Per-Ivar Olsson

Impact of Time-domain IP Pulse Length on Measured Data and Inverted Models

The duration of time domain (TD) induced polarization (IP) current injections has significant impact on the acquired IP data as well as on the inversion models, if the standard evaluation procedure is followed. However, it is still possible to retrieve similar inversion models if the waveform of the injected current and the IP response waveform are included in the inversion. The on-time also generally affects the signal-to-noise ratio (SNR) where an increased on-time gives higher SNR for the IP data.

Measuring Time Domain Spectral IP in the Ontime – Decreasing Acquisition Time and Increasing Signal-to-noise Ratio

Resistivity and time domain IP measurements using a 100% duty cycle square waveform current injection signal, and measuring the IP responses during current transmission, was modelled numerically and tested in the field against a traditional IP current injection with a 50% duty cycle waveform and measuring during the current off-time. The results show that the approach is practically applicable and can reduce the measuring time substantially as well as increase the signal-to-noise ratio by measuring the IP-response during the current on-time.

Data acquisition, processing and filtering for reliable 3D resistivity and time-domain induced polarisation tomography in an urban area: Field example of Vinsta, Stockholm

There is an increasing demand for trustworthy engineering geological conceptual models in urban areas due to an increasing trend in the underground infrastructure construction. Good-quality site investigations can reduce the risk of encountering unexpected geological conditions during construction. Geoelectrical measurements can be used as a tool for providing an overview of the site conditions and serve as a base for planning a geotechnical drilling program and for integration of the results. Geophysical surveys in urban environments may encounter problems due to strict logistical constraints and may be severely affected by electric and electromagnetic noise. Careful processing of the data is necessary to obtain a reliable estimation of the electrical properties of the ground, both electrical resistivity and chargeability. A large three-dimensional dataset was acquired in the suburban area of Stockholm (Sweden), with the aim of investigating a weak zone in the crystalline bedrock, which had been pointed out by prior geological and geotechnical surveys. Full waveforms of potential dipoles were recorded and processed for removing harmonic noise and background drift. Moreover, a statistical algorithm for handling the quality of the full-waveform shapes has been proposed. The goodness-of-fit test identifies full waveforms with noise that derives from direct current injections, caused by grounding spots of the adjacent metro line. The processed dataset is inverted for electrical resistivity and integral chargeability. The results image a large three-dimensional volume of the underground. The inverted distribution of geophysical quantities marks out the presence of a wide zone of weak rock, which was not identified by geotechnical probing in the site investigation but documented during the construction phase. Such zones can potentially cause severe problems during the construction of underground infrastructure.

Mapping geological structures in bedrock via large-scale direct current resistivity and time-domain induced polarization tomography

An investigation of geological conditions is always a key point for planning infrastructure constructions. Bedrock surface and rock quality must be estimated carefully in the designing process of infrastructures. A large direct-current resistivity and time-domain induced-polarization survey has been performed in Dalby, Lund Municipality, southern Sweden, with the aim of mapping lithological variations in bedrock. The geology at the site is characterised by Precambrian granitic gneisses and amphibolites, which are intensely deformed, fractured, and partly weathered. In addition, there are northwest-trending Permian dolerite dykes that are less deformed. Four 2D direct-current resistivity and time-domain induced-polarization profiles of about 1-km length have been carefully pre-processed to retrieve time-domain induced polarization responses and inverted to obtain the direct-current resistivity distribution of the subsoil and the phase of the complex conductivity using a constant-phase angle model. The joint interpretation of electrical resistivity and induced-polarization models leads to a better understanding of complex three-dimensional subsoil geometries. The results have been validated by lithological descriptions from several drillings. In addition, direct-current resistivity and time-domain induced-polarization logging has been carried out in two different boreholes, showing a good match with the results of the surface direct-current resistivity and time-domain induced-polarization profiles. The direct-current resistivity and time-domain induced-polarization methodology proved to be a suitable technique for extensively mapping weathered zones with poor geotechnical characteristics and tectonic structures, which can lead to severe problems for infrastructure construction and/or constitute risk zones for aquifer contamination.

Geophysical site investigation at Dalby-Önneslov using joint inversion

A geophysical site investigation using ERT and refraction seismic was done to detect the bedrock interface and possible weakness zones. For the purpose of a more realistic subsurface model a joint inversion was done.

