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CHAPTER VI

Applications of electrical geophysical methods in environmental and civil engineering

The precise information about spatial distributions of different soil properties is essential not only in agriculture, but also in civil and environmental engineering. This chapter features applications of electrical geophysical methods to the problems of environmental engineering, criminology, and remediation services requiring estimation of various soil properties.

Electrical geophysical methods provide a means to monitor the changes in hydrological conditions and to investigate subsurface soil properties in urban areas without any disturbance (VI.1.). Methods of self-potential, electrical profiling, vertical electrical sounding, and non-contacting electromagnetic profiling were applied to urban soils in Kiev, Ukraine, and Astrakhan’, Russia. Groundwater fluctuations in the areas were measured in details using the methods. The obtained information was incorporated in engineering projects; thus, the costs of site maintenance were dramatically reduced. The methods ensure quick yet non-destructive estimation of soil and hydrological conditions in cities assisting with successful restoration and preservation of urban soils and municipal constructions. The methods of four-electrode profiling and non-contact electromagnetic profiling are shown to successfully outline the areas polluted by petroleum byproducts and solutions used for petroleum and gas mining (VI.2). The test study conducted in northwest Siberia helped to distinguish the areas with different pollution levels from the native non-polluted soils. The VES method outlined the polluted soil layers and the permafrost horizons in Gelisols. The method of four-electrode probe was tested for quick in-situ search of non-metallic objects buried in soils and related to crimes (VI.3.). The application of the method was based on sharp differences in electrical resistivities of undisturbed and disturbed soils. This study was conducted upon a request of Russian Ministry of Internal Affairs.

VI.1. Geophysical methods for evaluating physical properties and hydrology of urban soils

Urban soils are highly disturbed, consist of mixed genetic horizons and non-soil materials, and have hydrological properties different from those of natural soils (Gennadiev et al., 1992). Construction activities in modern cities change soil, geological, and hydrological conditions and often cause groundwater rising and fluctuation (Stroganova and Agarkova, 1992). Rising groundwater destroys buildings and causes landslides. Timely and precise soil and hydrological information is essential to prevent destruction. Such information includes stratification of water-bearing and waterproof horizons, location of water charge and discharge areas, and estimation of groundwater levels. Obtaining the hydrological information with conventional methods, such as drilling and excavation, is difficult and destructive in urban areas. Therefore, rapid, precise, and non-destructive methods for the investigation of soils and hydrological conditions are highly desirable in modern cities.

The electrical geophysical methods, which allow us to evaluate various soil properties, are promising for the application in urban areas. As shown previously, the geophysical methods should be carefully chosen, designed, and adapted for a practical application. Especially, these methods have not been applied to measure soil properties in urban areas. The objectives of this chapter study are to test the suitability of the geophysical methods for measuring soil physical properties and groundwater fluctuations in urban areas and to find advantages and limitations of the proposed methods (Pozdnyakova et al., 1999a).

Materials and methods. The study was conducted in two cities: Kiev, Ukraine and Astrakhan’, Russia. Hazardous hydrological situation caused by unknown factors appeared in Kiev-Pechersk Lavra (Kiev, Ukraine) near the Church of Holy Cross Elevation in 1987 (Fig. 62). Kiev-Pechersk Lavra, also known as Monastery of Caves, is a center of Russian Christianity since IX century and still the residence of Ukrainian patriarch. The church was built in 1700 above the holy caves, a place of pilgrimage of Russian Christians since XI century. The caves have historical and art treasuries, such as XI century frescoes, living cell of Russian first annalist Nester, and the burial niches with the remains of civil and religious leaders. The caves extend 228 m in length, with various depths from 5 to 20 m. The groundwater penetrated in the caves and partly destroyed wall frescoes and other masterpieces in the caves and church interiors. To prevent groundwater seepage into the caves, a concrete tier wall was built around the church (Fig. 62). Unfortunately, the activity did not alter the hazardous situation. Therefore, it was necessary to analyze accurately hydrological conditions in the Patriarch garden. The methods of EP, VES, and SP were applied to investigate and solve the problem.

Fig. 62. Scheme of the investigated area within Patriarch garden, Kiev, Ukraine: the numbers in circles show VES locations, lines are routes for EP investigation (Route 1 from VES 1 to VES 21 and Route 2 from VES 27 to VES 21), and the darken place near the church is a concrete wall.

