The detection of concealed faults in the Ofanto Basin ...

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Tectonophysics 301 (1999) 321–332

The detection of concealed faults in the Ofanto Basin using the correlation between soil-gas fracture surveys G. Ciotoli a,Ł , G. Etiope b , M. Guerra b , S. Lombardi a a

Department of Earth Sciences, University of Rome ‘La Sapienza’, P. le A. Moro, 5-00185 Rome, Italy b Istituto Nazionale di Geofisica, Via di Vigna Murata, 605-00143 Rome, Italy Received 16 May 1997; accepted 25 August 1998

Abstract An integrated geochemical, morphological and structural analysis was applied to a basin filled with clayey sediments in southern Italy (Ofanto Valley) to delineate tectonic features. More than 100 soil-gas samples were collected and analysed for CO2 , Rn and He, and the resulting distribution was compared with the location and orientation of field-observed brittle deformations (faults and fractures), and air-photo interpreted morphotectonic features. The results show that the highest helium, radon and CO2 values occur preferentially along elongated zones similar to the most representative trends obtained by geomorphological and mesostructural analyses, i.e. anti-Apennine, Apennine and, secondarily, N–S orientations. Furthermore, the development of geostatistical techniques has allowed the semi-quantitative evaluation of the anisotropic soil-gas distribution. The gas-distribution pattern is considered to result from the combination of the anisotropic distribution of fracture traces and the randomly distributed background field. The correspondence between soil-gas distribution and geomorphological=mesostructural features, as well as the results from the geostatistical analysis, suggest that gas leakage towards the surface is controlled by the same structural pattern which also created some morphological features. This technique has been shown to be a useful tool for neotectonic studies; this is especially true in basins filled with clayey sediments, as soil gas is even able to define the leakage of deep-seated gases along tectonic discontinuities which have no surface expression.  1999 Elsevier Science B.V. All rights reserved. Keywords: soil gases; faults; geomorphology; neotectonics; geostatistics

1. Introduction The soil-gas method as applied to geological exploration is based on the determination of sub-terrestrial gases in the vadose zone that may provide information on the tectonic setting and fluid circulation of the subsurface environment (Jones and Drozd, 1983; Durrance and Gregory, 1990; Duddridge et al., 1991; Ł Corresponding

author. Fax: C39 06 49914919; E-mail: [email protected]

Klusman, 1993). Among the sub-terrestrial gases He, Rn and CO2 are the most reliable geochemical signals for fluid circulation, particularly as fault tracers. Since faults and fractures can act as preferential fluid-flow pathways, their locations may be assessed by detecting gases on the surface which use such discontinuities to move upward. Recent efforts indicate that the soil-gas method seems to provide good information on gas-bearing faults, even in basins filled with clayey sediments where the mapping of tectonic discontinuities by direct methods is made difficult by

0040-1951/99/$ – see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 1 9 5 1 ( 9 8 ) 0 0 2 2 0 - 0

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G. Ciotoli et al. / Tectonophysics 301 (1999) 321–332

the homogeneity and plastic behaviour of the clayey cover (Lombardi et al., 1993, 1996; Etiope, 1995). This paper focuses on a soil-gas survey conducted in a clay-filled basin in southern Italy. The goal of this study was to map faults whose presence have only been suggested by indirect stratigraphic and morphological evidence. To better understand the influence of buried structures on gas migration and concentration in the subsurface, He, Rn and CO2 soil-gas data have been compared with each other and with geo-structural data obtained by photo-interpretation and field-based structural mapping. Helium is chemically inert, physically stable, and highly mobile. Surface helium anomalies are generally attributed to upwardly-migrating deep-seated gas (e.g., Pogorsky and Quirt, 1981). During degassing helium mixes with other more abundant gases (carrier gases) and can be more easily transported towards the surface. The constant helium concentration in the atmosphere (5220 š 4 ppb (v=v), Holland and Emerson, 1990) results from the dynamic equilibrium between terrestrial helium produced by radioactive decay and helium escaping to outer space. Atmospheric helium is thus considered a reference standard, and soil-gas helium results are expressed in ppb (v=v) as the difference between sample and atmospheric concentrations. 222 Rn is a radioactive gas that is produced by the decay of radium (226 Ra) within the uranium (238 U) decay series. Typical soil air Rn values in Italian sedimentary basins (up to 15 Bq=l; Lombardi et al., 1996) are related to the content of parent radionuclides in the surface rocks (with 1–2 ppm of U). However, some surface radon anomalies can be related to the upward migration of gas along fault zones (e.g. Abdoh and Pilkington, 1989, and references therein). The short half-life of 222 Rn (3.85 days) limits its migration distance in the subsoil, and thus, radon measured in the soil air cannot be produced at great depth unless it is lifted upward by a relatively fast-flowing carrier gas, such as CO2 , CH4 , or N2 (Wilkening, 1980; Durrance and Gregory, 1990, and references therein). Carbon dioxide is thought to be the major component of endogenic gas and a suitable carrier for trace gases (Durrance and Gregory, 1990; Hermansson et al., 1991; Etiope and Lombardi, 1995a). It occurs in many geological environments especially in seismic