Mapping Bedrock Lithology in Urban Environment via Large Scale Time Domain Induced Polarization Tomography

The present work is an example of a Direct-Current resistivity and time-domain Induced Polarization (DCIP) survey, carried out in an urban environment. The main goal of the survey is to map a weak zone in the bedrock, through which a tunnel will be constructed. Geophysical measurements in urban environments can be very complicated due to the restrictions of available space and the high levels of noise. A way to deal with the latter is careful processing of the recorded waveforms, if they are sampled at a reasonably high frequency (sampling >= 1 KHz). A deeper understanding of raw data leads to a more accurate use of the measured quantities, improving the reliability of electrical resistivity and IP estimations.

BeFo 382 - Detailed Comparison Study of 3D-characterized Rock Mass and Geophysical Models

This paper presents a study where resistivity and integral chargeability three-dimensional (3D) models, representing a volume in the vicinity of a rock quarry is compared to a quasi-3D characterization of rock mass volume. By repeating geological mapping, i.e. with unmanned aerial vehicles, ground based high resolution photography and sampling, as the quarry operation reaches further into the rock volume mapped with geophysics, we retrieve a semi-continuous geological characterization of the same rock mass volume characterized by the geophysical investigations. This enables a detailed comparison between geophysical results and rock volume, where factors such as fracture and weathering zones or rock type is being considered.

Doubling the Spectrum of Time-domain Induced Polarization by Harmonic De-noising, Drift/Spike Removal and Tapered Gating

The extraction of spectral information in the inversion process of time-domain (TD) induced polarization (IP) data is changing the use of the TDIP method. Data interpretation is evolving from a qualitative description of the subsurface, able only to discriminate the presence of contrasts in chargeability parameters, towards a quantitative analysis of the investigated media, which allows for detailed soil- and rock-type characterization. Two major limitations restrict the extraction of the spectral information of TDIP data in the field: i) the difficulty of acquiring reliable early-time measurements, and ii) the self-potential drift in the measured potentials distorting the shape of the late time. For measuring at early times, we developed a new method for removing harmonic noise from the data. Furthermore, a new scheme for spike removal was developed and tapered windows are used in the data gating. For measuring at late times, we developed a drift-removal scheme that model the polarization effect and allows for preserving the shape of the IP responses at late times. Overall, the removal of harmonic noise, spikes, self-potential drift, tapered windowing, and the uncertainty estimation allows for doubling the usable range of TDIP data to almost four decades in time.

Resistivity inversion software comparison

The application of geophysics may assist in solving environmental, geotechnical and exploration problems. One commonly used method in such applications is electrical resistivity tomography (ERT). In order to interpret the results, inversion is needed to create models estimating distribution of resistivity in the ground. Several inversion programs are available for geophysicists to use. It is therefore important to investigate if different softwares give similar results for a given data set. In this paper, three different inversion softwares have been compared through inversion of the forward response of the same synthetic model. The programs used are Res2Dinv, Aarhusinv and BERT. Two synthetic forward models have been used. As expected, Res2Dinv generally tends to exaggerate structures vertically and Aarhusinv to exaggerate them vertically. BERT seems to be somewhere in between, but does not seem to resolve sharp features in the synthetic forward model, resulting in rounded anomalies. A square-like feature close to the ground surface in the forward model was well resolved by all inversion software. However, it was not possible to resolve inclined blocks in any of the inversions. Possible explanations might be poor data coverage at deeper depths in the model and model discretization that cannot resolve inclined geometries. (Less)

A Comparison of DCIP Inversion Software

Electrical resistivity tomography (ERT) is a commonly used geophysical method, which can conveniently be combined with measuring time-domain induced polarization. The combined measurement can be termed DCIP, and can be employed in a wide range of situations such as pre-investigations for construction, or contaminant monitoring to name a few. An integral part of a DCIP survey is the software used for processing and interpretation. There exists many such software packages. We compare the ubiquitous, commercial software, Res2DInv and two other common alternatives; the semi-academic AarhusInv and the academic, open source, BERT/GIMLi. The comparison is done by simulating measurements of resistivity and IP with a multiple gradient protocol over a model that represents a waste pond setting. The results show that there are quite a few similarities between the different softwares, but also some notable differences that one should be aware of during interpretation. This work is the first step towards an exhaustive comparison.