The hydrological conditions in the delta Volga, where Astrakhan’ city is located, are described in Chapter V.2. Not only the farming lands but also the urban areas in the delta Volga suffer from the destructive activity of the rising saline groundwater. The groundwater caused visible destruction of more than 20% of the buildings in Astrakhan’ city to the summer of 1994. The natural hazardous groundwater condition in delta Volga was further aggravated in the urban areas by the uncontrolled leakage from the canals and plumbing pipes. One hundred eighty monitoring wells were set up in Astrakhan’ city to measure groundwater fluctuations. Nevertheless, the number of wells was not enough to provide detail information of the groundwater level for the entire city, especially when groundwater rising was influenced simultaneously by many factors and, therefore, unpredictable. The methods of VES and NEP were tested for detail outlining of the groundwater level within the representative part of Astrakhan’ city (Fig. 63). The study area was located in the center of Astrakhan’ with a large change of elevation, which induced a high variation of groundwater level within the area.

Fig. 63. Scheme of the investigated area in Astrakhan’, Russia: the numbers show VES locations; arrows indicate the NEP routes, and black circle represents Dramatic Theater with active drainage.

We utilized geophysical methods of vertical electrical sounding and electrical profiling to estimate the soil layering, which influence water distributions and fluxes in soils. Method of self-potential was used to map directions and intensities of water fluxes in the shallow soil subsurface. Non-contact electromagnetic profiling seems promising for application in urban areas, since it does not require physical contact with the soil surface and can measure soil electrical resistivity through any firm pavement.

Results and discussion

Investigation in Kiev, Ukraine. In Kiev-Pechersk Lavra, the problem area was located on a hill far above the groundwater level. Therefore, the problem was attributable to temporary subsurface water fluxes fed by precipitation. The excess water accumulated in subsurface in spring because of snow melting and in summer during intensive rainfalls. Due to the hill topography, the water could accumulate in the soil covering the whole territory of Upper Lavra and then flow into the Patriarch Garden as shallow subsurface fluxes (Fig. 62). We used the VES and EP methods to investigate the properties of water-bearing and waterproof layers essential for the development of the subsurface water fluxes. The directions and intensities of the fluxes were determined with the SP method.

The VES and EP methods revealed the complex stratification of the hill slope in the Patriarch garden near the Church of Holy Cross Elevation (Fig. 62). All the VES curves were interpreted as three layer curves with ER1>ER2<ER3 (Fig. 64). The top layer was represented by the eroded Mollisols of coarse textures with electrical resistivity (ER1) about 125 ohm m for loamy sand and about 50 ohm m for sandy clay loam. The second layer was a thick clay layer (7 m) with low electrical resistivity (ER2) from 2 to 16 ohm m. The clay was saturated and gleyed in some places, which was indicated by 2 ohm m resistivity. The third layer with the resistivity (ER3) about 2000 ohm m was horizontally deposed sandstone.

Fig. 64. Typical distribution of electrical resistivity within soil profile measured by VES in Patriarch garden, Kiev, Ukraine.

Soil thickness was only about 20 cm on the top of the hill, but it increased toward the gallery gate up to 2 m (Fig. 62, VES 1 and 4). Soil thickness increased up to 2.5 m from the gallery gate further downhill (Fig. 62, VES 17). Along the same direction the texture in surface soil layer changed from loamy sand (VES 4) to clay loam and clay (VES 17 and 21). That change in the texture of the first layer was revealed by the decrease in electrical resistivity (ER1) measured by the EP along the Route 1 (Fig. 62). The electrical resistivity was about 100 ohm m for peaty sand at the top part of the Route 1 (VES 4), then decreased to 65 ohm m for sandy loam (VES 12), and further decreased to 35 ohm m on the surface and to 7 ohm m on the 0.5 m depth for clays near the tier wall (VES 21).

The thickness of clay deposit (second layer) decreased from 8 to 2 m along the line from the top of the hill to the tier wall (Fig. 62). The interpretation of the VES for the second layer revealed that the clay did not bear any intrusions of sand or sandy loam; therefore, water flow inside the layer was impossible. The water flow could be formed only in the topsoil over the layer of waterproof clay.