areas (Irwin and Barnes, 1980), and originates primarily from, decomposition of organic matter, metamorphism of marine carbonate sediments and mantle degassing. Sugisaki (1983) observed that in some cases (i.e. active faults) high carbon dioxide concentrations did not occur exclusively along a fault plane, but also in correspondence with associated fractures.

2. Geological overview The Ofanto Valley is located in a region of the Southern Apennine Mountains which was hit by a strong earthquake .Ms D 6:9/ on November 23, 1980 (Amato et al., 1989). It is a piggy-back basin mainly filled with over-consolidated clayey sediments of the Ariano cycle (Early Pliocene–Late Pliocene) and the Atessa cycle (Early Pliocene– Late Pleistocene) (Patacca et al., 1990). Inside these pelitic sequences several sandy gravel horizons occur with variable thickness and lateral continuity. The entire sequence has a maximum outcropping thickness of about 600 m. The pre-Pliocene substratum is a complex sequence of orogenic argillites and limestones. Recently Pliocene compressional structures have been displaced by important dip-slip and transtensive Upper Pliocene–mid-Lower Pleistocene Apennine and anti-Apennine tectonic discontinuities. Some of these elements have been inferred on stratigraphic and morphological bases; they are summarized in the geological sketch map (Fig. 1, modified after Gambino, 1993) and include the following: (1) A NW–SE-striking fault which cuts the arenaceous-conglomerate sequence near Cairano village. (2) A system of N–S-striking morphological steps, probably of tectonic origin, to the south of Andretta. (3) Two NW–SE-striking parallel faults which intersect the arenaceous-conglomerate formations near Cairano (one corresponding to the Ofanto River, the other, southwards, seems to continue toward Calitri). (4) A tectonic element has been identified in correspondence with the Ficocchia River (Fig. 3) (Budetta et al., 1990). Still-active Pleistocene extensional tectonics created the Ofanto Basin. Its recent effects are confirmed by the presence of normal faults cutting the Vulture Volcano products in the eastern sector of the area


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Fig. 1. Geological map of the Ofanto Basin (southern Italy) (modified after Gambino, 1993).

3. Methodology The method applied to study the relationships between tectonics and terrestrial gas geochemistry is based on the comparison of soil gas, photo-geological and field-based fracture mapping results. The endogenic-gas concentration in soil pores may be affected by soil moisture, soil and air temperature and barometric pressure (Hinkle, 1994); in order to minimise these effects, soil gas sampling was performed over a very short period of time and during stable meteorological conditions. More than one hundred soil-gas samples were collected from a depth of 0.5 m using a well-tested technique (Reimer, 1990; Lombardi and Reimer, 1990; Lombardi et al., 1993). Sampling was carried out according to a regular grid over an area of about 120 km2 (1 sample=km2 ).

