Near Surface 2008 - 14th EAGE European Meeting of Environmental and Engineering Geophysics

The microtremor and the MASW (Multi-channel Analysis of Surface Waves) survey were carried out to determine the shear wave velocity-depth information, resonance frequencies and the depths of the sedimentary layer in the Dinar city, Southwestern of Turkey. The shear-wave velocity profile of the basin sediments was estimated from the inversion of the microtremor horizontal-to-vertical spectrum based on surface waves from seismic noise at each site using a genetic algorithm. The average shear-wave velocities estimated from the multichannel analysis of surface waves experiments were given as constraints in the inversion. A new relationship was derived between the resonance frequency ( ) and thickness ( ) of the overlaying layer, which is given as

The length of existing foundation piles is unknown in many cases due to missing files and drawings. The investigation methods used in most cases today have certain limitations that require the application of new techniques. The parallel seismic method uses first arrivals of elastic waves generated by an impulse on the pile top registered by sensors in a nearby borehole. The travel time curves can be interpreted in terms of velocities and foundation depth. Numerical simulation (ray-tracing and full waveform modelling) helps to understand the results achieved at non-standard foundations or under complex subsurface conditions. The method is successfully applied in research and commercial projects.

With a general knowledge about the geological setting it is possible to use numerical modelling of geoelectrical and IP data to obtain useful experience for the interpretation of real measurements. In this case the vertical extension of a dolerite dyke has been modelled and compared with real data. It is obvious from the resistivity modelling that it in this particular case not is possible to say anything about the vertical extension of the low resistive zone in the real measurements. Based on the experience from the IP modelling it is much clearer that the dyke extends to the bottom of the model. By combining different methods which exploits different physical properties a better and more reliable interpretation can clearly be done.

This extended abstract will concentrate on results of POHVA I which lasted from May 2005 to the end of 2007. POHVA I consisted of assessing the means and their reliability to measure subsurface conditions where water content distribution was the single most important factor. A model for transforming resistivities to water contents was established. During the project a new index testing method for soil stabilisation testing was developed and an extensive soil stabilisation database was constructed and based on this database statistical models for shear strength improvement to be obtained by given amount of binder in given environmental conditions was created.

We give an overview on possible transform functions (TF) used in geophysical inversion. For three different geophysical methods - dc resistivity sounding, surface nmr sounding and travel time tomography - we investigate how TF's affect the results of synthetic data, the singular value spectrum and resolution properties.

The well-known finite-difference time-domain (FDTD) approach to the numerical solution of Maxwell’s equations for ground penetrating radar (GPR) modeling has a conditionally stable nature and this is one of the major limitations of the FDTD scheme. For the stability condition to be fulfilled, the maximum allowable time-step for the numerical computations is limited by the minimum cell size in the computational domain. Therefore, when detailed structures need to be simulated and/or when there are small features or regions of limited extent in the overall computational mesh with values of high dielectric constant - supporting propagation of waves at very short wavelengths - the spatial-step should be small enough so as all the objects in the computational domain are adequately resolved. For such cases, the FDTD solution employing a small, uniform spatial-step in the whole computational domain would lead to substantial computer memory requirements and execution time increase. This problem can be overcome and efficiency can be increased by introducing subgrids, into the FDTD method. Examples of areas of applications of these subgrids include the efficient modelling of fractures and other small targets as well as targets with high values of dielectric constant.

We present a new approach to imaging shallow subsurface objects in 3D space based on weighted multipath summation of seismic waves diffracted by the target objects. The method is a further development of the recently proposed multipath summation technique, where the imaging is performed by stacking the diffracted waves along all possible diffraction time surfaces having a common apex at the given point. The multipath approach does not require any explicit information on velocities since the summation is performed for all possible velocity values within a wide specified range. We show that the images obtained by the multipath summation can be considerably improved by applying a proper weighting function. Moreover, the weighted multipath summation can also produce a reliable estimate of velocity above the diffractor. The obtained velocity can be used to further improve the image quality. Application of the proposed method is demonstrated by synthetic and real data examples.

Seismic refraction tomography is an attractive strategy for modeling near surface presenting lateral and vertical gradients and also producing static corrections, where traditional seismic refraction fails. We evaluate a hybrid inversion method that combines a global search with a local optimization method to build the velocity model from first break pickings without ray tracing. In the initial stage of the inversion, the Monte Carlo technique searches for the region of the model space where the solution is. This solution is then further improved by using a simultaneous iterative reconstruction technique (SIRT). The travel times and ray paths required for the inversion are calculated by a highly efficient grid ray tracing technique based on the Fresnel volume approach. First, we evaluate the method with synthetic data and show that the hybrid optimization method satisfactorily reconstructs the model shape and the velocity range. Then, we analyse the method with real seismic data to verify that, in both cases, the code performs at a level similar to the commercial software RAYFRACT

The “Villa del Casale” is a wonderful Roman Villa excavated in the archaeological site of Piazza Armerina (Sicily) in 1929. Its interest is mainly to be referred to the floor mosaics, among the largest and most beautiful ones of Roman times, of superlative quality. The floor of the Corridor is covered by a wonderful mosaic depicting the hunt and capture of wild animals, so that it is called the “Corridor of the Great Hunting Scene”. The Villa is now under restoration, and during the planning of the restoration a 3D electrical resistivity tomography was performed on the corridor by using an acquisition grid composed by 704 electrodes. Furthermore 25 GPR profiles with 400 MHz antenna were acquired. The 3D ERT survey was carried out by utilizing a new 3D array, named “Maximum Yield Grid” (MYG), that allows a great decrease of the acquisition time and of the invasivity of the ERT investigation in respect of classical arrays. The results show shallow anomalies, that correspond to the canalizations for water drainage and to the concrete basement built below the corridor during a previous restoration intervention, and a diminution of resistivity with depth, related to the aquifer below the Villa.

Magnetic prospecting is a rapid, economic and noninvasive method for studying buried near-surface structures of archaeological sites. A new generation of magnetic equipment for field data acquisition and advanced methodology for their data analysis allows to reveal a broad range of buried archaeological targets: walls, columns, foundations, caves, tunnels, tombs, water pipes, kilns, furnaces, ovens, and other objects. However, magnetic survey at archaeological sites is commonly affected by a number of natural and artificial “noise” factors. Such complex conditions require an optimization of current methodology of magnetic survey. The implication of magnetic prospecting for the study of archaeological sites is based on the difference (magnetic contrast) in magnetic properties of archaeological targets and surrounding medium. Two archaeomagnetic provinces were defined according to the magnetic characteristics of archaeological elements, background soils and rocks, and pattern of magnetic anomalies. This allowed us to develop an optimized methodology for high-resolution and reliable archaeomagnetic prospecting in each archaeomagnetic province.