Although undetectable on the surface, three gullies were revealed in the second layer of waterproof clays by the VES and EP methods. The subsurface water flow could be formed in such gullies. The first gully was detected near the gallery wall along the Route 1. The subsurface water flowed in the gully from the Upper Lavra downhill. Waterproof clay occurred on the surface about 6 m apart from the gallery wall (VES 12) causing bend of the subsurface water flow toward the Church of Holly Cross Elevation. The second gully formed along the garden part, which was indicated by the peat deposition. The narrow (about 17 m) peat band with the depth about 30 cm was formed along the garden path (Fig. 62, VES 1, 27, and 32). Formation of the peat on such steep slope designated that water saturation had occurred here for at least ten years. The second gully was also directed toward the Church of Holly Cross Elevation. The third gully was formed near the rampart and separated from the first and second gullies. Thus, the stratification of the investigated area with the EP and VES methods shown three possible ways for water flow in the soil: (1) near the gallery, (2) along the garden path, and (3) near the rampart.

The method of self-potential was used to measure water flux directions and intensities. An iso-potential map (Fig. 65) was developed with the method on a 5 x 10 m grid; 299 locations were measured with five replications. Three major iso-potential areas detected from Fig. 65. Two areas with negative potentials were formed near rampart and along the gallery (including the garden path) and indicated the areas of water infiltration and development of groundwater flow (Fig. 65, I, II, and III). The third area with positive potentials outlined the seepage zone near the Church of Holy Cross Elevation (Fig. 65, V). The most negative potentials (-250 mV) along the garden path indicated the most intensive subsurface water flow in this area (Fig. 65, IIA, IIB1, and IIB2). The –250-mV iso-potential area developed in surface peaty sand with electrical resistivity (ER1) about 170 ohm m. The less negative potentials (-150 mV) and, therefore, the less intensive water flow occurred near the gallery (Fig. 65, IA and IB). The same negative potential areas were detected in the middle of the garden path and near the rampart (Fig. 65, IIIA, IIIB, and IV). The seepage area was outlined by the 0-mV iso-potential near the Church of Holy Cross Elevation (Fig. 65, V). The seepage area was enriched with clay material having electrical resistivity about 5 ohm m. The percentage of clay in the soil increased toward the corner of the church along with the electrical potential.

Fig. 65. Result of the SP measurements within Patriarch garden, Kiev, Ukraine. The curves are iso-potential lines. The numbers and letters indicate fluxes, zones of infiltration and saturation.

The directions of water flow were predicted from the increasing electrical potential. Three main water flux directions were detected by self-potential method: flux I near the gallery, flux II along the garden path, and flux III near the rampart (Fig. 65). Flux I was formed in thick loam sand about 1.5 m depth and characterized by a stable slow rate. Flux II could be highly intensive during rainfalls, since it was formed in coarse sand layer with a thickness about 0.3 m. Further downhill flux II was separated into two sub-fluxes (IIB1 and IIB2). Fluxes IB1 and IIIB2 merged during the intensive precipitation and seeped near the church infiltrating into the holly caves. Flux III was separated from fluxes I and II and did not influence the seepage near the Church of Holy Cross Elevation.

To protect the Church of Holy Cross Elevation, the following procedures were proposed based on our geophysical exploration near the architecture memorial. First, a hedge should be constructed across the gate to the garden to prevent the surface water flow to the Patriarch Garden from the pavement of Upper Lavra. Second, a small dike should be built perpendicular to the gallery and the garden path to direct subsurface water flow from fluxes II and I into the drain system. Third, to enhance evapotranspiration, trees and bushes with the intensive transpiration ability, such as willows and poplars, should be planted along the gallery and rampart, especially in the areas indicated by low potentials.

Investigation in Astrakhan’, Russia. The VES measurements indicated that the depth of groundwater table decreased downhill for the investigated area (Fig. 66). The groundwater tables were 0.4, 0.75, 3.1, and >5 m for locations 1, 3, 4, and 6, respectively (Fig. 66). Although the determination of the groundwater table by the VES method is much faster than by conventional drilling methods, it still requires considerable time to cover the extent area. The VES application in cities is restricted to the areas with open soil surface. Hence, the VES method alone can not provide a complete map of groundwater levels in urban areas.

Fig. 66. Electrical resistivity distributions within soil profile measured by VES in Astrakhan’, Russia.

