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Evaluation of the topographic effect on the results of mapping by electrical resistivity method: application to rubber tree plantation on Ban Non Tun experimental site in north-east of Thailand

Abstract : Since 1991, Thailand has been the largest country in the world for natural rubber production and its export. More than 80% of Thailand natural rubber is produced by smallholders. It is obvious that the production of natural rubber has a very important economic interest. For rubber-tree, the normal climatic and environmental conditions are: mean annual temperature of 26°C ± 2°C, and high rainfall (from 1800 to 3000 mm/year) regularly distributed throughout the year (with 120 - 40 rainy days per year) [Gonçalves et al., 1999]. Worldwide demand for natural rubber is increasing every year and so is its price in the world market. In addition of this, the governmental subsidizations in Thailand have created beneficial conditions for the expansion of rubber-tree (called "hevea") plantations even in areas where climatic conditions are not really auspicious for this plant. This applies to the NE of Thailand and particularly in the Khon Kaen region, characterized by a semi-arid tropical climate where the annual average air temperature is 26°C and the total precipitation does not exceed 1300mm/year, almost all of it falling during the rainy season from April to October [Watanabe & al., 2004]. If the temperature is favorable for rubber-tree growth, the rainfall remains below the requested moisture and its distribution is irregular. The effects of water deficiency or excess can lead to reduced rates of tree growth, low latex yields [Rao et al., 1998], and in severe cases, the water stress may affect the plantations resulting in "hevea" diseases or total plant failure. In this area the sandy soils are not rich in organic matters and other nutrients. For this region the soil and the climate can be considered as marginal for rubber-tree cultivation. Traditional management of intensive cropping in this area should include specific studies on soil - water - plant - atmosphere interactions. Failure to do so could cause irreversible damage. An increase in rubber-tree plantation surfaces in the long term can lead to important changes of water resources and no foreseeable impact on soils. Several scientists from different professional fields are working for many years to find and propose solutions to grow this tree in the described above conditions. The ongoing TICA project "Impact assessment of planting rubber trees on sandy soils in NE Thailand, Phase 2 " (started since September 2010, for 3 years), which is a cooperation project between the Land Development Department of Thailand and the French Research Institute for Development (IRD), has for purpose to contribute to the strategy of LDD, particularly by studying the soil variability and find solution in rehabilitation of marginal up-lands of North-East Thailand in order to enlarge the area of rubber-tree plantations and to improve living conditions for Thai smallholders. In order to study the soil variability on the experimental site "Ban Non Tun", an electrical resistivity mapping was carried out. This map showed that the lines of trees are characterized by resistant anomalies compared with "interline" zones. But it is not obvious to link these anomalies directly with the root systems of trees, because between the "lines of trees" and the "interlines" exists a topographical unevenness specially created by the farmer to protect trees roots from water excess during rainy periods. Under these conditions, the differences in elevation between the "lines" and "interlines" may themselves generate apparent electrical resistivity anomalies, even when the ground is homogeneous. In this case, we have at least two sources that can create anomalies in the same direction (resistivity increasing). The conceptual framework of this project includes a detailed analysis of the apparent electrical resistivity anomalies in order to estimate the contributions from various sources. A comparison of data measured in the field with those of modeling allows us to assess the topographic effects on electrical resistivity values obtained by mapping. Several studies have been done concerning the influence of topography on the electrical resistivity measurements [Fox et al., 1980], but the proposed approaches and correction methods are difficult to apply for the site "Ban Non Tun", due to very inhomogeneous distribution of soil's electrical resistivity and topographical irregularities. This project aims to estimate the contribution of different sources that can generate these anomalies. The influence of topographic structures on the apparent electrical resistivity measurements is evaluated using numerical calculations. Field detailed measurements, using different electrode configurations, assesses the degree of adaptation of different arrays for geophysical surveys in rubber tree plantations. It also helps for a deeper understanding of phenomena, and a better interpretation of the results of electrical resistivity mapping. Comparison of the Ban Non Tun site topographic map and electrical resistivity mapping shows a clear coherence between ranks of trees and extended resistant anomalies. More detailed topographic and electrical resistivity mapping have been done on a small surface in order to better identify the sources of the anomalies observed on the electrical resistivity mapping. This area, centered on a rank of trees, includes both extended resistant anomalies and a conductive anomaly on the topographic structure. Results of apparent electrical resistivity mapping obtained using the "pole-pole", "Wenner-α" and "dipole-dipole" arrays with the same inter-electrode spacing were compared to study the probable differences between responses observed on the same structures. We used results obtained in pole-pole configuration with an inter-electrode spacing of 25 cm to calculate the index of anisotropy in the horizontal plane. Index values are on average very close to 1, which shows that the topographical effect is very small compared to the influence of superficial layers resistivities. Raised portions of relief corresponding to the ranks of trees were established by extracting soil in the inter-ranks areas to transfer it on the lines of tree. This has certainly led to a significant deterioration of the structure of these soils, which could result in an increase in their electrical resistivity. Results of topographic effect modeling show consistent resistant anomalies corresponding to the topographic structure. Yet, amplitudes of these anomalies (around 5-10 %) are lower than those observed on the site (more than 150 %). Thus, the topographic effect cannot be the only source of the extended resistant anomalies observed on the results of electrical resistivity mapping on the "Ban Non Tun" site. These results allow us to improve our knowledge concerning sources and physical bases of the recorded resistive anomalies along the ranks of trees.
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Sylvain Pasquet. Evaluation of the topographic effect on the results of mapping by electrical resistivity method: application to rubber tree plantation on Ban Non Tun experimental site in north-east of Thailand. 2011. ⟨hal-00794616⟩

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