F. A. Monteiro Santos

Resolving the spatial distribution of the true electrical conductivity with depth using EM38 and EM31 signal data and a laterally constrained inversion model

The ability to map the spatial distribution of average soil property values using geophysical methods at the field and district level has been well described. This includes the use of electromagnetic (EM) instruments which measure bulk soil electrical conductivity (σa). However, soil is a 3-dimensional medium. In order to better represent the spatial distribution of soil properties with depth, various methods of inverting EM instrument data have been attempted and include Tikhonov regularisation and layered earth models. In this paper we employ a 1-D inversion algorithm with 2-D smoothness constraints to predict the true electrical conductivity (σ) using σa data collected along a transect in an irrigated cotton field in the lower Namoi valley. The primary σa data include the root-zone measuring EM38 and the vadose-zone sensing EM31, in the vertical (v) and horizontal (h) dipole modes and at heights of 0.2 and 1.0 m, respectively. In addition, we collected σa with the EM38 at heights of 0.4 and 0.6 m. In order to compare and contrast the value of the various σa data we carry out individual inversions of EM38v and EM38h collected at heights of 0.2, 0.4, and 0.6 m, and EM31v and EM31h at 1.0 m. In addition, we conduct joint inversions of various combinations of EM38 σa data available at various heights (e.g. 0.2 and 0.4 m). Last we conduct joint inversions of the EM38v and EM38h σa data at 0.2, 0.4, and 0.6 m with the EM31v and EM31h at 1.0 m. We find that the values of σ achieved along the transect studied represent the duplex nature of the soil. In general, the EM38v and EM38h collected at a height of 0.2, 0.4, and 0.6 m assist in resolving solum and root-zone variability of the cation exchange capacity (cmol(+)/kg of soil solids) and the electrical conductivity of a saturated soil paste extract (ECe, dS/m), while the use of the EM31v and EM31h at 1.0 m assists in characterising the vadose zone and the likely location of a shallow perched-water table. In terms of identifying an optimal set of EM σa data for inversion we found that a joint inversion of the EM38 at a height of 0.6 m and EM31 signal data provided the best correlation with electrical conductivity of a saturated soil paste (ECp, dS/m) and ECe (respectively, 0.81 and 0.77) closely followed by a joint inversion of all the EM38 and EM31 σa data available (0.77 and 0.56).

Electromagnetic imaging of a transpressional tectonics in SW Iberia

Forty-one magnetotelluric soundings were carried out along a 200 km-long profile (approximately NNE-SSW) across the three major geotectonic units in SW Iberia. A model obtained from two-dimensional inversion of the magnetotelluric data set reveals high conductivity zones in the middle-lower crust (10–30 km). Two of these zones correspond to the transition between the main geotectonic units: one between the South Portuguese Zone and the Ossa Morena Zone, interpreted as having been caused by metasediments, and the other between the Ossa Morena Zone and the Central Iberia Zone associated with a shear zone and metasediments. Another high conductivity anomaly related to black shales with major graphite impregnation was detected within the Ossa Morena Zone. The resistive features, located preferentially in the upper crust (1–10 km), coincide with gabbroic and granitic complexes.

2-Dimensional soil and vadose-zone representation using an EM38 and EM34 and a laterally constrained inversion model

The network of prior streams and palaeochannels common across the Riverine Plains of the Murray-Darling Basin act as conduits for the redistribution of water and soluble salts beneath the root-zone. To improve scientific understanding of these hydrological processes there is the need to better represent and map the connectivity and spatial extent of these physiographic and stratigraphic features. Groundbased electromagnetic (EM) instruments, which measure bulk soil electrical conductivity (sa), have been used widely to map their areal distribution across the landscape. However, methods to resolve their location with depth have rarely been attempted. In this paper we employ a 1-D inversion algorithm with 2-D smoothness constraints to predict the true electrical conductivity (s) at discrete depth increments using EM data. The EM data we use include the root-zone measuring EM38 and the deeper sensing EM34. We collected EM38 data in the vertical (EM38v) and horizontal (EM38h) dipole modes and EM34 data in the horizontal mode and coil spacing of 10, 20, and 40m (respectively, EM34-10, EM34-20, and EM34-40). In order to compare and contrast the value of the various EM data we carried out multiple inversions using different combinations, which include: independent inversions of (i) EM38 (root-zone) and (ii) EM34 data (vadose-zone), and in combination using (iii) EM38v, EM38h, and EM34-10 (near-surface), and (iv) all 5 EM datasets (regolith) available. The general patterns of s are shown to compare favourably with the known pedoderms, physiographic, and stratigraphic features and soil particle size fractions collected from calibration cores drilled across the lower Macquarie Valley study area. In general we find that the EM38 assists in resolving root-zone variability, specifically duplex soil profiles and physiographic features such as prior streams, while the use of the EM34 assists in resolving the stratigraphic nature of the vadose-zone and specifically the likely location of palaeochannels and subsurface anomalies that may indicate the location of good quality groundwater and/or clay aquitards. In this case, our potential to use s to predict clay content is limited by the non-linearity of the cumulative functions. In order to improve on the non-linearity of our inversion we need to develop a full solution of the forward problem.