In practice, to outline the area where the groundwater table is higher than safe level is, sometimes, more important than to determine the exact groundwater levels at individual locations. The NEP method can be used to outline areas where groundwater level is higher than a threshold level as shown in Chapter V.2. The threshold value is determined by the distance between the radiating and receiving antennas (Chapter II.8). The 9-m inter-antenna distance is set to measure the electrical resistivity within the top 1-m layer, while the 16-m distance can be used to evaluate 2.5-m layer. The general decrease of the resistivity downhill was revealed by the NEP (Fig. 67). Profile A indicated that the saline groundwater risen higher than 1 m to the surface at the bottom of the slope (Fig. 67), whereas at the top part of the slope, near the Kremlin, groundwater table was deeper than 5 m, as indicated by the VES measurement (Fig. 66, VES 6). Notably, the NEP profiles designated even local fluctuations of the groundwater level. For example, the area near the Dramatic Theater was drained by the specially constructed active drainage as shown by the resistivity increase in Fig. 67. Local increases in electrical resistivity were observed on the crossroads at the lower part of the slope (Fig. 67). These increases appeared, probably, due to draining effect of sand and cloth isolations of the pipes gathering under the crossroads. Such detailed outline of the groundwater levels was obtained by the NEP method in less than 30 minutes. The NEP profile provided continuous and detailed hydrological information about the soil subsurface even through the concrete pavement.

Fig. 67. Profiles of electrical resistivity measured by the NEP along the slope in Astrakhan’, Russia: profiling with (A) 9-m distance between antennas and with (B) 16-m distance between antennas. Vertical lines show location of crossroads, (!) indicates local increase of resistivity near the Dramatic Theater, and (!!) indicates increase of resistivity at the crossroads in the low part of the hill.

Hazard hydrological conditions, such as spatial and temporal groundwater fluctuations, cause problems in many cities. Geophysical methods of stationary and non-stationary electrical fields can be used to evaluate hydrological conditions and soil physical properties quickly, precisely, and non-destructively. Various geophysical methods were applied in urban areas of Kiev, Ukraine and Astrakhan’, Russia, to detect subsurface soil and hydrological properties. Vertical electrical sounding was successfully utilized to delineate preferential water permeability paths in stratified soil profiles as well as to determine saline groundwater table in a uniform soil. The method of self-potential revealed the directions and intensities of the subsurface water fluxes. The methods of the electrical and electromagnetic profiling (EP and NEP) were used to outline the areas with different subsurface resistivities, which indicated different hydrology conditions in soils. Particularly, the NEP could outline saturated areas even through firm pavement materials. Thus, geophysical methods of the stationary and non-stationary electrical fields are convenient and powerful tools to investigate hydrology and soil properties and develop plans for building maintenance in urban areas.

VI.2. Evaluation of soil pollution during gas and petroleum mining

Electrical geophysical methods were successfully used for exploration of gas and oil fields (Kalenev, 1970). However, the methods are not widely used for estimation of the soil pollution with petroleum products (Znamensky, 1980; Pozdnyakov et al., 1996a). The possibility of using the methods of electrical resistivity to evaluate the places of petroleum pollution or natural petroleum and gas deposits is based on highly different resistivities of soil and petroleum products. Petroleum and various products of petroleum manufacture, such as oil, gasoline, bitumen, and kerosene have very high electrical resistivity compared with soils. Electrical resistivity of petroleum varies from 104 to 1019 ohm m (Fedinsky, 1967), whereas resistivity of petroleum-saturated sand is much lower (2200 ohm m) (Znamensky, 1980), but is still higher than that of any non-polluted soil. For any soil type the resistivity is within several hundred ohm m (Pozdnyakov et al., 1996a; Chapter IV). Therefore, the objective of this study was to evaluate the methods of four-electrode probe, VES, and NEP for fast outlining of areas polluted during petroleum mining, manufacturing, and transportation.

Materials and methods. Soil pollution by the products of gas and petroleum mining was studied near Urengoi in northwest Siberia, Russia. The virgin soils, Glacic and Aquic Haplorthels, were extremely polluted with various by-products of petroleum extraction and manufacturing, such as bitumen, gasoline, kerosene, and mining brine solutions. The study area of about 30 acres included sub-areas polluted with light and heavy fractions of petroleum, places of dumping brine mining solutions, sub-areas affected by leakage from them, places of burned petroleum products, and non-polluted sub-areas with native disturbed and undisturbed soils. The area had complex topography with hills, small wetland, and creek. The study area was thoroughly investigated with four-electrode profiling on 1.2-m array, vertical electrical sounding, and non-contacting electromagnetic profiling.

