- Home
- A-Z Publications
- Near Surface Geophysics
- Previous Issues
- Volume 8, Issue 1, 2010
Near Surface Geophysics - Volume 8, Issue 1, 2010
Volume 8, Issue 1, 2010
-
-
Fast approximate 1D inversion of frequency domain electromagnetic data
Authors Niels B. Christensen, Andrew Fitzpatrick and Tim MundayABSTRACTWe present a fast approximate method for 1D inversion of frequency domain data and apply it to frequency domain helicopter‐borne data from the Bookpurnong area of the Murray River, South Australia. The method is based on fast approximate forward computation of transient electromagnetic step responses and their derivatives with respect to the model parameters of a 1D model, with the frequency domain responses and derivatives then found through Fourier transformation of the time‐domain counterparts. The inversion is carried out with multi‐layer models in an iterative, constrained least‐squares inversion scheme including explicit formulation of the model regularization through a model covariance matrix. The method is 30 times faster than conventional full inversion for a layered earth model and produces model sections of concatenated 1D models and contoured maps of mean conductivity in elevation intervals almost indistinguishable from those of a conventional full inversion. In a theoretical forward and inverse modelling study, the fast approximate and conventional computation methods are compared demonstrating the applicability of the approximate method and its limitations.
Applied to the Bookpurnong RESOLVE® FDHEM data set from South Australia, the inversion produces model sections and conductivity maps that reveal the distribution of conductivity in the area and thereby the distribution of salinity. This information is crucial for any remediation effort aimed at alleviating the salinization of the river and the degradation of floodplain vegetation and associated ecosystems.
-
-
-
Three‐dimensional magneto‐seismic reconstruction of the ‘Torre del Greco’ submerged volcanic structure (Naples Bay, Southern Tyrrhenian Sea, Italy): implications for Vesuvius’s marine geophysics and volcanology
Authors Gemma Aiello, Ennio Marsella and Ruggieri StefanoABSTRACTA three‐dimensional reconstruction of a large volcanic structure located offshore the town of Torre del Greco (Naples Bay, Southern Tyrrhenian Sea, Italy) and representing the seaward prolongation of the Vesuvius volcano has been carried out using integrated geological interpretation of existing densely‐spaced high‐resolution seismic and magnetic profiles. This reconstruction has provided new insights into the knowledge of marine geophysics and volcanology of Vesuvius, one of most studied volcanoes in the world.
Seismic stratigraphy of the volcanic structures shows acoustically‐transparent seismic facies and high contrasts of acoustic impedance with respect to the overlying sediments. The structures have mound‐shaped external geometry and average dimensions measurable in terms of kilometres. The base of the Torre del Greco volcanic structure overlies the volcanic seismic unit correlated to the ‘Campanian Ignimbrite’ pyroclastic flow deposits. Consequently, no contrast of acoustic impedance between the two volcanic units is clearly evident on the seismic profiles. The top of the structure is irregular and eroded and shows several culminations.
The three‐dimensional reconstruction of the Torre del Greco structure has been carried out using seismic constraints and correlations of volcanic structure to bathymetry. The sea floor topography is compared to the top of the Torre del Greco volcanic structure represented by contour depth of corresponding seismic horizons. Proceeding from south‐east to north‐west there is a good correspondence between the sea floor topographic surface and the top of the volcanic structure, which does not crop out at the sea‐bottom. The rising of the volcanic structure in proximity to the sea‐bottom corresponds to the occurrence of topographic undulations of up to ten metres. This evidence is confirmed by the interpretation of seismic profiles, showing three main vertical culminations of the volcanic structure, where the overlying sediment drape is significantly reduced. These culminations are linked to magnetic anomaly extremes, having values ranging between 250–350 nT. A striking coincidence between the long‐shaped topographic high on the sea floor (towards the east) and the high in the top of the volcanic structure underneath can be explained as a consequence of sea floor deformation due to the underlying volcanic culminations. Smaller volcanic mounds, buried and fossilized by marine sediments, are not related to any magnetic anomaly, probably due to their composition of volcanic tuffs.