Magnetotelluric observations over the Chaves geothermal field (NE Portugal)—Preliminary results

Abstract On the Portuguese mainland the most important low enthalpy field occurs in the graben of the Chaves region, where magnetotelluric measurements have been carried out to determine the crustal electrical conductivity distribution. From the impedance tensor decomposition two distinct regional directions (N-S and N65° ± 10°E) have been found. These directions are consistent with the geology and related to the main faults. The apparent resistivity and phase curves derived from the determinant of the impedance tensor were used to obtain 1-D and 2-D interpretations. The regional models indicate the existence of a conductive layer at a depth between 7 and 12 km (resistivity in the range of 100–300 Ω m, from 1-D modelling and 60 Ω m from 2-D model). The 2-D interpretation provides a first estimate of the regional geothermal gradient (28°C km −1 ) in the Hercynian granite. The 1-D models from the soundings located within the graben suggest that local tectonics could play a significant role in the CO 2 extraction and in migration of the deep CO 2 , as indicated from isotopic data.

Comparative study of local versus global methods for 1D joint inversion of direct current resistivity and time-domain electromagnetic data

Direct current resistivity and time-domain electromagnetic (TDEM) surveys are often used in environmental, hydrological and mining evaluation. The interpretation of the data acquired with each of these geophysical methods, assuming one-dimensional models, frequently produces ambiguous results. The joint inversion of TDEM and direct current resistivity data is considered by several authors as an efficient method to reduce the ambiguity inherent to each of these methods. This paper presents the results of a comparative study on the use of a local optimization method, iteratively reweighted least squares and global optimization methods of simulated annealing and particle swarm optimization in the joint inversion of TDEM and direct current resistivity data. The models obtained from joint inversion of synthetic data (H- and K-type) and of experimental data using the three methods yield similar results. The iteratively reweighted least squares method is the fastest while the simulated annealing is the most time-consuming. Simulated annealing and particle swarm optimization are the most efficient methods when studying equivalence problems.

Urban speleology applied to groundwater and geo-engineering studies: underground topographic surveying of the ancient Arca D'Água galleries catchworks (Porto, NW Portugal)

The Porto settlement (Northwest Portugal, Iberian Peninsula) was originally built in the twelfth century and has been developed on granitic hill slopes of the Douro riverside, being one of the oldest cities in Europe. In the urban area of Porto, the second most important city of the Portuguese mainland, there is a population of about 216,000 inhabitants. This study highlights the importance of urban speleological mapping applied to groundwater and geo-engineering studies. All the water that flows from the so-called Paranhos or Arca D’Agua springs is captured by catchwork galleries and their utilization date back around 1120 AD. Paranhos spring galleries catchworks (c. 3,3 km extension and a -21m below ground level) was one of the main water supplies to Porto City for more than six centuries and, nowadays, these waters are still appropriate for irrigation uses. Topographical, geological, geophysical and hydrogeological data were collected and interpreted, allowing the definition of a hydrogeotechnical zoning. All these features were mapped and overlaid using GIS mapping techniques. This multidisciplinary approach offers a good potential for reliable urban speleological and geo-engineering studies of Arca D’Agua site.

Using resistivity logs to estimate hydraulic conductivity of a Nubian sandstone aquifer in southern Egypt

The present study was carried out in the Khor-El-Ramlah area, in a Nubian sandstone aquifer. It is located in the western bank of Nasser Lake, southern Egypt, in a tectonically active area, which is dissected by many regional faults extending towards the lake. For this reason, the Nubian sandstone aquifer in this area is expected to be recharged by a large amount of seepage water from the lake. One important parameter used to quantify seepage is hydraulic conductivity. Since in this area there is no available information about hydraulic conductivity via pumping tests or laboratory examina- tion of core samples, an attempt is made in this study to estimate hydraulic conductivity using available resistivity logs by applying the Kozeny-Carman model. The clay contamination effect on porosity estimation has been removed via application of the Waxman-Smits model. The hydraulic conductivity is estimated to be 9.5 ± 2.5 m/day for the Nubian sandstone aquifer in this area. The present result is in agreement with the estimated and measured hydraulic conductivity values for the same aquifer in nearby areas and could be used to estimate seepage in this zone. The application of geophysical well logging often provides a cost-effective and efficient alternative to estimate aquifer parameters. out. Therefore, no information about hydraulic conductivity or seepage water quantity is known for that area. In particular, the seepage quantity is expected to be higher than in other parts around the lake due to the local dense fault system. The main objective of the present study is to estimate the hydraulic conductivity of the Nubian sandstone aquifer for this area, from the measured resistivity logs of well RML, in order to make it available for any future estimation of seepage.

