Research and Papers

Soil dynamic parameters such as shear wave velocity and damping ratio are of major interest in earthquake engineering. While the shear wave velocity, directly linked to the shear modulus, can be determined by a number of laboratory and in-situ tests with satisfying accuracy the damping ratio is much more difficult to obtain. Especially the results of in-situ experiments show often large variations. This is in general due to the troublesome determination of precise signal amplitudes whether in time or frequency domain related with these techniques. The paper presented comes back to a relationship between attenuation and velocity dispersion of body waves which replaces the measurement of the amplitude characteristics of seismic signals by a frequency dependent velocity function. The implementation of this method has previously shown to be difficult because of the very small levels of dispersion observed in seismic data. Our approach aims to overcome the problem by applying a multi-channel spectral analysis which is widely used in surface wave testing to calculate a velocity dispersion. Multi-channel measurements have shown to be more tolerant to erroneous phase characteristics of single seismic traces than the more common two station measurements. The velocity dispersion curve is extracted from a phase velocity–frequency spectrum and the damping ratio is calculated by fitting a theoretical dispersion curve to the extracted curve. The method is demonstrated on correlated data of a seismic downhole test performed using a S-wave vibrator source. The obtained results show a reasonable agreement with damping ratios found in the literature for similar soils.

2019

Joint P- and S-wave measurements for tomographic cross-borehole analysis can offer more reliable interpretational insight concerning lithologic and geotechnical parameter variations compared with P-wave measurements on their own. However, anisotropy can have a large influence on S-wave measurements, with the S-wave splitting into two modes. We have developed an inversion for parameters of transversely isotropic with a vertical symmetry axis (VTI) media. Our inversion is based on the traveltime perturbation equation, using cross-gradient constraints to ensure structural similarity for the resulting VTI parameters. We first determine the inversion on a synthetic data set consisting of P-waves and vertically and horizontally polarized S-waves. Subsequently, we evaluate inversion results for a data set comprising jointly measured P-waves and vertically and horizontally polarized S-waves that were acquired in a near-surface (<50 m) aquifer environment (the Safira research site, Germany). The inverted models indicate that the anisotropy parameters and are close to zero, with no P-wave anisotropy present. A high VP/VS ratio of up to nine causes considerable SV-wave anisotropy despite the low magnitudes for and. The SH-wave anisotropy parameter γ is estimated to be between 0.05 and 0.15 in the clay and lignite seams. The S-wave splitting is confirmed by polarization analysis prior to the inversion. The results suggest that S-wave anisotropy may be more severe than P-wave anisotropy in near-surface environments and should be taken into account when interpreting cross-borehole S-wave data.

P-wave, as well as horizontally and vertically polarized S-wave, tomographic data were collected between two borehole pairs. This enabled the joint-inversion of the three datasets. By employing structural constraints, the S-wave traveltimes were coupled to the more accurate P-wave traveltimes during the inversion. Thereby, the traveltime and anisotropic artefacts, initially observed in the individually inverted S-wave tomograms, were significantly reduced and the correlation with the borehole logs improved, while the resolution of the jointly inverted P-wave tomogram was only marginally affected. The joint inversion proves successful in determining the Swave velocity distribution more accurately than individual inversions. In addition, the jointly inverted tomograms were used to detect aquifer heterogeneities, caused by differences in clay content, and to distinguish areas of relatively high effective pressure. Comparison of the jointly inverted S-wave tomograms suggests the effect of S-wave anisotropy, which showed substantial velocity differences of approximately −10% to +10%. The anisotropy may have been caused by the presence of water-filled pores, micro-cracks and preferred mineral alignment (mainly clay) in the media.

An integrated P- and S-wave cross-borehole tomographic survey was performed in the city center of Kuala Lumpur, Malaysia, with the aim of exploring a karstic limestone area near an area that previously encountered cavities. Horizontally polarized shear waves were generated with two opposing, perpendicular strike directions and recorded with a multi-level, threecomponent receiver array. This allowed a high quality picking of the traveltimes, whereby the wave train reverses at the time of the S-wave arrival. In addition, high quality sparker generated P-waves were recorded. The P- and S-wave traveltimes were used to invert for two co-located tomograms. These tomograms enabled a better interpretation capability than a P- or S-wave tomogram on its own. The tomograms enabled the calculation of the elastic parameters, i.e., P- to S-wave velocity (Vp/Vs) ratio, Poisson’s ratio, bulk modulus, Young’s modulus and the shear modulus, on a 2D surface between the boreholes. This further aided the interpretation, as areas with limited traveltime accuracy and thus, an increase in tomographic error, could be easily identified, and the extent of a large cavity could be estimated. The interpretation of the tomograms was constrained by two additional boreholes, which provided more confidence on the delineation and location of cavities at depths. The survey shows the benefit of co-locating P- and S-wave tomography surveys.