-
-
-
Geoelectrical mapping of aquifer contamination: a case study from Roorkee, India
Authors Sudha, B. Tezkan, M. Israil, D.C. Singhal and J. RaiABSTRACTThe impact of untreated sewage disposal and sewage irrigation practice around Roorkee, Uttarakhand, India, on groundwater contamination is studied using surface geoelectrical techniques. Electrical resistivity tomography (ERT) and transient electromagnetic (TEM) soundings data were recorded in and around a sewage‐irrigated area in Saliyar and Khanjarpur and a remote site where groundwater contamination due to sewage was not expected. The sewage is mainly composed of domestic and municipal waste. Lithology data was used for correlating the resistivity values, which were derived from 2D inversion of resistivity and 1D Occam inversion of TEM data, with the subsurface formation. These two methods were used jointly to increase the reliability of the results in mapping of ground‐water contamination. It was thus observed that the sewage pollutants, infiltrated through the soil, reach the shallow unconfined aquifer in and around the polluted area, up to a limited distance from the existent waste disposal sites. The present study suggests that the groundwater present in the shallow unconfined aquifer (<10 m depth) close to waste disposal sites is contaminated. It was noticed that the decrease in the resistivity of a shallow unconfined aquifer is up to 50% in comparison with that of the shallow unconfined aquifer at a remote site located about 20 km north‐west of the disposal sites. The extent of the polluted aquifer zone was delineated by comparing its resistivity with that of the same aquifer at a point away from the disposal sites. Thus, the extent of groundwater pollution and relative direction of its flow were also determined qualitatively.
-
-
-
Joint inversion interpretation for gravity and resistivity data: a case study at New Heliopolis City, Cairo, Egypt
More LessABSTRACTGravity and resistivity data were used in the present study for groundwater exploration using a joint inversion process. One hundred fifty three gravity stations and thirty two vertical electrical soundings were used to determine the thickness and resistivity distribution of the different geological units in the study area. Then, the gravity and resistivity data were utilized concurrently to identify the structural elements, stratigraphic units and groundwater potential in the study area. The results of the joint inverse interpretation have strengthened the analysis and consistency of the outcome. The results revealed the presence of four layers. The first is the Quaternary deposits of thickness ranging from 0.5–2 m; the second layer is the Hommath Formation of Middle Miocene age deposits, which have resistivity ranging from and thicknesses ranging from 5–40 m. The third layer is the Gabal Ahmar Formation (Oligocene age), which has resistivity ranging from and a thickness ranging from 10–342 m. The fourth layer is limestone of the Upper Eocene age that exhibits high resistivity values ranging from , depths ranging from 55–350 m and bulk density 2600 kg/m3. The results also indicate that the area is dissected by different fault elements trending N‐S, NE‐SW, NE‐SW and E‐W.
-
-
-
Migration and interpretation of first arrival inflection points due to lateral variations
By H.H. SeisaABSTRACTThe majority of shallow refraction seismic algorithms work well only in cases of a horizontal plane or inclined refractors. In case of piecewise lateral variation either in dip angle and/or in velocity along the refracting interfaces, the applications of these conventional methods are usually problematic. This is because of the existence of two different inflection points along the first arrival travel‐time curve. The first inflection point corresponds to the crossover point while the later inflection points are due to these lateral variations. The terms true and apparent inflection points will be used in this study to refer to inflection points corresponding to crossover points and those due to lateral variations, respectively. The intersection of the backward extrapolation from inflection points with the time axes will give true intercept times from true inflection points and apparent intercept times from apparent inflection points.
Migration of apparent inflection points aims to remove the distortion due to lateral variations of refracted rays from the first arrival traveltime curves by moving them to their proper locations. After the migration of the apparent inflection points, the intercept time technique can be applied in a conventional way to map shallow subsurface refracting interfaces with piecewise lateral variations. The common traveltime elements recorded from the same refracting interfaces and the layer reciprocal time are required for this migration process.
