Landslide deformations involve approximately all geological materials (natural rocks, soil, artificial fill, or combinations of these materials) and can occur and develop in a large variety of volumes and shapes. The characterization of the material inhomogeneities and their properties, the study of the deformation processes, and the delimitation of boundaries and potential slip surfaces are not simple goals. Since the ‘70s, the international community (mainly geophysicists and lower geologists and geological engineers) has begun to employ, together with other techniques, geophysical methods to characterize and monitor landslides. Both the associated advantages and limitations have been highlighted over the years, and some drawbacks are still open. This review is focused on works of the last twelve years (2007-2018), and the main goal is to analyse the geophysical community efforts toward overcoming the geophysical technique limitations highlighted in the 2007 geophysics and landslide review. To achieve this aim, contrary to previous reviews that analysed the advantages and limitations of each technique using a “technique approach,” the analysis was carried out using a “material landslide approach” on the basis of the more recent landslides classification.

/pdf/a-review-of-the-advantages-and-limitations-of-geophysical-2o2wrchk29.pdf

A Review of the Advantages and Limitations of Geophysical Investigations in Landslide Studies

Mesoscopic structure PFC∼2D model of soil rock mixture based on digital image

A three-dimensional gravity modelling of the Carpatho-Pannonian region was carried out to get a better image of the Moho boundary and the most prominent intra-crustal density heterogeneities. At first, only the major density boundaries were considered: the bottom of the Tertiary basin fill, the Moho discontinuity and the lithosphere to asthenosphere boundary. Density contrasts were represented by relative densities. The improved density model shows a transitional unit of high density at the base of the crust along the Teisseyre-Tornquist Zone. In the Western Carpathians, an extensive, relatively low-density unit was inferred in mid-crustal levels. The border zone between the Southern Carpathians and the Transylvanian basin is characterized by a sharp, step-like contact of the two crustal units. The Moho configuration reveals important information on the tectonic evolution of the region. Zones of continental collision are represented by thick Moho roots (Eastern Alps, Eastern Carpathians). Transpressional orogenic segments, however, are different: in the Western Carpathians, the Moho is a flat surface; in the Dinarides, a medium Moho root is observed; the Southern Carpathians are characterized by a thick crustal root. The differences are explained with the presence or absence of “subductible” oceanic crust along the Carpathians during the extrusion of Pannonian blocks.

Crustal structure in the Carpatho-Pannonian region: insights from three-dimensional gravity modelling and their geodynamic significance

Landslides are natural hazards that cause severe casualties and financial losses. There are various methods used to analyze landslides; among these, geotechnical and geophysical methods are widely used due to their accuracy and low cost, respectively. In this study, 2D electrical resistivity tomography (ERT) surveys, geotechnical, and field data are used to define the subsurface structure and the geometry of the Nargeschal landslide. To determine the most appropriate array, ERT measurements were performed by Wenner-alpha (Wa), Wenner-Schlumberger (WS), and dipole-dipole (DD) arrays. Furthermore, the relationship between electrical resistivity with change in the degree of saturation and landslide hydrology was investigated by completing multiple surveys at the same location, at different times. Landslide 3D geometry and hydrology were identified by ERT results. Moreover, geotechnical data was used to investigate the Nargeschal landslide and to interpret the ERT pseudo sections. The data from boreholes were used to constrain the depth and consequently the resistivity range that characterize the basal slip surface of the landslide in the ERT pseudo sections. The results of geotechnical investigations indicated that the landslide material had moisture content very close to the plastic limit, with a higher clay fraction and low shear strength at the slip surface. Finally, it must be noted that the simultaneous use of the ERT survey, geotechnical methods, and field investigations led to a complete and accurate characterization of the Nargeschal landslide.

Reconstruction of landslide model from ERT, geotechnical, and field data, Nargeschal landslide, Iran

Volume calculations based on a digital elevation model (DEM) are widely used instead of traditional handmade cross-sectioning methods on contour maps. Cross sections can be derived from a DEM performed in a computer, when the cross-sectioning method is preferred. Although many studies have been performed on the accuracy of DEM, accuracy of volume calculation based on a DEM is still unfamiliar to users. Therefore, in the concept of this study, accuracies of the DEM based volume calculations are tested on four selected theoretical surfaces for various data patterns. Volume calculations with triangular prisms, rectangular prisms, and cross sections are applied. Rational true errors are derived for the DEM based volume calculations.