Helium (expressed as 1He, ppbv) and carbon dioxide (% v=v) were analysed in the laboratory using a fixed-mass spectrometer (Leak Detector Varian 938-41) and a quadrupole mass spectrometer (VG SX-200), respectively; radon activity (Bq=l) was measured in the field using a portable scintillation counter (EDA RDA 200). Field-based fracture analysis was performed at 18 stations, yielding a total of 581 measured elements; their distribution is inhomogeneous for lack of outcrops. The surveyed outcrops were restricted to the ‘Ariano cycle’ and rocks potentially subjected to landslides or recognised as olistolites were not considered. Structural data (strike and dip) were graphically and statistically processed for each station using plane projection on a Schmidt stereo net. Data from stations located in correspondence with, or near, each soil-gas anomaly have been

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grouped to obtain the most representative directions for comparison with the geochemical results. Interpretation of 1 : 33,000 scale air photos consisted of the identification and selection of lineaments that may be genetically associated with structural discontinuities based on their morphological characteristics. All rectilinear or gently curved topographic features (such as scarps, valley-floors, ridge crests, drainage lines, etc.) were marked on the aerial photographs. All lineaments with an assumed tectonic origin (654 elements) were transferred from the photos to a small-scale map. The information thus obtained was subsequently checked in the field. Only lineaments occurring within soil-gas anomalies were considered for this analysis in order to achieve a more detailed resolution between soil-gas and photo-lineament data; in this way five areas have been marked and for each area a rose-diagram plotted. Statistical treatment of gas distribution was performed in two stages. First, contour maps of the entire area were derived by a default ‘kriging’ method; second, a variogram surface analysis was performed on a selected area in order to: (1) refine the assessment of the spatial continuity of gas concentration anomalies; (2) achieve useful parameters for the construction of a contour-line map, corrected for the appearance of elongated anomalies due to the gridding procedure rather than to the effective anisotropy (Isaaks and Srivastava, 1989). Soil-gas distribution is affected by several factors (climate, soil texture and water content, organic matter content, structural features). Some of these factors produce an isotropic or random distribution of gases that tends to mask the anisotropic pattern caused by aligned structural features. The anisotropy ratio enables one to evaluate the effectiveness and ‘strength’ of anisotropic factors versus factors which generate a random or ‘background’ soil-gas distribution. The anisotropy ratio is computed between the maximum and the minimum elongation of the isovariance closed contour lines on the variogram surface map which results by the combinations of directional variograms along all directions. The variogram is the basic geostatistical tool used in geosciences for visualising, modelling and exploiting the spatial autocorrelation of a regionalised variable (i.e. soil-gas concentration). As the name

implies, a variogram is a measure of variance. Most people intuitively know that two values in space that are close together tend to be more similar than two values farther apart. Univariate statistics cannot take this into account. Two distributions might have the same mean and variance, but differ in the way they are correlated with each other. Geostatistics using the variogram allows to quantify the correlation between any two values separated by distance h (usually called the lag distance) and uses this information to make prediction at unsampled locations. Variogram modelling is a prerequisite for kriging, or making predictions; it is a modelling of such spatial correlation structure. The variogram function is defined as follows:

.h/ D Var [Z .x C h/

Z .x/] =2n

where Z .x/ and Z .x C h/ are the gas values at points x and x C h, respectively, and n is the number of sampled differences. A detailed description of variography techniques is reported elsewhere (Isaaks and Srivastava, 1989; Pannatier, 1996).

4. Results Soil gas statistics are summarised in Table 1. About 19% of He, 13% of CO2 and 27% of Rn values are considered anomalous, i.e. exceeding the mean value plus 1=2 standard deviation (80 ppb for He, 2.4% for CO2 and 22 Bq=l for Rn). For radon it is possible to establish a ‘geochemical’ threshold computed according to the formula: CRn D CRa ž²n

1

where CRn and CRa are radon and radium concentrations in soil gas (Bq=l) and in soil (Bq=kg), respectively, ž is the emanation coefficient, ² is the soil density (kg=dm3 ) and n is the effective soil porosity. Soil Ra concentrations for the Ofanto Basin are not Table 1 Soil gas statistics from the Ofanto Basin Gas

Mean St. Dev Mode Max.

1He (ppbv) 19 131 Rn (Bq=l) 17 17.2 CO2 (% v=v) 1.33 2.10

0 7.4 0.98

402 97.3 15.51

Min.