2D sections of porosity and water saturation from integrated resistivity and seismic surveys

Porosity and degree of saturation – or the water content – are important parameters for hydrogeological, geotechnical and environmental studies. Geophysical methods, especially the resistivity method, are routinely employed to study the spatial variations of these parameters. Resistivity is highly influenced by the presence of water in pore spaces and hence is well suited for studying the presence of fluids on a site and its saturation condition. However, the non-uniqueness of the solution of resistivity models has led to the joint use of more than one geophysical method in order to reach more accurate geophysical models. In this research, we combined resistivity and seismic refraction profiles. The integration of these methods was particularly aimed to obtain 2D sections for estimating the porosity, water saturation and volumetric water content rather than to obtain a better geophysical solution. Independently inverted resistivity and P-wave refraction sections are the input data to an iterative analysis process using simulated annealing. Empirical equations taken from the literature are used to relate both the seismic velocity and the electrical resistivity with porosity and water saturation. Several parameters, such as the resistivity of water and clay; the velocity of water, clay, air and matrix; clay percentage and Archie’s parameters remain constant throughout the process. The values considered for each parameter are derive from both the literature and laboratory measurements. Resistivity and seismic refraction profiles were performed on a site located within the LNEC campus. At this site, field measurements of void ratio and volumetric water content were performed at different depths. Soil samples were also collected at three different depths in order to perform laboratory measurements of these parameters and to determine soil composition. The laboratory results were compared with the 2D sections of each parameter. The proposed approach was also applied to two other locations with different and well characterized geology. These tests also allowed us to characterize the dependency of the clay content on the resistivity. This research has potential fields of application in environmental studies, in particular, the determination of probable pathways of pollutants; in hydrological investigations, where it can be useful to transport of nutrients studies; and in geotechnical studies, where, for example, it will be able to give a continuous image of the saturation degree of an embankment.

Mapping soil water dynamics and a moving wetting front by spatiotemporal inversion of electromagnetic induction data

Characterization of the spatiotemporal distribution of soil volumetric water content (θ) is fundamental to agriculture, ecology, and earth science. Given the labor intensive and inefficient nature of determining θ, apparent electrical conductivity (ECa) measured by electromagnetic induction has been used as a proxy. A number of previous studies have employed inversion algorithms to convert ECa data to depth-specific electrical conductivity (σ) which could then be correlated to soil θ and other soil properties. The purpose of this study was to develop a spatiotemporal inversion algorithm which accounts for the temporal continuity of ECa. The algorithm was applied to a case study where time-lapse ECa was collected on a 350 m transect on seven different days on an alfalfa farm in the USA. Results showed that the approach was able to map the location of moving wetting front along the transect. Results also showed that the spatiotemporal inversion algorithm was more precise (RMSE = 0.0457 cm3/cm3) and less biased (ME = −0.0023 cm3/cm3) as compared with the nonspatiotemporal inversion approach (0.0483 cm3/cm3 and ME = −0.0030 cm3/cm3, respectively). In addition, the spatiotemporal inversion algorithm allows for a reduced set of ECa surveys to be used when non abrupt changes of soil water content occur with time. To apply this spatiotemporal inversion algorithm beyond low induction number condition, full solution of the EM induction phenomena can be studied in the future.

Application of the resistivity/gravity joint inversion technique for Nubian sandstone aquifer assessment on the area located at the central part of Sinai, Egypt

Eleven deep vertical electrical soundings of AB/2 spacing ranging from 5 to 3000 m were carried out to investigate the upper part of the Nubian groundwater aquifer at the central part of Sinai, Egypt. These soundings have been jointly inverted using the SA algorithm with 160 gravity stations measured in the study area, assuming that density and resistivity contrast are represented by coincident interfaces. One hundred and sixty magnetic stations were executed at the same locations as gravity measurements to estimate the depth of basement rocks. The results of the joint interpretation indicated that the depth of the groundwater aquifer ranges from 500 to 800 m with resistivity values ranging from 6 to 562 Ω m, suggesting that the fresh water is of good quality towards the northern part of the area. The top of the basement, which is mainly defined by gravity and magnetic data, lies at a depth ranging from 830 to 2788 m. The results also show that the aquifer configuration is controlled by different regional faults in the NNW–SSE direction.