Detailed information on shallow sediment distribution in basins is required to achieve solutions for problems in Quaternary geology, geomorphology, neotectonics, (geo)archaeology, and climatology. Usually, detailed information is obtained by studying outcrops and shallow drillings. Unfortunately, such data are often sparsely distributed and thus cannot characterise entire basins in detail. Therefore, they are frequently combined with remote sensing methods to overcome this limitation. Remote sensing can cover entire basins but provides information of the land surface only. Geophysical methods can close the gap between detailed sequences of the shallow sediment inventory from drillings at a few spots and continuous surface information from remote sensing. However, their interpretation in terms of sediment types is often challenging, especially if permafrost conditions complicate their interpretation. Here we present an approach for the joint interpretation of the geophysical methods ground penetrating radar (GPR) and capacitive coupled resistivity (CCR), drill core, and remote sensing data. The methods GPR and CCR were chosen because they allow relatively fast surveying and provide complementary information. We apply the approach to the middle Orkhon Valley in central Mongolia where fluvial, alluvial, and aeolian processes led to complex sediment architecture. The GPR and CCR data, measured on profiles with a total length of about 60 km, indicate the presence of two distinct layers over the complete surveying area: (i) a thawed layer at the surface, and (ii) a frozen layer below. In a first interpretation step, we establish a geophysical classification by considering the geophysical signatures of both layers. We use sedimentological information from core logs to relate the geophysical classes to sediment types. This analysis reveals internal structures of Orkhon River sediments, such as channels and floodplain sediments. We also distinguish alluvial fan deposits and aeolian sediments by their distinct geophysical signature. With this procedure we map aeolian sediments, debris flow sediments, floodplains, and channel sediments along the measured profiles in the entire basin. We show that the joint interpretation of drillings and geophysical profile measurements matches the information from remote sensing data, i.e., the sediment architecture of vast areas can be characterised by combining these techniques. The method presented here proves powerful for characterising large areas with minimal effort and can be applied to similar settings.

In areas with an unknown geology, boreholes are usually placed either at the planned location of buildings and infrastructure or following a semiregular pattern. The number of boreholes is typically limited by installation cost, especially the number of boreholes to be used for geophysical testing, such as those used for downhole, crosshole, or tomographic analyses. An alternative approach to conventional drilling is the use of mobile pushing devices, i.e., directpush procedures. By placing geophysical tools into the pushing rods, geophysical methods become more flexible and adaptive during drilling, and investigation techniques can be implemented more expeditiously. From a geoengineering perspective, the in-situ tests are relatively efficient because they generate near continuous data and are considerably more accurate in comparison to laboratory consolidation tests. In this paper we present a combination of a direct-push system with seismic crosshole measurements as a cost effective alternative to standard investigation techniques. The new methodology was successfully tested at the site for Technical Safety (TTS) in Horstwalde, Germany. A complete crosshole dataset of P-, SV- and SH-waves was acquired between previously installed PVC cased boreholes and the direct-push borehole. Furthermore, the in-situ profiles of paired shear wave velocity profiles (SH and SV) were used to evaluate the stress history of the soils.

Geophysical methods are able to fill the gap between boreholes and to provide a measure of the spatial continuity of structures. Borehole seismic tomography promises the highest resolution when applied at a local scale of a few tens of meters. Currently, almost exclusively P-wave tomography is employed in geotechnical oriented tomographic surveys. However, from the geotechnical perspective, the benefit of P-wave tomography is rather limited. It is the S-wave structure of the ground which is crucial to derive geotechnical relevant parameters, such as shear strength or other elastic moduli. Up to now, only little effort has been made to develop the borehole S-wave crosshole tomographic method. S-wave tomography has some clear advantages compared to conventional P-wave tomography, such as a better spatial resolution and a higher sensitivity to material changes. Furthermore, Swaves are only slightly influenced by the ground water table and S-waves passing this zone are not much affected by ray bending compared to P-waves. Within this paper we present first results of a newly developed Swave tomography system and field results from different test sites.

Identification of spatial hydraulic properties of an aquifer, such as porosity and hydraulic conductivity and their associated structures play an important role in contaminant risk assessment. At local scale average sediment properties may not be appropriate to describe the subsurface where the contaminant migrates through. A detailed aquifer characterisation is needed to delineate the preferential flow paths. Furthermore, hydraulic properties may vary over several decades over small distances in river aquifer sediments. Only a few, highly connective and conductive zones may dominate the overall groundwater flow regime. To compensate for the poor lateral resolution of sparsely spaced drillings geophysical cross-hole methods could be used to characterise the subsurface with high resolution. Combining data from borehole samples as direct measures with their excellent vertical resolution and “soft” high-resolution geophysical data, i.e. the tomographic data as indirect measurements, may overcome some major problems in accessing the subsurface environment to describe groundwater flow and transport of migrating solutes. In this paper we present a field experiment for identification of flow- and transport paths in a shallow unconsolidated aquifer. Cross-hole seismic tomography data together with hydraulic borehole data are combined to model a field tracer experiment carried out at the Belau test site in Northern Germany.