Count

509 110 0.7 110 0.13 106


available, nevertheless an average U content of 2 ppm (equivalent to 25 Bq=kg of Ra at the equilibrium), typical of other Italian Plio–Pleistocene clay basins (Brondi and Voltaggio pers. commun.), can be used. Introducing into the equation the values of the other parameters characteristic of the surveyed soils (ž D 0:25, ² D 2, n D 0:25) yields about 50 Bq=l of Rn. Measured values lower than this computed threshold are considered background, while values exceeding it are thought to result from radon migration from deeper strata. Negative helium values (i.e. values lower than the atmospheric reference) constitute about 40% of the whole set. In spite of what has been reported in Reimer and Otton (1976), Reimer (1990) and Duddridge et al. (1991), negative anomalies in the studied area do not seem to be linked to tectonic or morphological features. Therefore more likely they result from a disequilibrium between soil gases and the atmosphere, as a consequence of differential mobility of the involved gaseous species. The correlation matrix appears to be independent of gas type. A better knowledge of gas behaviour versus geology is obtained by considering contour maps given in Fig. 2. This figure shows a different distribution pattern for CO2 , radon and helium over the surveyed area. Nevertheless an apparent anti-Apennine trending anomaly northeast of S. Andrea di Conza is common for all three gases, in correspondence with a fault inferred on the basis of stratigraphic and morphological data (Gambino, 1993); the greatest helium and radon values occur in this sector (402 ppb and 97.3 Bq=l, respectively). In addition the N–S fault parallel to the Ficocchia River is well marked by CO2 distribution and, to a lesser extent, by radon and helium. Figs. 3 and 4 show the statistical distribution of field-surveyed brittle deformations and photo-lineaments, respectively. Comparing these data with the soil-gas distribution, the following observations can be made: (1) Stations 1, 2 and 3 (Fig. 3) and the photolineament directions in the area near Andretta (1, Fig. 4) show trends of N0º–10ºW, approximately the same orientation of the helium and radon contour lines. (2) Stations 5, 6, 7, 8 and 18 show a major N50ºE trend coinciding with the linear helium, radon and carbon dioxide anomalies (N45ºE) between S.

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Andrea di Conza and the Orata River. In this area the photo-lineament directions, plotted in the diagram of area 2, also show a dominant trend of N40ºE. (3) Stations 15, 16 and 17 show a preferential fracture trend of N55ºE which corresponds to the direction of radon anomalies in the western area of Rapone. Such an anti-Apennine discontinuity is also seen in the statistical analysis of photo-lineaments (diagram for area 5) and in the regional tectonic pattern of the area (for example the ‘S. Fele-Vulture Line’ Ortolani and Pagliuca, 1988a,b). No correlation is observed along the Ficocchia River between the fracture trends of stations 9, 10 and 14 and the weak linear anomalies, yet the linearity of the river, probably of tectonic origin, is revealed by the N5ºE peak in the rose-diagram relative to the area (4). The variogram surface maps (Fig. 5), restricted to the area between S. Andrea di Conza and Calitri to better investigate the common helium, radon and carbon dioxide anomaly, show the following features: (a) Minimum values for radon, corresponding to the directions of maximum data continuity, extend along the anti-Apennine direction (N45ºE) in correspondence with a fault zone recognised by a previous geological survey (Gambino, 1993). This feature is also well marked in the contour map (Fig. 2a) obtained by ordinary kriging (i.e. ignoring the effective anisotropy of radon distribution). Along N–S directions, values show a weak ‘hole’, the origin of which is discussed in the next section. (b) Helium and CO2 maps show two main, perpendicular anisotropy directions which approximate Apennine and anti-Apennine trends. The Apennine anisotropy is poorly represented in the maps of Fig. 2, suggesting that variogram surface exploration is a more powerful tool for detecting anisotropic trends than contour-line maps computed by ordinary kriging. (c) The variogram surface maps are useful to quantify the anisotropy ratio .R/ for each gas distribution. R is given by the relation Amax =Amin , where Amax and Amin are the lengths of the maximum and minimum anisotropy semiaxes (Fig. 5). Data clearly show different Rn, He (R D 6:25 and R D 5:6, respectively) and CO2 .R D 2:5/ anisotropy ratios, suggesting that anisotropy-inducing factors are more effective for He and Rn than for CO2 . The length

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Fig. 3. Field-based fracture analysis of the Ofanto basin. Data from nearby stations are grouped and plotted in rose diagrams to obtain the most representative directions for comparison with geochemical results.

of the maximum semiaxes calculated for Rn and He variogram surface maps suggests that the fault spatial ‘domain’ influences gas distribution over the entire selected area.

