Summary. The finite element method is a powerful tool for 3 D DC resistivity modelling and inversion. The solution accuracy and computational efficiency are cr

We investigate a simple scheme for full-waveform amplitude spectrum inversion of crosshole seismic data with an unknown source wavelet. The method is based on our 2D/2.5D finite-element method of acoustic-wave modeling. The normalized amplitude data, defined as the spectral ratio of the original trace amplitude to the average amplitude for the entire common shot gather, are used for full-waveform inversion in the frequency domain. In essence, the normalization of amplitudes removes the source wavelet and is easily carried out in the time domain or frequency domain from crosshole seismic surveying. Two synthetic models, simulating, respectively, a dipping fracture model and a complicated sedimentary structure, are examined with the inversion scheme. The numerical results show that clear images of the targets can be obtained by using the single- and multiple-frequency data. For comparison, two other amplitude inversion schemes in which the source signatures are known and estimated with an iterative procedure were carried out for the same models. The results show that the iteratively source-estimated procedure also produces satisfactory images of the velocity structure and yields an approximate amplitude of the source wavelet. With the multifrequency data, three such schemes yield very competitive results.

Repeated resistivity measurement is a potentially powerful method for monitoring development of internal erosion and anomalous seepage in earth embankment dams. This study is part of a project to improve current longterm monitoring routines and data interpretation and increasing the understanding when interpreting existing data. This is accomplished by modeling various occurrences typical of embankment structures using properties from two rockfill embankment dams with central till cores in the north of Sweden. The study evaluates the influence from 3D effects created by specific dam geometry and effects of water level fluctuations in the reservoir. Moreover, a comparison between different layout locations is carried out, and detectability of internal erosion scenarios is estimated through modeling of simulated damage situations. Software was especially developed to model apparent resistivity for geometries and material distributions for embankment dams. The model shows that the 3D effect from the embankment geometry is clearly significant when measuring along dam crests. For dams constructed with a conductive core of fine-grained soil and high-resistive rockfill, the effect becomes greatly enhanced. Also, water level fluctuations have a clear effect on apparent resistivities. Only small differences were found between the investigated arrays. A layout along the top of the crest is optimal for monitoring on existing dams, where intrusive investigations are normally avoided, because it is important to pass the current through the conductive core, which is often the main target of investigation. The investigation technique has proven beneficial for improving monitoring routines and increasing the understanding of results from the ongoing monitoring programs. Although the technique and software are developed for dam modeling, it could be used for estimation of 3D influence on any elongated structure with a 2D cross section.