Results and discussion. Four-electrode profiling was conducted for a transect through the most common pollution features within the area. Figure 68 shows a clear distinction between non-polluted areas and areas with bitumen or brine pollution. The salty mining solutions can decrease resistivity of Gelisols to 20-50 ohm m, and wetland formed with salty mining solutions is outlined by the lowest resistivity in the profile. The places polluted by bitumen, on contrary, have the very high resistivity, about 3000 ohm m. Non-polluted soils are indicated by resistivity of about 1000-1500 ohm m.

Fig. 68. Electrical profiling with four-electrode probe on a transect in area polluted with petroleum products and mining solutions. Measuring interval 1 m. Urengoi area, northwest Siberia.

Non-contact electromagnetic profiling was used to outline the pollution features faster. The profile on Fig. 69 was measured over the places polluted with the stream bearing the salty mining solutions leaching from the brine solution collector. The profile was started from the salty wetland with low resistivity and ended in the middle of the muddy area formed after the dumping of "clean" non-salty soil over the old mining solution collector.

The variation in electrical resistivity indicating the pollution distribution in soil profiles can be seen on VES profiles (Fig. 70). Pollution by heavy fraction of petroleum, such as bitumen appeared at the top part of soil profile and was indicated by electrical resistivity as high as 6´ 105 ohm m (Fig. 70c). The pollution by salty mining solutions lowered soil electrical resistivity. The resistivity of the soil near the stream where brine mining solution was discharged, varied from 50 to 200 ohm m (Fig. 70b). The surface soil at the brine collector has resistivity as low as 20 ohm m (Fig. 70d), while the electrical resistivity of the native pergelic soils was about 1000 ohm m at the surface (Fig. 70a). Some non-polluted native soils shown increase in electrical resistivity up to 8000 ohm m at the AB/2=2.4 m (about 0.6-m depth) indicated the presence of permafrost in soil profile (Fig. 70a). The depth of the permafrost was verified by soil excavation.

Fig. 69. Non-contact electromagnetic profiling through the areas polluted with salty mining solutions used during petroleum and gas mining. Urengoi area, northwest Siberia.

Fig. 70. Profile distributions of electrical resistivity measured by VES method in soils of northwest Siberia polluted by petroleum and gas mining: (a) non-polluted Gelisols, (b) soil near the stream with mining solution discharge, (c) soil polluted with bitumen, (d) soil in a brine collector.

Thus, the methods of EP, VES, and NEP are suitable for outlining areas polluted during petroleum and gas mining and manufacturing. The estimation of pollution is possible, since the pollutants have specific values of electrical resistivity distinct from non-polluted soils. Table 22 shows the average values of electrical resistivity of natural non-polluted soils (Glacic and Aquic Haplorthels) and soils polluted during petroleum and gas mining in northwest Siberia. In this particular case the pollution by petroleum products highly increased the soil electrical resistivity, whereas brine solutions used for the mining considerably decreased soil resistivity. The polluted areas were well distinguished from the surface and deep layers of non-polluted Gelisols (Table 22).

Table 22. Electrical resistivity of native and polluted soils in northwest Siberia

Soil

Electrical resistivity

—— ohm m ——

Surface layers of non-polluted Gelisols

2´ 102 – 2´ 103

Permafrost

4´ 103 – 8´ 103

Polluted by bitumen and other heavy fraction of oil

1´ 105 – 6´ 105

Polluted by gasoline

1´ 104 – 4´ 104

Polluted by salty mining water

2´ 10 – 2´ 102

 

VI.3. Method of four-electrode probe for identifying burial places of criminal origin

In criminology practice difficulties often arise when it is necessary to find some objects hidden in soils. Criminals usually carefully mask places of burial or make false disturbances of the topsoil layer. Search for non-metallic objects hidden in soils such as buried decomposed corpses, documents, jewelry, and drugs, is troublesome with the conventional police methods. So far only metal objects if buried just below the surface can be found in soils with a help of magnetometers or metal detectors (Murray and Tedrow, 1991). Such techniques, although effective in specific cases, fails to detect non-metallic objects (Murray and Tedrow, 1991). Davenport et al. (1990) conducted research on detection of corpses with ground-penetrating radar (GPR) in Colorado, USA. The GPR method, which utilized the high frequency radio waves, fails if the hidden object is small, buried on higher depth, or in clay/salt rich soil (Liner and Liner, 1997). The GPR measures the difference in radio waves penetration between soil and hidden object, therefore, the GPR response on burial corps decreases with the increase of time after burial as the decomposition progresses.