5. Discussion The assumption that the recognised morphological features are controlled by Plio–Pleistocene tectonic activity seems to be confirmed by the good agreement between morphological and structural data. Many factors can affect gas migration and retention in soils, resulting in the irregular CO2 , radon and helium distribution patterns observed in the study area. Climatic influences (soil moisture and

temperature, changes in atmospheric pressure) were minimised by sampling within a short period of time (few days) during stable weather conditions. Likewise the adopted sampling density may result in the loss of some information, but 1 sample=km2 has proven to be suitable for regional-scale studies (Lombardi et al., 1996). The influence of other parameters, mainly the different geochemical behaviour of gases (mobility, reactiveness, solubility etc.), is beyond the scope of this study. In spite of the above-mentioned limitation, wherever soil-gas distribution is similar for all the three gases, a main dominating factor must be invoked to control gas leakage towards the surface. Geochemical trends coincide with geomorphological and structural features, e.g., the He–Rn–CO2 trend be-

Fig. 2. Soil-gas distribution in the Ofanto Basin: (a) radon (Bq=l), (b) helium (ppb v=v), and (c) carbon dioxide (%). The hatched line includes the area selected for the variogram surface analysis.

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Fig. 4. Photo-lineaments within the Ofanto Basin. Rose diagrams include the elements recognised in the marked areas.

tween S. Andrea di Conza and the Orata River is coherent with the local SW–NE fault (Fig. 2), the dominant fracture strike (rose diagram from stations 5, 6, 7, 8 and 18; Fig. 3) and morphological evidence (Fig. 4). This suggests that the enhanced gas permeability zones are connected to the structural pattern (i.e. fault and fracture locations) of the area; in this manner tectonics controls the leakage and the distribution of terrestrial gas in soil pores. Results

also show that the gas-bearing properties of faults depend on the enhanced permeability of fracture systems; however, due to structural heterogeneity and self-sealing processes, such a permeability is not continuous throughout the faults leading to spotted distribution of soil-gas anomalies. Trace gas anomalies in correspondence with high-CO2 structural elements suggest that CO2 can act as carrier along such discontinuities, in accor-

Fig. 5. Contour maps of the variogram surface from the selected area (location is showed in Fig. 2) between S. Andrea di Conza and Calitri for Rn (a), He (b) and CO2 (c). Contour lines depict the function .h/ of sample pairs grouped according their distance (lag h D 1 km) and the direction of the segment connecting the points of pair. These maps stress the directional anisotropies in the spatial distribution of the investigated variables. For each gas, the directional variogram was derived from the maximum axis of anisotropy (directions 45º, 135º, 40º for Rn, He, and CO2 , respectively). The modelling of the experimental variograms along these directions allows the evaluation of parameters (nugget, range, sill) necessary to describe the continuity of the investigated variable in the 2D space.


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dance with theoretical considerations and field data (Durrance and Gregory, 1990; Etiope and Lombardi, 1995b). From a theoretical point of view carrier transport would result in a quicker advective migration of trace gases (helium and radon) that, if alone, would migrate only by a very slow diffusion mechanism. As 222 Rn is an unstable nuclide with a very short half-life (3.8 days), a relatively high flow rate (or alternatively a very shallow source of Rn) is required to explain the surplus of radon activity with respect to the calculated geochemical threshold. This geochemical constraint provides further evidence for carrier-gas-based migration along structural discontinuities acting as enhanced permeability pathways. The application of geostatistical techniques in the test area between S. Andrea di Conza and Calitri confirms the anisotropic behaviour of soil gas distribution along the anti-Apennine direction, and emphasised the presence of an Apennine trend for

