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1.
Magnetotelluric (MT) and ground magnetic surveys were conducted on the Mahallat geothermal field situated in Markazi province, central Iran, as a primary part of the explorations and developments of a geothermal energy investigation program in the region. Mahallat region has the greatest geothermal fields in Iran. MT survey was performed in November 2011 on an 8 km profile crossing the hot springs with a total of 17 stations. The 2D inversion of the determinant MT data was performed using a 2D inversion routine based on the Occam approach. The 2D resistivity model obtained from the determinant data shows a low resistivity zone at 800-2000 m depth and a higher resistivity zone above the low resistivity zone, interpreted as geothermal reservoir and cap rock, respectively. It also revealed two major concealed faults which are acting as preferential paths for the circulation of hydrothermal fluids. To obtain more geophysical evidence, a ground magnetic survey with 5000 stations was also performed over an area of 200 km2 around the MT profile. Magnetic measurements show a main positive anomaly of about +1000 nT over the study area, which could be interpreted as an intrusive body with the high magnetic susceptibility (i.e. mafic and ultramafic rocks) into the sedimentary host rocks. We interpret the body as the heat source of the geothermal system. Structural index and depth estimation of the anomaly indicate that the intrusive body is similar to a cylinder extending from about one kilometer depth down to greater depths. The results of MT and magnetic investigations indicate a geothermal reservoir which proves the preliminary geological observations to a great extent.  相似文献   

2.
In order to gain a better understanding of the geometry of surface faults, five Controlled Source/Radio Magnetotelluric (CSRMT) profiles were measured across the Volvi basin, 45 km northeast of the city of Thessaloniki in Greece. The data were collected in two frequency ranges: a) 1–12.5 kHz using a remotely controlled double horizontal magnetic dipole transmitter (CSAMT measurements), and b) 15–250 kHz using the signal from distant radio transmitters (RMT measurements). The transition from the RMT band to the CSAMT band was smooth and continuous allowing us to combine both datasets for plane-wave modeling. The surface geology shows a predominantly 2D structure, and therefore we planned the survey into profiles perpendicular to the geological strike. We have used a 2D interpretation tool to model the data in TE, TM, TE + TM and determinant modes. Using a 4% error floor on the impedance, 2D resistivity models from inversion of the determinant data provide lower RMS data fits (4.2 and 1.2 for resistivity and phase, respectively) compared to the combined TE + TM data (4.4, 2.8, overall resistivity and phase, respectively). 2D inversion of the measured tensor data shows a sharp change in the depth to the top of resistive gneiss–schist basement that is overlain by a less resistive overburden at southern basin flanks. The change in depth to the bedrock is clearly seen in all 2D models along the measured profiles suggesting the existence of normal faults with strike directions of NE–SW to E–W. The 2D electrical resistivity models suggest that the bedrock deepens towards south-west. The resistivity models are also compared with the existing borehole information in the area and show a reasonable correlation. For example the sharp change of depth to the bedrock towards the center of the basin as seen in the resistivity models are also confirmed by the borehole data.  相似文献   

3.
Magnetic measurements with an unmanned aerial vehicle are ideal for filling the gap between ground and airborne magnetic surveying. However, to obtain accurate aeromagnetic data, the compensation of magnetic effects of the unmanned aerial vehicle is a challenge. Typically, scalar magnetometers are towed several metres under the unmanned aerial vehicle to minimize its magnetic field. In this study, a fluxgate three-component magnetometer is attached 42 cm in front of the unmanned aerial vehicle at the tip of a composite pipe. Using a scalar calibration, the sensor can be calibrated, and the permanent and induced magnetic fields of the unmanned aerial vehicle can be compensated. The contributions of the magnetic measurements at different altitudes to the unmanned aerial vehicle results were tested over an area of 1 km² in the Northern Vosges Mountains. The area is located in a hamlet surrounded by a forest where few geological outcrops are observed. Three magnetic surveys of the same area are obtained at different altitudes: 100, 30 and 1 m above the ground. The unmanned aerial vehicle magnetic data are compared with a helicopter aeromagnetic survey at 300 m above the ground and a ground magnetic survey using upward continuations of the maps to compare the results. The magnetic maps (300, 100, 30 and 1 m above the ground) show very different magnetic anomaly patterns (e.g. amplitude, shape, wavelength and orientation). The magnetic data at different altitudes improve the understanding of the geology from the local to more general scales.  相似文献   