We proposed electrical geophysical methods to measure the disturbance of soil together with properties of a hidden object itself. The methods can be helpful for detection of small non-metallic objects buried in soil yet theoretically should not be influenced much by properties of the bury soil and time after soil disturbance. The method is based on measurements of soil bulk electrical resistivity and principles of soil formation. The bulk electrical resistivity of soil can be measured with a number of different geophysical methods (Chapter II). The most simple and common method is four-electrode probe or constant current electrical profiling. No analogous method was applied in forensic science for detection of non-metallic objects hidden in soil and associated soil disturbance. Therefore, the objectives of our study were: (i) to investigate the theoretical base of using the four-electrode probe to access soil disturbance, (ii) to test the probe for criminology search of non-metallic objects hidden in soil, and (iii) to modify the probe for routine forensic applications.

Materials and methods. The study was conducted by a suggestion from the Russian Ministry of Internal Affairs to test methods for fast outlining soil disturbance places to help criminological search. The test study of 1995 appeared promising and was developed into a long-term collaboration with scientists from the Russian Research Criminology Center. To test the method of four-electrode probe for mapping soil disturbance we measured bulk electrical resistivity within Chashnikovo area serving several years as the training polygon for soil scientists from Moscow State University. The places and time of pit digging were well documented by the soil scientists. The soil pits, usually about a size of common grave, were filled up with the same material after investigations. The ages of filled pits varied from 1 month to 27 years. Together with operative criminology groups we also participated in a number of case studies for search of hidden corpses and other objects of criminal origin within suspected areas.

For the criminology applications we slightly modified the method of four-electrode probe. Our hand-made device consisted of the measuring unit, which was in turn composed of potentiometer and amperemeter mounted in one small case, and four-electrode array. The package included three different equally spaced arrays (AM=MN=NB) in Wenner configuration (Kirkham and Taylor, 1949) with distances between AB electrodes equal to 45, 120, and 240 cm. The proposed electrode arrays measured bulk electrical resistivities of soil volume from the surface to the approximate depths 7.5, 20, and 40 cm, respectively (Chapter II). The electrodes were conveniently mounted on the wooden bases, therefore, the grounding of the all four electrodes at one array could be made at once within seconds. The disturbances, located on some depth, could be found even if no signs of them presented on the soil surface. The larger distance between electrodes the less influence on measurement was by disturbance of uppermost layer and by natural variability of soil properties. Therefore, the detection of objects hidden deeper in soil were easier with larger probes.

Results and discussion. A number of soil properties, such as humus content, cation exchange capacity, bulk density, structure, and texture, affect soil bulk electrical resistivity (Chapter III). All these properties differ considerably in upper soil horizons from those in lower horizons (Chapter IV). Due to digging or mixing of soil materials the resistivity of soils in disturbed places differ significantly from the resistivity of surrounding undisturbed soils. The effect is more pronounced if topsoil and subsoil extremely vary in the electrical resistivity, but differences can be noted practically in any soil. We investigated various soil types. The main attention was given to Alfisols (sod-podzolic in Russian classification), the most widespread in Moscow area and in a number of other humid regions in Russia. The soil profile is strongly differentiated in morphology, texture, and chemical properties, therefore, in electrical resistivity onto three horizons (Chapter IV).

The similar distribution of bulk electrical resistivity within eluvial-illuvial soil profile was described in the literature and occurs in a number of soils, such as Spodosols, Alfisols, Ultisols, Oxisols, and some Aridosols (Pozdnyakov et al., 1996a). Furthermore, in many other soils the layers with different resistivities exists due to other soil processes or differences in the parent materials. The importance of such natural soil feature for criminology search is that with infringement of soil the horizons are mixed, hence the place of disturbance shows the different electrical resistivity compared with undisturbed locations. The difference exists for a considerable time, as long as it takes to create the same layered soil profile as at undisturbed locations around, i.e. thousands of years. Therefore, even the disturbance that occurred several years ago can be detected.