2016

Over a period of several years subsidence of pavement and infrastructure of a gas plant located on an island in the Middle East were observed. It is known that various areas of the industrial plant were built on highly weathered sandstone and weathered limestone. Furthermore, to achieve a plane working surface the area was initially flattened by a cut and fill procedure resulting in inhomogeneous backfilled areas. Thus, loosening zones and cavities have two different origins. Foundation settlement already occurred, which made a large scale exploration and risk assessment essential for the continued safe operation of the plant. A combination of ground-penetrating radar (GPR), electrical resistivity tomography (ERT) as well as surface wave seismic (MASW) were used to benefit from the individual resolution, depth penetration and advantages of the geophysical methods. Accompanying small trial pits were excavated, as well as drillings were performed and the subsequently explored cavities were inspected using downhole video. All data gathered were presented in a user-friendly GIS format, which is now also available for the client for future projects.

Jet grouting is a geotechnical method of ground improvement to increase shear strength and stiffness of soils. The method is typically used to construct in-situ geometries of grouted soil such as panels or columns. The diameter of grouted columns and its material strength depend on various process parameters and the subsurface soil properties. It is only vaguely possible to predict the final column diameter. Therefore, it is a general practice to excavate a test column and perform a visual examination. However, an excavation to control the in situ diameter is often impossible, especially under complex site conditions, such as a high ground water table. Therefore, as part of a research project, borehole seismic measurements (crosshole, downhole and tomography) were tested as a quality control to verify the extent of the column and to monitor the influence of the jet grout injection on the soil over time. The field surveys were conducted before and after the jet grouting process at different time intervals. The acquired seismic data show clear traveltime differences which allow the determination of the specific column depth and diameter. The tomogram measured in the natural soil and the tomograms of the measurements after the injection process were used to visualize the time dependent effects of the jet grout injection on the soil.

In seismic borehole tomography, the interpretation of the results is commonly limited to the comparison of the velocity map, the ray coverage, and the global root-mean-square RMS residual. However, the quality of the seismic data has a significant influence on the accuracy of the arrival time picking, but is generally not considered in the inversion. This paper presents an enhancement of the inversion taking into account the data quality, based on the signal-to-noise ratio, by using it to weight the traveltime residuals in each iteration step. This implementation also calculates the spatial distribution of the data quality and the distribution of the residual remaining at the end of the inversion, which are used to support the evaluation of a velocity map. The effect of the data weighting is studied on a field data set. Quality and residual maps are given and their relevance for the interpretation is discussed. The results indicate that areas of exceptionally high signal attenuation can be identified by means of the quality information.

The need for effective and reliable methods to survey and monitor the structure of earth-fill dams recently became pressing in light of the increasing number of flood events in central Europe. Among geophysical techniques, dam imaging using electrical resistivity methods is applied in most cases. Occasionally, ground-penetrating radar is applied in the framework of the search for subsurface facilities. Seismic methods are rarely used. This paper focuses on the multichannel analysis of the surface waves (MASW) method to determine dynamic soil properties and aims to extend its application field to dyke and dam structures. The standard processing procedure of the MASW assumes a flat free surface of infinite extension. The flat surfaces of a dyke, in contrast, are in the order of 1–10 times smaller than the wavelengths in the soil; disturbing side reflections will occur. Even though MASW has already been applied on a few dyke sites, the effect of such an obvious breach of preconditions needs to be studied before the method can be recommended. In this paper the influences of the dyke’s topography on the test results are studied by means of a numerical analysis. Typical cross-sections are modelled using 2.5D finite and boundary elements. The results of models taking the topography into account are compared with models neglecting the topography. The differences are evaluated on the level of the dispersion curves and for one crosssection on the level of the S-wave velocity. They were found to be insignificant for dykes with a width-to-height ratio larger than four. A testing campaign was conducted providing the chance to collect experience in the practical use of the MASW method on dykes. Test results obtained at two test sites are selected and compared to the results of borehole logs and cone penetration tests. A remarkable relation between the S-wave velocity and the consistency of the clay sealing was found at one site; a distinct positive correlation to the measured cone tip resistances was achieved on the other test site. Valuable information on the composition of the dyke body and base could be obtained but the resolution of the method to identify small areas of inhomogeneity should not be overestimated.