4.
Helicopter-borne frequency-domain electromagnetic (HEM) surveys are used for fast high-resolution, three-dimensional resistivity mapping. Standard interpretation tools are often based on layered earth inversion procedures which, in general, explain the HEM data sufficiently. As a HEM system is moved while measuring, noise on the data is a common problem. Generally, noisy data will be smoothed prior to inversion using appropriate low-pass filters and consequently information may be lost.For the first time the laterally constrained inversion (LCI) technique has been applied to HEM data combined with the automatic generation of dynamic starting models. The latter is important because it takes the penetration depth of the electromagnetic fields, which can heavily vary in survey areas with different geological settings, into account. The LCI technique, which has been applied to diverse airborne and ground geophysical data sets, has proven to be able to improve the HEM inversion results of layered earth structures. Although single-site 1-D inversion is generally faster and — in case of strong lateral resistivity variations — more flexible, LCI produces resistivity — depth sections which are nearly identical to those derived from noise-free data.The LCI results are compared with standard single-site Marquardt–Levenberg inversion procedures on the basis of synthetic data as well as field data. The model chosen for the generation of synthetic data represents a layered earth structure having an inhomogeneous top layer in order to study the influence of shallow resistivity variations on the resolution of deep horizontal conductors in one-dimensional inversion results. The field data example comprises a wide resistivity range in a sedimentary as well as hard-rock environment.If a sufficient resistivity contrast between air and subsurface exists, the LCI technique is also very useful in correcting for incorrect system altitude measurements by using the altitude as a constrained inversion parameter.  相似文献   

5.
The spatial distribution of the electrical resistivity data provides useful information for investigating and modeling the fluid transport processes. 3D electrical resistivity distribution provides information about water flow and changes in electrical resistivity of the pore fluid.Therefore, to assist in understanding and modeling of the fluid transport process, 3D spatial distribution of the electrical resistivity data with the corresponded 3D geological section were mapped and interpreted in the test site located in western Germany. A process of deriving electrical resistivity values from the mechanical and radioactive parameters of cone penetration tests (CPT) and geological information of boreholes was presented. A reliable method which gives accurate resistivity values in cases of near surface sediments was introduced. Then a field test was executed where the calculated resistivity values were compared with the measured CPTe resistivity data. The CPTe (cone penetration test with electrical extension) data were also used in correlating to the ERT (electrical resistivity tomography) data. Consequently, obtained dense CPT surveys give us the possibility to determine a high resolution resistivity distribution of the investigated area.  相似文献   

6.
Magnetotelluric surveys were conducted to investigate the structure in the El Hito Anticline in the southern Loranca Basin, Spain. The structure of this anticline is partly known from nearby data (geological cross sections, geological maps, well-logs) from vintage geophysical and geological basin-scale surveys. Unfortunately, these surveys do not have the appropriate resolution to determine certain characteristics of the anticline accurately, such as the thickness or geometry of geological units. To address this deficiency, magnetotelluric data were acquired at 51 sites along three profiles to image the electrical resistivity of the anticline. To identify a geologically reasonable resistivity model, a conceptual model based on previous geological and geophysical information (cross sections and well-log data) was generated. Several inversions were performed using the conceptual model, which played a key role in the interpretation of the magnetotelluric data, and the construction of a suitable initial model was essential in producing geologically meaningful models. Using these results, we obtained 2-D resistivity models that provide information on the main geological units and structures. We also performed sensitivity tests to understand the morphology and structure of the resistive basement better. The results indicate the presence of basement highs that show different structural styles for the basement and its cover, and an increase in the basement depth towards the south. The final 2-D resistivity models provide new information regarding the structure of the anticline and demonstrate a clear correlation between the main geological units and resistivity, as well as correlations with mapped surface faults.  相似文献   