He and, secondarily, for CO2 . Different R values obtained for the investigated gases can be explained by the variable contribution of anisotropic phenomena acting along specific directions versus isotropic phenomena acting randomly. The former are supposed to be linked to gas leakage along fault systems, whereas the latter are mainly related to surface phenomena producing the background pattern. High Rn and He anisotropy ratios are the result of combining relatively high gas-value oscillations induced by the anisotropy effect (i.e. faults) with the minor background oscillations (Fig. 6a). Low anisotropy ratios, such as those observed for CO2 in Fig. 5c, are the effect of random background oscillations prevailing over statistically oriented spikes caused by preferential gas leakage (Fig. 6b). High background oscillations for CO2 are probably related to the many mechanisms which can

Fig. 6. Hypothetical cross-sections of Rn (a) and CO2 (b) concentrations cutting a fault transversally. Profiles are a conceptualization of the relationship between the component of background field (dashed line) and fault-related, anisotropy field (full line). (a) The high contrast between spatial continuity along specific directions and background field, as evaluated by experimental variogram surface (Fig. 5a), indicates for radon that background fluctuations are generally minor that aligned, fault-related Rn anomalies. Radon ‘ridge’ observed near a fault originates a maximum concentration gradient perpendicular to the fault-line and a minimum gradient axially to the structure (see the well definite shape of -contour lines in Fig. 5a). (b) Observing the less-shaped anisotropy axes of CO2 (Fig. 5c), a minor contrast between background field (dashed line) and fault-related field (full line) can be hypothesized. This can originate by the occurrence of many parameters affecting CO2 soil-gas concentration at the surface, which tend to mask the deep, fault-related CO2 input.


generate large amounts of this gas (root respiration, organic matter decomposition, rock weathering etc.). They are reflected in the shorter length of the elongated low- contour line (Fig. 5c) and in the less well-defined anomalous shapes obtained by ordinary kriging (Fig. 2c). The ‘hole effect’ observed in the radon variogram surface map, marked by a bump along the N–S direction (point H in Fig. 5a), may be caused by too few pairs of points or to natural fluctuation in the variograms (Armstrong, 1995). In some cases, however, it can be explained physically as: (1) the presence of the discontinuous gas-bearing fault properties (possibly related to self sealing phenomena along the structure), yielding a decreased variance for all pairs separated by a distance equal to the corresponding lag (i.e. lag D 2.5 km in direction N–S in Fig. 5a); (2) the interaction of two or more faults having different directions. The length of the maximum axis of anisotropy is more than 6 km, suggesting that the fault influences radon distribution over the entire selected area.

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nation of the effective ‘forces’ which generate the anisotropic distribution and the random background field. The former are concluded to be linked to the gas-bearing properties of the brittle deformations in the area. These results show how soil-gas research may enhance geological studies aimed at delineating tectonic discontinuities, even where clay sequences mask surface structural features. For a better understanding of the relationships between gas leakage and the structural and morphological setting of an area, the analysis of a broad spectrum of gaseous species (e.g. Ar, H2 , CH4 ), as well as a further refinement of geostatistical techniques, are strongly recommended.

Acknowledgements We wish to thank A. Baccani and E. Grillanda for technical support, A. Corami and L. Paltrinieri for their help in the field and S. Beaubien for the valuable suggestions and correction of the manuscript.

6. Conclusions References In spite of the many parameters which affect the occurrence of deep-seated gases in soil, the main helium, radon and CO2 soil gas anomalies in the Ofanto Valley were found to be linearly distributed along Apennine, anti-Apennine and N–S trends. Such directions agree well with those obtained by means of the analysis of fracture patterns and the interpretation of photogeology. High gas leakage suggests that these areas are affected by greater fracturing, i.e. that tectonic discontinuities act as preferential paths for deep-seated gas leakage. It follows, therefore, that the clay sequence, if fractured, does not form an impermeable barrier to terrestrial gases in spite of its great thickness, plasticity and low intrinsic permeability. The advection of gaseous carriers, transporting trace gases through fractured rocks, seems to be the probable mechanism for soil-gas anomalies. The geostatistical approach, based on the plotting of concentration-contour maps and variogram surface maps, has permitted the semi-quantitative evaluation of the anisotropy pattern of soil-gas distribution. This pattern is produced by the combi-

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