The detection of disturbed place was based on measuring and analyzing spatial distributions of soil bulk electrical resistivity. We measured the bulk electrical resistivity on the soil surface over the former pits and on the surrounding territory in Chashnikovo area. Even the 27-year old soil pits were easily located with the method (Fig. 71).

The pronounced differences between electrical resistivities of non-disturbed and disturbed areas were noticed on the forest and grassland soils (Fig. 72). The resistivity of disturbed soils was higher than the resistivity of the control areas in two cases (Fig. 72, 1 and 4), and lower in other two cases (Fig. 72, 2 and 3). This appeared, probably, because of different degree of disturbance and mixing of the soil layers and their water contents for the different cases. The less difference between the resistivity of control areas and the area above the former soil pit was noticed for the plowland probably, because of initial disturbance of the top-soil to the depth about 30 cm on the plowland. The array with larger difference between the electrods should be used on plowlands to overcome this problem. Nevertheless, the distinctions between the disturbed and undisturbed sites were statistically authentic for all four cases (Fig 72).

Fig. 71. Spatial variability of electrical resistivity over the disturbed Typic Cryboralf. Rectangular boxes (0.5x1.0 m) indicate the location of filled soil pit; numbers are the values of electrical resistivity (ohm m), and the shaded rectangle outlines the location of former (5 years old) soil pit.

Fig. 72. Graph representation of summary statistics of bulk electrical resistivity measured (1) on control and (1a) over former soil pit on Cumulic Cryboralf, plowland; (2) on control and (2a) over former soil pit on Typic Cryboralf, coniferous-deciduous forest; (3) on control and (3a) over former soil pit on Sapric Haplohemist, grassland; and (4) on control and (4a) over former soil pit on Typic Cryboralf, grassland. All soil pits are about 2-month age.

The criminologist should be aware of the natural variability of soil. If an anomaly in electrical resistivity is detected several measurements should be taken at closer locations to check if they replicate the similar anomaly. The repeated measurements can help to outline the area of disturbance. One should be especially suspected if the disturbance has a size and form of grave (Fig. 71). The smaller sized anomalies can also be important depending on what an expert is looking for. Using different electrode spacing various volumes of soil can be measured (Chapter II). Thus, we can judge whether the potential soil disturbance is at the very surface or goes deeper. The places with deeper disturbance should be given special attention.

Case study. The method was used for a search of a hidden grave near a lake. The preliminary measurements revealed three abnormal places where the values of bulk electrical resistivity were considerably differed from the background. The soil within the territory (» 90 acres) was extensively disturbed by tourists. The average electrical resistivity at abnormal places within the territory is shown in Table 23. For exact localization of the grave and establishment of its borders the detailed measurement of bulk electrical resistivity was carried out at 0.5x0.5 m grid.

Table 23. Mean electrical resistivity (ohm m) of soils on a scene of a crime and surrounding territory

Specific location

Distance AB (cm)

45

120

45

120

Disturbed location

Surrounding territory

Burial of trash

107

131

179

104

Disturbance of undetected origin

188

107

474

131

Burial of corpse

226

340

69

127

As a result of the measurements a corpse was found. At this case, the soil was carefully repacked and the humus layer was accurately reestablished on the site masking the disturbance. Nevertheless, the natural alternation of soil horizons was broken and mixed. Therefore, even with the subsequent compaction and stacking of the upper horizon, the value of bulk electrical resistivity in the place of a corpse burial was about three times higher than that in the surrounding territory. With measurement of electrical resistivity to the 20-cm depth (AB = 120 cm) the distinction between the undisturbed and disturbed soils was statistically authentic.

The method of four-electrode probe has been shown to be a successful method for criminology search of some non-metallic objects, primarily corpses, buried in soil. The method outlines the differences in electrical resistivity between disturbed and non-disturbed soils, therefore, does not depend on the properties of the hidden object itself and the properties of bury soil. Although the proposed method is not as quick as metal detectors, magnetometers, or ground penetration radar, the method is free of their drawbacks. The efficiency of the method can be future improved by modifications: combined probes with an automatic switch between different arrays, automatic data logging and calculations of resistivity, and incorporating of a sound signal sensitive to sharp changes in measured electrical resistivity. The development of such equipment and its brand manufacturing is pending now in Russia. Although the geophysical techniques employed for criminology search might be not totally successful in finding hidden graves and buried objects, they can be very useful in allowing law-enforcement officers to screen large areas and eliminate many potential targets.