7.
A detailed magnetotelluric survey was conducted in 2013 in the Sehqanat oil field, southwestern Iran to map the geoelectrical structures of the sedimentary Zagros zone, particularly the boundary between the Gachsaran Formation acting as cap rock and the Asmari Formation as the reservoir. According to the electrical well logs, a large resistivity contrast exists between the two formations. The Gachsaran Formation is formed by tens to hundreds of metres of evaporites and it is highly conductive (ca. 1 Ωm–10 Ωm), and the Asmari Formation consists of dense carbonates, which are considerably more resistive (more than 100 Ωm). Broadband magnetotelluric data were collected along five southwest–northeast directed parallel lines with more than 600 stations crossing the main geological trend. Although dimensionality and strike analysis of the magnetotelluric transfer functions showed that overall they satisfied local 2D conditions, there were also strong 3D conditions found in some of the sites. Therefore, in order to obtain a more reliable image of the resistivity distribution in the Sehqanat oil field, in addition to standard 2D inversion, we investigated to what extent 3D inversion of the data was feasible and what improvements in the resistivity image could be obtained. The 2D inversion models using the determinant average of the impedance tensor depict the main resistivity structures well, whereas the estimated 3D model shows significantly more details although problems were encountered in fitting the data with the latter. Both approaches resolved the Gachsaran–Asmari transition from high conductivity to moderate conductivity. The well‐known Sehqanat anticline could also be delineated throughout the 2D and 3D resistivity models as a resistive dome‐shaped body in the middle parts of the magnetotelluric profiles.  相似文献   

8.
Airborne electromagnetic methods are most commonly used in mineral exploration. However, new developments, such as multifrequency capability and digital on-board field recording, as well as improvements in instrumentation resulting in high signal-to-noise ratios in recorded data, have made their application in geological mapping possible. A three-frequency airborne EM survey carried out over an area northwest of Timmins, Ontario, was interpreted in terms of thickness and resistivity of the layers of a two-layer earth section. Since both in-phase and quadrature components are measured, this provides six independent parameters at each point in space. Based on prior geological information and a preliminary interpretation of the field records, two two-layer models of the subsurface seemed to be appropriate for most of the survey area. An automatic computerized interpretation procedure was devised to interpret the field data at each point in terms of thickness and resistivity parameters of those two models. When the geology is more complex, the data do not fit the models and no interpretations are made. Two maps illustrating the variation of the resistivity and the thicknesses of the layers were constructed from the interpreted data. These maps agree with the known geological information about the distribution of glacial clay in the area. Areas where the layered models do not fit are known to be areas where the geology is complex with a large number of dykes and other lateral inhomogeneities. The study shows that multifrequency airborne EM surveys can be very useful in geological mapping over inaccessible terrain and can significantly help the mapping geologist where outcrops are scarce.  相似文献   

9.
In order to couple spatial data from frequency‐domain helicopter‐borne electromagnetics with electromagnetic measurements from ground geophysics (transient electromagnetics and radiomagnetotellurics), a common 1D weighted joint inversion algorithm for helicopter‐borne electromagnetics, transient electromagnetics and radiomagnetotellurics data has been developed. The depth of investigation of helicopter‐borne electromagnetics data is rather limited compared to time‐domain electromagnetics sounding methods on the ground. In order to improve the accuracy of model parameters of shallow depth as well as of greater depth, the helicopter‐borne electromagnetics, transient electromagnetics, and radiomagnetotellurics measurements can be combined by using a joint inversion methodology. The 1D joint inversion algorithm is tested for synthetic data of helicopter‐borne electromagnetics, transient electromagnetics and radiomagnetotellurics. The proposed concept of the joint inversion takes advantage of each method, thus providing the capability to resolve near surface (radiomagnetotellurics) and deeper electrical conductivity structures (transient electromagnetics) in combination with valuable spatial information (helicopter‐borne electromagnetics). Furthermore, the joint inversion has been applied on the field data (helicopter‐borne electromagnetics and transient electromagnetics) measured in the Cuxhaven area, Germany. In order to avoid the lessening of the resolution capacities of one data type, and thus balancing the use of inherent and ideally complementary information content, a parameter reweighting scheme that is based on the exploration depth ranges of the specific methods is proposed. A comparison of the conventional joint inversion algorithm, proposed by Jupp and Vozoff ( 1975 ), and of the newly developed algorithm is presented. The new algorithm employs the weighting on different model parameters differently. It is inferred from the synthetic and field data examples that the weighted joint inversion is more successful in explaining the subsurface than the classical joint inversion approach. In addition to this, the data fittings in weighted joint inversion are also improved.  相似文献   