Digital Elevation Model Based Volume Calculations using Topographical Data

The present work deals with an important theoretical problem of geodesy: we are looking for a mathematical dependency between two spatial coordinate systems utilizing common pairs of points whose coordinates are given in both systems. In geodesy and photogrammetry the most often used procedure to move from one coordinate system to the other is the 3D, 7 parameter (Helmert) transformation. Up to recent times this task was solved either by iteration, or by applying the Bursa–Wolf model. Producers of GPS/GNSS receivers install these algorithms in their systems to achieve a quick processing of data. But nowadays algebraic methods of mathematics give closed form solutions of this problem, which require high level computer technology background. In everyday usage, the closed form solutions are much more simple and have a higher precision than earlier procedures and thus it can be predicted that these new solutions will find their place in the practice. The paper discusses various methods for calculating the scale factor and it also compares solutions based on quaternion with those that are based on rotation matrix making use of skew-symmetric matrix.

/pdf/a-comparison-of-different-solutions-of-the-bursa-wolf-model-1aea756aw7.pdf

A comparison of different solutions of the Bursa–Wolf model and of the 3D, 7-parameter datum transformation

DTM-based surface and volume approximation: geophysical applications

We have studied a slowly moving loess landslide along the River Danube in South Hungary. In contrast with other efforts, we aimed to determine its fracture system. Due to the homogeneous composition of the loess, it seems to be the only possibility to get information about the landslide and its further evolution. Beside of the well-known Electrical Resistivity Tomography (ERT) the so-called Pressure Probe (PreP) method was applied to characterise the supposedly dense fracture system. This method was developed to detect and characterise mechanically weak zones, which may not visible from the surface, and may occur e.g. due to landslides. Fracture zones had been especially well localised by the ERT, enabling the prediction of the positions of future rupture surfaces and thus also the delineation of the endangered zones. PreP was able to give a very detailed image about the surface projection of the fractures. Both methods proved to be good to characterise the fracture system of such a landslide area. Geophysical predictions have been verified also in reality: the mass movements occurred about 1½ years after the measurements. Therefore, to provide early risk warnings and to avoid damage to constructions or endangering human life, the application of the ERT and PreP methods is highly recommended.

/pdf/characterisation-of-a-landslide-by-its-fracture-system-using-3x8ww95q28.pdf

Characterisation of a landslide by its fracture system using Electric Resistivity Tomography and Pressure Probe methods

Beyond rectangular prism polyhedron, as a discrete volume element, can also be used to model the density distribution inside 3D geological structures. The calculation of the closed formulae given for the gravitational potential and its higher-order derivatives, however, needs twice more runtime than that of the rectangular prism computations. Although the more detailed the better principle is generally accepted it is basically true only for errorless data. As soon as errors are present any forward gravitational calculation from the model is only a possible realization of the true force field on the significance level determined by the errors. So if one really considers the reliability of input data used in the calculations then sometimes the “less” can be equivalent to the “more” in statistical sense. As a consequence the processing time of the related complex formulae can be significantly reduced by the optimization of the number of volume elements based on the accuracy estimates of the input data. New algorithms are proposed to minimize the number of model elements defined both in local and in global coordinate systems. Common gravity field modelling programs generate optimized models for every computation points (dynamic approach), whereas the static approach provides only one optimized model for all. Based on the static approach two different algorithms were developed. The grid-based algorithm starts with the maximum resolution polyhedral model defined by 3–3 points of each grid cell and generates a new polyhedral surface defined by points selected from the grid. The other algorithm is more general; it works also for irregularly distributed data (scattered points) connected by triangulation. Beyond the description of the optimization schemes some applications of these algorithms in regional and local gravity field modelling are presented too. The efficiency of the static approaches may provide even more than 90% reduction in computation time in favourable situation without the loss of reliability of the calculated gravity field parameters.

/pdf/generalization-techniques-to-reduce-the-number-of-volume-2u5bgrolou.pdf

János Kalmár

Papers

A comparison of different solutions of the Bursa–Wolf model and of the 3D, 7-parameter datum transformation

DTM-based surface and volume approximation: geophysical applications

Characterisation of a landslide by its fracture system using Electric Resistivity Tomography and Pressure Probe methods

Generalization techniques to reduce the number of volume elements for terrain effect calculations in fully analytical gravitational modelling

Investigation of sediment compaction in the Pannonian basin using 3D gravity modelling