- Home
- A-Z Publications
- Near Surface Geophysics
- Previous Issues
- Volume 3, Issue 2, 2005
Near Surface Geophysics - Volume 3, Issue 2, 2005
Volume 3, Issue 2, 2005
-
-
Evaluation of electrical methods, seismic refraction and ground‐penetrating radar to identify clays below sands ‐ Two case studies in SW Sweden
Authors Anita Turesson and Gustaf LindABSTRACTBecause of their geophysical properties, making a detailed interpretation of clay layers beneath surface layers of sand or gravel is sometimes a challenge. We chose two sites in south‐western Sweden where sand over clay has been documented by drilling, to test the effectiveness of four different geophysical methods in mapping the buried clay. We used geoelectrical methods (resistivity and induced polarization), seismic refraction and ground‐penetrating radar. Resistivity was found to be the best method for identifying the presence of the clay layer. Induced polarization (IP), in combination with resistivity, provides additional information but can give erroneous results if IP responses are weak. The seismic refraction method was not effective in detecting the clay layer due to the low contrast in seismic velocities between saturated sand and clay, although the method did give useful constraints on resistivity modelling. With ground‐penetrating radar, it was possible to map the upper boundary of clay with good accuracy.
-
-
-
Combination of 1D laterally constrained inversion and 2D smooth inversion of resistivity data with a priori data from boreholes
Authors Roger Wisén, Esben Auken and Torleif DahlinABSTRACTResistivity imaging in combination with borehole information is a powerful tool for site investigation. We show that the combination of 1D laterally constrained inversion (1D‐LCI) with the use of a priori information from borehole data and 2D smooth inversion adds significant value to the interpretation of continuous vertical electrical sounding (CVES) data. The 1D‐LCI offers an analysis of the resolution of the model parameters. This is helpful when evaluating the integrity of the model. Furthermore, with the 1D‐LCI it is possible to constrain model parameters with a priori information, e.g. depth‐to‐layer interfaces, based on borehole information.
We show that 2D smooth inversion resolves lateral changes well, while 1D‐LCI results in well‐defined horizontal layer interfaces. In geological environments where the lateral variations are not too pronounced, the 1D‐LCI contributes to a geological interpretation of the resistivity measurements. Depths to layers can be interpreted with greater certainty than if using results from 2D smooth inversion only. The inclusion of a priori information in the inversion reveals further details and enhances the geological interpretation significantly.
-
-
-
Characterisation of a NAPL‐contaminated former quarry site using electrical impedance tomography
Authors J.E. Chambers, P.I. Meldrum, R.D. Ogilvy and P.B. WilkinsonABSTRACTElectrical impedance tomography (EIT) was used to characterize two tar pits, containing a range of NAPL and aqueous contaminants, located within a former quarry site. The site, which is situated on Coal Measures strata near Belper, Derbyshire, UK, has been the subject of an extensive conventional site investigation. Consequently, the geometry and contents of the pits are relatively well defined. The aims of the survey were to test 3D EIT in field conditions against good ground‐truth data and, in combination with intrusive and historical site records, to improve the characterization of the waste pits.
The EIT survey employed surface electrodes within a network of linear survey lines. Impedance magnitude and phase data were collected at frequencies of 0.6, 2, 10, 40 and 125 Hz, from which resistivity and phase models were calculated using a 3D smoothness‐constrained least‐squares inversion technique.
Resistivity models displayed only weak frequency‐dependent variations. The resistivity images were proved to correlate well with the ground‐truth data, and were used to further constrain models of the distribution of waste and bedrock contamination. Furthermore, interrogation of the 0.6 Hz resistivity model permitted estimates of waste and contaminated bedrock volumes for the two pits. A reliable phase model could be produced only from the 0.6 Hz data, due to the increased noise levels affecting the data at higher frequencies. The 0.6 Hz phase model displayed phase variations consistent with the known distribution of the waste. The mechanisms causing the observed phase anomalies could not be conclusively determined; however, it is probable that they are related to the presence of metallic waste.
EIT was shown to be a particularly effective tool for site characterization when used in conjunction with intrusive and historical data for the purposes of model calibration and interpretation.
-
-
-
Using trapped waves for mapping shallow fault zones
Authors V. Shtivelman, S. Marco, M. Reshef, A. Agnon and Y. HamielAbstractRecent studies have shown that shallow fault zones can be identified by anomalous behaviour of the wavefield recorded by high‐resolution seismic surveys. Analysis of seismic records acquired using off‐line shooting geometry, where sources and receivers are located along two separate parallel lines crossing a suspected location of a fault zone, reveals prominent anomalies which may be identified with the waves trapped within the zone. On seismic records, trapped waves usually appear as regular low‐frequency high‐amplitude wavetrains. In order to facilitate the identification of the trapped‐wave‐related anomalies, we propose two procedures utilizing specific properties of the trapped waves. The first procedure is based on stacking time‐scaled seismic traces in the common‐shot and common‐receiver domains with the subsequent application of eigenimage analysis. In the second procedure, band‐limited spectral energy of time‐scaled traces is represented as a map in the source–receiver coordinates. By detecting and mapping trapped‐wave anomalies in the source and receiver domains, the spatial location of a fault zone can be estimated by linear interpolation between the locations of the corresponding anomalies on the source and receiver lines.
Application of the above procedures is illustrated by a number of synthetic and real data examples.
-
-
-
Weathering zone determination in the province of Rio Negro, Argentina, with high resolution gravimetry
Authors M. Gimenez, P. Martínez, F. Ruíz, A. Introcaso and J. La VecchiaABSTRACTThis investigation demonstrates the advantages of microgravity results obtained with a digital gravimeter. On this basis, we determined the physical parameters of the weathering zone used in the calculation of static corrections in hydrocarbon exploration. A significant result presented here is, that by combining information from upholes with the application of microgravity results, the accuracy of the determination of the weathering thickness was improved by 5‐6 metres. It is also shown that application of this technique is not only time‐saving and economical, but it is also a more efficient method of determining anomalous bodies, which are overlooked by the conventional techniques currently used in prospecting.
-
-
-
Use of antenna arrays for GPR surveying in archaeology
More LessABSTRACTAmong the different geophysical prospecting methods applied in archaeology, ground‐penetrating radar is one that still requires a lot of time to carry out field recordings. A duo‐antenna configuration is proposed that will halve the time needed in the field. Tests have shown that this can be done without any interference between the two antennae. The signals from different antennae do not have the same characteristics and the spectrum and amplitude of the traces are not the same. The use of spectral whitening and energy matching for adjacent traces or samples makes the data sets as similar as possible to allow for unperturbed time‐ or depth‐slices. As a side effect, spectral whitening enhances the vertical resolution and penetration depth of the method.
-
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)