10.
Electromagnetic signals from distant radio transmitters in the frequency range 15–250 kHz were measured to model an electrical resistivity structure beneath 7 profiles in the vicinity of the Karinu limestone quarry in Estonia with the aim to map the extent of the economically exploitable limestone. The resistivity models from a 2D inversion of determinant resistivity and phase values using an Occam type of regularization contained reasonably accurate information about the geometry, namely depth to the top and the bottom of the target high‐resistivity limestone. The resistivity models correlated well with existing geological evidences as well as information from closely located boreholes. However, the sharp lithological boundaries seen in the boreholes were not resolved exactly in the resistivity models. This is probably because of the smoothing regularization used in the inversion process. Combined use of borehole data together with resistivity models resulted in two major geological interpretations; a) towards the western part of the existing limestone quarry there is a NNW to NS striking fault, covered by post‐glacial sediments, b) a potential cost‐effective exploitable area containing high quality highly resistive limestone is located south of the existing quarry. This case study shows the applicability of the reasonably fast radio magnetotelluric (RMT) method for the exploration of near‐surface resources.  相似文献   

11.
Z‐axis tipper electromagnetic and broadband magnetotelluric data were used to determine three‐dimensional electrical resistivity models of the Morrison porphyry Cu–Au–Mo deposit in British Columbia. Z‐axis tipper electromagnetic data are collected with a helicopter, thus allowing rapid surveys with uniform spatial sampling. Ground‐based magnetotelluric surveys can achieve a greater exploration depth than Z‐axis tipper electromagnetic surveys, but data collection is slower and can be limited by difficult terrain. The airborne Z‐axis tipper electromagnetic tipper data and the ground magnetotelluric tipper data show good agreement at the Morrison deposit despite differences in the data collection method, spatial sampling, and collection date. Resistivity models derived from individual inversions of the Z‐axis tipper electromagnetic tipper data and magnetotelluric impedance data contain some similar features, but the Z‐axis tipper electromagnetic model appears to lack resolution below a depth of 1 km, and the magnetotelluric model suffers from non‐uniform and relatively sparse spatial sampling. The joint Z‐axis tipper electromagnetic inversion solves these issues by combining the dense spatial sampling of the airborne Z‐axis tipper electromagnetic technique and the deeper penetration of the lower frequency magnetotelluric data. The resulting joint resistivity model correlates well with the known geology and distribution of alteration at the Morrison deposit. Higher resistivity is associated with the potassic alteration zone and volcanic country rocks, whereas areas of lower resistivity agree with known faults and sedimentary units. The pyrite halo and ≥0.3% Cu zone have the moderate resistivity that is expected of disseminated sulphides. The joint Z‐axis tipper electromagnetic inversion provides an improved resistivity model by enhancing the lateral and depth resolution of resistivity features compared with the individual Z‐axis tipper electromagnetic and magnetotelluric inversions. This case study shows that a joint Z‐axis tipper electromagnetic–magnetotelluric approach effectively images the interpreted mineralised zone at the Morrison deposit and could be beneficial in exploration for disseminated sulphides at other porphyry deposits.  相似文献   

12.
Mud volcanism is commonly observed in Azerbaijan and the surrounding South Caspian Basin. This natural phenomenon is very similar to magmatic volcanoes but differs in one considerable aspect: Magmatic volcanoes are generally the result of ascending molten rock within the Earth's crust, whereas mud volcanoes are characterised by expelling mixtures of water, mud, and gas. The majority of mud volcanoes have been observed on ocean floors or in deep sedimentary basins, such as those found in Azerbaijan. Furthermore, their occurrences in Azerbaijan are generally closely associated with hydrocarbon reservoirs and are therefore of immense economic and geological interest. The broadside long‐offset transient electromagnetic method and the central‐loop transient electromagnetic method were applied to study the inner structure of such mud volcanoes and to determine the depth of a resistive geological formation that is predicted to contain the majority of the hydrocarbon reservoirs in the survey area. One‐dimensional joint inversion of central‐loop and long‐offset transient electromagnetic data was performed using the inversion schemes of Occam and Marquardt. By using the joint inversion models, a subsurface resistivity structure ranging from the surface to a depth of approximately 7 km was determined. Along a profile running perpendicular to the assumed strike direction, lateral resistivity variations could only be determined in the shallow depth range using the transient electromagnetic data. An attempt to resolve further two‐dimensional/three‐dimensional resistivity structures, representing possible mud migration paths at large depths using the long‐offset transient electromagnetic data, failed. Moreover, the joint inversion models led to ambiguous results regarding the depth and resistivity of the hydrocarbon target formation due to poor resolution at great depths (>5 km). Thus, 1D/2D modelling studies were subsequently performed to investigate the influence of the resistive terminating half‐space on the measured long‐offset transient electromagnetic data. The 1D joint inversion models were utilised as starting models for both the 1D and 2D modelling studies. The results tend to show that a resistive terminating half‐space, implying the presence of the target formation, is the favourable geological setting. Furthermore, the 2D modelling study aimed to fit all measured long‐offset transient electromagnetic Ex transients along the profile simultaneously. Consequently, 3125 2D forward calculations were necessary to determine the best‐fit resistivity model. The results are consistent with the 1D inversion, indicating that the data are best described by a resistive terminating half‐space, although the resistivity and depth cannot be determined clearly.  相似文献   

