and some in solution, but as any water evaporates, they must revert to vapor state or sorb to surface particles. 

Dried Puddles Concentrate Molecules In our common experience, we have all observed the formation of puddles after a rain. We realize, without much analysis, that the puddle is not formed from rain that fell in only that location. It contains water that fell nearby and flowed to that area, which is at the locally lowest elevation. If the rain falls on an area that has buried sources of explosive molecules, then some of those that were sorbed to the surface particles above the source will be dissolved and carried into the puddle. When puddles dry they leave a concentration of molecules on the surface soil particles. Thus, an irregularly shaped area of relatively high concentration of molecules may appear some distance from any buried source. See the discussion on Figure 8.2 p 182. 

This concentration in puddles does not require a buried source. If a source on, or above, the surface receives rain, concentrating puddles can form. Since the presumed reason for searching for trace explosive molecules is to locate the source, some ingenuity may need to be exercised to complete that task. In fact, Phelan and Webb [1, pp. 70, 71] report on work by Hewitt et al. [16] where they buried mines on a gentle slope. The signatures from these mines were found to form in patterns where concentration decreased with distance (a few feet) from the mine as the surface water flowed down the slope away from the mine. 

Molecules that originate from a buried source, upon reaching the surface, may be carried away by wind currents. This is not always the case since they emerge from their upward journey through the pores in the soil into a region of relatively stagnant air called a boundary layer. 

Plumes in Air It is clear that, if the supply of molecules is adequate, plumes can form in air as well. While there may not be enough concentration in the air above buried sources to form plumes, in the case of unburied explosives, such as in IEDs or UXO covered loosely with rubble, there may be exploitable plumes. When such plumes develop, there will often be some level of urgency associated with locating the source. Sometimes, when logs seem to be following... 

Search strategies that exploit the natural tendency of the molecules to remain adsorbed on surface particles and to become trapped in the boundary layers near the surface are likely to be most productive. If odor plumes from buried sources do develop, they are likely to remain close to the surface until they dissipate from turbulence. 

When seeking buried sources, the material presented here should make it clear that some times and some weather conditions are more favorable than others. Alternately, when circumstances do not permit waiting for a more propitious time or weather condition, it may be that artificially increasing either moisture or temperature will prove useful. 

Several types of computer models have been developed for estimating the expected concentrations of the chemicals of interest as they move away from the source. Soil transport models attempt to estimate the expected concentration at the surface above buried sources. Plume transport models attempt to estimate the concentrations within a plume, along with its shape and position. A different form of model is designed to guide a search pattern for employing a sensing system to trace a plume. 

Upward penetration distance (in metres) of radon from a buried source in terms of bulk diffusion coefficient (D ), fraction of radon detectable (N/Nq) and thickness of inactive cover above the source (from Novikov and Kapkov, 1965)... 

Indeed all our evidence from studying the filling histories of fields (cf. Karlsen Larter 1989, 1991 Horstad et al. 1990 Karlsen et al. 1993, 1995 Skalnes 1993 Skalnes et al. 1993 Horstad 1995 Larter Aplin 1995 Angard 1996 Steinhoff 1996 Bhullar et al. 1998, 1999a,b Sletten 2003 Winterstad 2003 Xu 2003) indicate that structures which in the distant past (e.g. 70-50 Ma bp), were linked to establish oil wetting migration systems, will continue to utilize reservoir-proximal parts of this system even when other more shallowly buried source rocks become mature. 

Within subsiding continental basins, groundwater can be induced by sediment compaction (pressure water) and by driving forces from topographic relief or from variations in fluid density. The intensive water flow can disturb the thermal profile and change the time-temperature history of buried source rocks. Maximal rate of pressure-water expulsion (Rpwe) can be estimated relatively easily using Eqs. 6.i and 6.2 for the case of one-dimensional consolidation of homogeneous sediments on immobile basement ... 

Corrosion due to stray current—the metal is attacked at the point where the current leaves. Typically, this kind of damage can be observed in buried stmctures in the vicinity of cathodic protection systems or the DC stray current can stem from railway traction sources. 

AU inactive sites where wastes have been disposed of in tbe past should be inventoried. Data should be gathered on buried wastes, including types, quantity, and sources. A groimdwater-monitoriug program should be developed. 

Examples of the sacrificial-anode method include the use of zinc, magnesium, or aluminum as anodes in electrical contact with the metal to be protected. These may be anodes buried in the ground for protection of underground pipe lines or attachments to the surfaces of equipment such as condenser water boxes or on ship hulls. The current required is generated in this method by corrosion of the sacrificial-anode material. In the case of the impressed emf, the direct current is provided by external sources and is passed through the system by use of essentially nonsacrificial anodes such as carbon, noncor-rodible alloys, or platinum buried in the ground or suspended in the electrolyte in the case of aqueous systems. 

Electromagnetic (EM) Conductivity Measures the electrical conductivity of materials in microohms over a range of depths determined by the spacing and orientation of the transmitter and receiver coils, and the nature of the earth materials. Delineates areas of soil and groundwater contamination and the depth to bedrock or buried objects. Surveys to depths of SO to 100 ft are possible. Power lines, underground cables, transformers and other electrical sources severely distort the measurements. Low resistivities of surficial materials makes interpretation difficult. The top layers act as a shunt to the introduction of energy info lower layers. Capabilities for defining the variation of resistivity with depth are limited. In cases where the desired result is to map a contaminated plume in a sand layer beneath a surficial clayey soil in an area of cultural interference, or where chemicals have been spilled on the surface, or where clay soils are present it is probably not worth the effort to conduct the survey. 

Bituminous This term is used for products obtained from both petroleum and coal tar sources but the petroleum products are the more widely used. These materials are very resistant to moisture and tolerant to poor surface preparation. They are only available as black, dark brown or aluminum pigmented. The last has reasonable outdoor durability but, without the aluminum, the film will crack and craze under the influence of sunlight. Normally they cannot be over-coated with any other type of paint, because not only will harder materials used for over-coating tend to crack or craze but there is also a possibility that the bitumen will bleed through subsequent coats. The best use is as a cheap waterproofing for items buried or out of direct sunlight. 

Disadvantages Possible interaction effects on other buried structures (Section 10.6) subject to the availability of a suitable a.c. supply source or other source of d.c. regular electrical maintenance checks and inspection required running costs for electrical supply (usually not very high except in the case of bare marine structures and in power stations where structures are often bare and include bimetallic couples) subject to power shutdowns and failures. 

Nevertheless, special consideration should be given to any measured small positive changes in structure/soil potential on a nearby buried pipe or cable if there is reason to believe that the secondary structure is already corroding because of local soil conditions, or as a result of stray currents from another source. 

Groundbed in cathodic protection of underground structures, a buried mass of inert material (e.g. carbon), or scrap metal connected to the positive terminal of a source of e.m.f. to a structure. 

Primary Structure a buried or immersed structure cathodically protected by a system that rpay constitute a source of corrosion interaction with another (secondary) structure. 

In more recent years, 7-ray scattering techniques have been considered for buried mine detection. As an initial source of bombarding energy, 7-radiation is more attractive than neutrons because it is easily available in the form of relatively inexpensive long-lived radioisotopes. 

One of the most direct methods is photoelectron spectroscopy (PES), an adaptation of the photoelectric effect (Section 1.2). A photoelectron spectrometer (see illustration below) contains a source of high-frequency, short-wavelength radiation. Ultraviolet radiation is used most often for molecules, but x-rays are used to explore orbitals buried deeply inside solids. Photons in both frequency ranges have so much energy that they can eject electrons from the molecular orbitals they occupy. 

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