This algorithm will be generalized after the removal of the effects due to the apparent inflection points and calculation of the corresponding migrated traveltime elements. Each pair of the common migrated traveltime elements from forward and reverse directions will represent a simple traveltime curve of two planes. Therefore, most shallow refraction algorithms can be applied to this simple traveltime curve using their own equations to map refracting interfaces recorded from media with piecewise lateral variations.
-
-
-
Application of electrical resistivity tomography to the environmental characterization of abandoned massive sulphide mine ponds (Iberian Pyrite Belt, SW Spain)
More LessABSTRACTMining activity in the Iberian Pyrite Belt, on the south‐west of the Iberian Peninsula, has generated a great amount of mine tailing ponds, which once the extractive activity is finished, are abandoned and become a serious environmental problem. Here we present the results of applying the electrical resistivity tomography (ERT) technique to characterize the abandoned mine ponds in two sites: Monte Romero and Mina Concepción. ERT has allowed us to determine both the general geometry of the pond’s base and the maximum thickness of the mine tailings. In all cases, the resistivity contrast between the infilling and the bedrock is high enough to clearly define the bottom pond boundary. The low‐resistivity values (lower than ) obtained for the infilling are explained by the high concentration of pyrite in the tailings and the occurrence of acid waters. Whereas the Monte Romero mine pond is almost completely saturated with water, in Mina Concepción it has been possible to identify the presence of inner acid water flows, the outlet of which through the damaged dyke originates a spilling of acid waters to the Odiel River. No low‐resistivity water flows through the base of the ponds into the bedrock have been observed, indicating a good isolation of the base of the studied mine ponds.
-
-
-
Shear‐wave velocity profiling at sites with high stiffness contrasts: a comparison between invasive and non‐invasive methods†
ABSTRACTThree sites of the Italian Strong Motion Network (RAN) have been selected for detailed S‐wave profiling, using both borehole and surface wave seismic methods. At these sites, the presence of stiffness contrasts within the soil column is found to influence the surface wave propagation profoundly. Advanced aspects in surface wave inversion such as resolution, accuracy and higher‐mode interpretation must be properly taken into account to obtain realistic results from the surface wave dispersion observations. The possibility of mode misidentification and the loss of resolution with depth in surface wave interpretation are explored using synthetic modelling together with active and passive seismic data sets. With high stiffness contrasts, the possibility of mode jumps and higher mode dominance over specific frequency ranges is very probable. This is true also for normally dispersive sites, where the shear velocity increases with depth, though higher mode dominance is recognized as more common in the case of a shear‐wave velocity inversion within the soil column and the sensitivity of the dispersion curves with respect to those layers beneath the low‐velocity zone may be significantly reduced. Pitfalls in the inversion resulting from mode misidentification can be avoided by investigating the effective phase velocity distribution, using active data sets and full waveform seismic modelling. When an unambiguous modal identification is achieved, the results obtained by surface wave inversion are very satisfactorily consistent with borehole data.
-
Volumes & issues
-
Volume 22 (2024)
-
Volume 21 (2023)
-
Volume 20 (2022)
-
Volume 19 (2021)
-
Volume 18 (2020)
-
Volume 17 (2019)
-
Volume 16 (2018)
-
Volume 15 (2017)
-
Volume 14 (2015 - 2016)
-
Volume 13 (2015)
-
Volume 12 (2013 - 2014)
-
Volume 11 (2013)
-
Volume 10 (2012)
-
Volume 9 (2011)
-
Volume 8 (2010)
-
Volume 7 (2009)
-
Volume 6 (2008)
-
Volume 5 (2007)
-
Volume 4 (2006)
-
Volume 3 (2005)
-
Volume 2 (2004)
-
Volume 1 (2003)