13.
Magnetotelluric data are collected along a NW-SE trending and about 900km long profile within northeastern boundary areas of the North China craton(NCC). This profile extends from the Hegenshan belt within the Central Asian orogenic belt(CAOB), across the Baolidao arc, Solonker-Linxi suture zone, Ondor Sum accretion complex, Bainaimiao arc, Inner Mongolia paleo-uplift, Yanshan belt, and ends on the Liaohe depression of the NCC. Impedance tensor decomposition methods are used to study the dimensionality and geo-electric strike of MT data of the region. Two-dimension (2D) analysis is appropriate for this profile. The 2-D subsurface electrical resistivity structure along profile is obtained using the non-linear conjugate gradient (NLCG) algorithm. The electrical resistivity structure is characterized by lateral segmentation, and divided into high resistive, low resistive, and high resistive areas; The lateral variation of electrical resistivity is significant within the CAOB, but it is smooth in the NCC; The extensive high conductive body(HRB)is observed in the mid-low crust beneath the Solonker-Linxi suture zone and Inner Mongolia paleo-uplift, respectively; The low resistivity could be due to the partial melts and crustal flows. Based on our electrical resistivity structure and other geological, geophysical observations, we speculate that (1)the final suturing of the Siberian craton to the NCC could be along the areas between Xilinhot Fault and Xar Moron Fault; (2)the relatively thick high resistive body beneath the Yanshan belt may serve as a tectonic barrier separating the on-craton and off-craton regions into different upper mantle convection system, and lower the effect of tectonic evolution of CAOB on the destruction to NCC.  相似文献   

14.
For about three decades helicopter-borne electromagnetic (HEM) measurements have been used to reveal the resistivity distribution of the upper one hundred metres of the earth's subsurface. HEM systems record secondary fields, which are 3–6 orders of magnitude smaller than the transmitted primary fields. As both the primary fields and the secondary fields are present at the receivers, well-designed bucking coils are often used to reduce the primary fields at the receivers to a minimum. Remaining parts of the primary fields, the zero levels, are generally corrected by subtracting field values recorded at high altitudes (standard zero levelling) or estimated from resistivities of neighbouring lines or from resistivity maps (advanced zero levelling). These zero-levelling techniques enable the correction for long-term, quasi-linear instrumental drift. Short-term variations caused by temperature changes due to altitude variations, however, cannot be completely corrected by this procedure resulting in stripe patterns on thematic maps.Statistical methods and/or 2-D filter techniques called statistical levelling (tie-line levelling) and empirical levelling (microlevelling), respectively, used to correct stripe patterns in airborne geophysical data sets are, in general, not directly applicable to HEM data. Because HEM data levelling faces the problem that the parameter affected by zero-level errors, the secondary field, differs from the parameter generally levelled, the apparent resistivity. Furthermore, the dependency of the secondary field on both the resistivity of the subsurface and the sensor altitude is strongly nonlinear.A reasonable compromise is to microlevel both half-space parameters: apparent resistivity and apparent depth, followed by a recalculation of the secondary field components based on the half-space parameters levelled. Advantages and disadvantages of the diverse levelling techniques are discussed using a HEM data set obtained in a hilly region along the Saale River between the cities of Saalfeld and Jena in central Germany. It turns out from a comparison of apparent resistivity and apparent depth maps derived from levelled HEM data that manually advanced zero levelling of major level errors and automatic microlevelling of remaining minor level errors yield the best results.  相似文献   

15.
Since its development some thirty years ago, the airborne electromagnetic (AEM) method has been primarily used as a tool for mineral prospecting. However, advanced AEM systems are capable of other tasks, such as geological mapping and groundwater exploration. Excellent correlation between maps of apparent conductivity and geological maps was observed in several regions of Brazil where AEM surveys were performed. The degree of correlation seems to depend on the local climate. In humid and subhumid tropical regions, a weathered layer develops whose thickness and conductivity depend upon bedrock lithology. Therefore certain lithological types can be recognized from their conductivity signature; e.g., granites and Precambrian coarse clastic rocks are resistive, metavolcanic (particularly mafic) and volcanic rocks are conductive, Phanerozoic sediments are generally highly conductive. Two geophysical surveys are analyzed in the paper. The first was conducted with the time-domain, towed-bird AEM system in the Itapicuru greenstone belt in the state of Bahia. The apparent conductivity map correlated better with the local lithology than the magnetic map. Results of the AEM survey were successfully used to improve the regional geological map. A helicopter EM system was used in the second survey, which covered a portion of the Precambrian shield of Rio Grande do Sul. Also in this region ground checks confirmed the usefulness of conductivity surveys in geological mapping. The technique outlined in the paper holds great promise for countries of humid tropical climate, where few outcrops exist and access is often difficult. The tests performed in Brazil indicate that by executing AEM/aeromagnetic surveys during initial stages of mapping and exploration programs, time and expenditure required for geological field work can be considerably reduced. The resulting geological maps are more accurate and the inventory of mineral occurrences becomes more complete.  相似文献   

16.
To minimize the number of solutions in 3D resistivity inversion, an inherent problem in inversion, the amount of data considered have to be large and prior constraints need to be applied. Geological and geophysical data regarding the extent of a geological anomaly are important prior information. We propose the use of shape constraints in 3D electrical resistivity inversion, Three weighted orthogonal vectors （a normal and two tangent vectors） were used to control the resistivity differences at the boundaries of the anomaly. The spatial shape of the anomaly and the constraints on the boundaries of the anomaly are thus established. We incorporated the spatial shape constraints in the objective function of the 3D resistivity inversion and constructed the 3D resistivity inversion equation with spatial shape constraints. Subsequently, we used numerical modeling based on prior spatial shape data to constrain the direction vectors and weights of the 3D resistivity inversion. We established a reasonable range between the direction vectors and weights, and verified the feasibility and effectiveness of using spatial shape prior constraints in reducing excessive structures and the number of solutions. We applied the prior spatially shape-constrained inversion method to locate the aquifer at the Guangzhou subway. The spatial shape constraints were taken from ground penetrating radar data. The inversion results for the location and shape of the aquifer agree well with drilling data, and the number of inversion solutions is significantly reduced.  相似文献   

17.
Jamal Asfahani 《水文研究》2007,21(8):1085-1097
A resistivity survey is conducted in Khanasser Valley, a semi‐arid region in northern Syria, to delineate the characteristics of ground water affected by the salt‐water intrusion related to Al‐Jaboul Sabkha. Existing wells were used to measure salinity and conductivity of water samplings. Vertical electrical sounding was carried out near the existing wells. The combination of resistivity and hydrogeological data enables the establishment of empirical relationships between earth resistivity, water resistivity, and the amount of total dissolved solids. These relationships are then used in order to derive salinity maps for electrode spacings of 70, 100, and 150 m. The distribution of fresh, brackish and salt‐water zones and their variations in space along two longitudinal profiles (LP1 and LP2) are established through converting subsurface depth–resistivity models into different ground‐water areas. The constructed ground‐water area maps allow interfaces between different water zones to be determined. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

18.
Salt water intrusion models are commonly used to support groundwater resource management in coastal aquifers. Concentration data used for model calibration are often sparse and limited in spatial extent. With airborne and ground‐based electromagnetic surveys, electrical resistivity models can be obtained to provide high‐resolution three‐dimensional models of subsurface resistivity variations that can be related to geology and salt concentrations on a regional scale. Several previous studies have calibrated salt water intrusion models with geophysical data, but are typically limited to the use of the inverted electrical resistivity models without considering the measured geophysical data directly. This induces a number of errors related to inconsistent scales between the geophysical and hydrologic models and the applied regularization constraints in the geophysical inversion. To overcome these errors, we perform a coupled hydrogeophysical inversion (CHI) in which we use a salt water intrusion model to interpret the geophysical data and guide the geophysical inversion. We refer to this methodology as a Coupled Hydrogeophysical Inversion‐State (CHI‐S), in which simulated salt concentrations are transformed to an electrical resistivity model, after which a geophysical forward response is calculated and compared with the measured geophysical data. This approach was applied for a field site in Santa Cruz County, California, where a time‐domain electromagnetic (TDEM) dataset was collected. For this location, a simple two‐dimensional cross‐sectional salt water intrusion model was developed, for which we estimated five uniform aquifer properties, incorporating the porosity that was also part of the employed petrophysical relationship. In addition, one geophysical parameter was estimated. The six parameters could be resolved well by fitting more than 300 apparent resistivities that were comprised by the TDEM dataset. Except for three sounding locations, all the TDEM data could be fitted close to a root‐mean‐square error of 1. Possible explanations for the poor fit of these soundings are the assumption of spatial uniformity, fixed boundary conditions and the neglecting of 3D effects in the groundwater model and the TDEM forward responses.  相似文献   

19.
The aim of this work is to introduce the application of the fuzzy ordered weighted averaging method as a straightforward knowledge‐driven approach to explore porphyry copper deposits in an airborne prospect. In this paper, the proposed method is applied to airborne geophysical (potassium radiometry, magnetometry, and frequency‐domain electromagnetic) data, geological layers (fault and host rock zones), and various extracted alteration layers from remote sensing images. The central Iranian volcanic–sedimentary belt in Kerman province of Iran that is located within the Urumieh–Dokhtar (Sahand–Bazman) magmatic arc is chosen for this study. This region has high potential of mineral occurrences, especially porphyry copper, containing some active world‐class copper mines such as Sarcheshmeh. Two evidential layers, including the downward continued map and the analytic signal of such filtered magnetic data, are generated to be used as geophysical plausible traces of porphyry copper occurrences. The low values of the resistivity layer acquired from airborne frequency‐domain electromagnetic data are also used as an electrical criterion in this study. Four remote sensing evidential layers, including argillic, phyllic, propylitic, and hydroxyl alterations, are extracted from Advanced Spaceborne Thermal Emission and Reflection Radiometer images in order to map the altered areas associated with porphyry copper deposits. The Enhanced Thematic Mapper plus images are used to map iron oxide layer. Since potassium alteration is the mainstay of copper alteration, the airborne potassium radiometry data are used. Here, the fuzzy ordered weighted averaging method uses a wide range of decision strategies in order to generate numerous mineral potential/prospectivity maps. The final mineral potential map based upon desired geo‐data set indicates adequately matching of high‐potential zones with previous working mines and copper deposits.  相似文献   

20.
Helicopter electromagnetic (HEM) systems are commonly used for conductivity mapping and the data are often interpreted using an isotropic horizontally layered earth model. However, in regions with distinct dipping stratification, it is useful to extend the model to a layered earth with general anisotropy by assigning each layer a symmetrical 3 × 3 resistivity tensor. The electromagnetic (EM) field is represented by two scalar potentials, which describe the poloidal and toroidal parts of the magnetic field. Via a 2D Fourier transform, we obtain two coupled ordinary differential equations in the vertical coordinate. To stabilize the numerical calculation, the wavenumber domain is divided into two parts associated with small and large wavenumbers. The EM field for small wavenumbers is continued from layer to layer with the continuity conditions. For large wavenumbers, the EM field behaves like a DC field and therefore cannot be sensed by airborne EM systems. Thus, the contribution from the large wavenumbers is simply ignored. The magnetic fields are calculated for the vertical coaxial (VCX), horizontal coplanar (HCP) and vertical coplanar (VCP) coil configurations for a helicopter EM system. The apparent resistivities defined from the VCX, VCP and HCP coil responses, when plotted in polar coordinates, clearly identify the principal anisotropic axes of an anisotropic earth. The field example from the Edwards Aquifer recharge area in Texas confirms that the polar plots of the apparent resistivities identify the principal anisotropic axes that coincide well with the direction of the underground structures.  相似文献   

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