Quantication of magnetic coercivity components by the analysis of acquisition curves of isothermal remanent magnetisation

A new method of analysing isothermal remanent magnetisation (IRM) acquisition curves based on cumulative log Gaussian analysis [Robertson and France, Phys. Earth Planet. Inter. 82 (1994) 223-234] is proposed. It is based on the curve fitting of the IRM acquisition curve versus the logarithm of the applied field with: (i) the acquisition curve on a linear scale, (ii) the acquisition curve expressed as a gradient, and (iii) the acquisition curve on a probability scale. Even when a sample is not saturated, its magnetic properties can be defined, although with less certainty. The number of magnetic components required for an optimal fit to a measured IRM acquisition curve is evaluated statistically. The method discriminates on the basis of different mineral coercivity. Therefore, additional rock-magnetic tests are still required to separate minerals with similar coercivities

Unravelling the magnetic record in marine and continental sediments : climatic versus geomagnetic signals

Magnetic iron oxides and iron sulfides are present as trace constituents in sediments. These magnetic particles can carry a natural remanent magnetisation (NRM). When the sediment consolidates and lithifies after deposition, the direction of the then ambient field is preserved. The amount of magnetic material is often related to climate. Moreover, the original magnetic material can be altered by diagenetic processes which are often linked to climate or the sedimentary environment. The NRM therefore contains information on the Earths magnetic field and on climate or sedimentary environment in the geologic past. The geomagnetic and the climatic signal, however, are often entangled. These signals need to be separated to be able to deduce information on either the geomagnetic field, or the climate and environment. For a correct interpretation of the NRM, its origin has to be known in detail, especially for variations on short geologic time scales of several hundreds to thousands of years. The aim of this thesis is twofold: first, we try to understand the acquisition of the NRM in various sedimentary environments, including the processes which might affect the magnetic signal (long) after deposition. Second, when the primary origin of the magnetic signal has been confirmed, the variations of the Earths magnetic field during the geologic past are studied. A new rock-magnetic method has been developed which analyses the magnetic mineral content of sediments and rocks. The magnetic minerals carry (part of) the NRM and therefore might shed a light on the origin of the NRM. This method is applied to a number of marine and continental sections to identify the magnetic remanence carriers and their contributions to the NRM. First, an Eastern Mediterranean organic rich layer, a so-called sapropel, is studied. The magnetic content is shown to be a combination of dust magnetite and hematite. Exclusively in the oxidised part of the sapropel, the presence of an additional magnetite fraction was demonstrated with very specific rock-magnetic characteristics - presumably magnetosomes. For a late Miocene section, consisting of red beds from a distal alluvial fan, the new method indicated that a recorded subchron (which was not identified before) is real and not a sedimentary artefact. In the Messinian alluvial/palustrine section of Librilla (SE Spain) the method demonstrated the presence of goethite in the alluvial lithology. The regular alternation of alluvial and palustrine lithologies could be linked to precession and therefore to climate. In the second part of the thesis, the variations of the geomagnetic field are investigated. The secular variation during the Permo-Carboniferous Reversed Superchron, as recorded by Permian red beds, appeared to be non-suppressed. This observation supports geodynamo models which regard a Superchron as a high-energy state of the geodynamo, rather than a low-energy state. The paleointensity of the last 276 kyr is studied in a marine core from near the Azores. The conventional normalisation paleointensity method is compared to the recently developed pseudo-Thellier method. Moreover, the climate influence on this core is investigated using fuzzy c-means analysis on rock-magnetic and geochemical parameters

Developing a model for the prediction of ground motions due to earthquakes in the Groningen gas field

AbstractMajor efforts are being undertaken to quantify seismic hazard and risk due to production-induced earthquakes in the Groningen gas field as the basis for rational decision-making about mitigation measures. An essential element is a model to estimate surface ground motions expected at any location for each earthquake originating within the gas reservoir. Taking advantage of the excellent geological and geophysical characterisation of the field and a growing database of ground-motion recordings, models have been developed for predicting response spectral accelerations, peak ground velocity and ground-motion durations for a wide range of magnitudes. The models reflect the unique source and travel path characteristics of the Groningen earthquakes, and account for the inevitable uncertainty in extrapolating from the small observed magnitudes to potential larger events. The predictions of ground-motion amplitudes include the effects of nonlinear site response of the relatively soft near-surface deposits throughout the field.</jats:p

Characterisation of ground motion recording stations in the Groningen gas field

The seismic hazard and risk analysis for the onshore Groningen gas field requires information about local soil properties, in particular shear-wave velocity (VS). A fieldwork campaign was conducted at 18 surface accelerograph stations of the monitoring network. The subsurface in the region consists of unconsolidated sediments and is heterogeneous in composition and properties. A range of different methods was applied to acquire in situ VS values to a target depth of at least 30 m. The techniques include seismic cone penetration tests (SCPT) with varying source offsets, multichannel analysis of surface waves (MASW) on Rayleigh waves with different processing approaches, microtremor array, cross-hole tomography and suspension P-S logging. The offset SCPT, cross-hole tomography and common midpoint cross-correlation (CMPcc) processing of MASW data all revealed lateral variations on length scales of several to tens of metres in this geological setting. SCPTs resulted in very detailed VS profiles with depth, but represent point measurements in a heterogeneous environment. The MASW results represent VS information on a larger spatial scale and smooth some of the heterogeneity encountered at the sites. The combination of MASW and SCPT proved to be a powerful and cost-effective approach in determining representative VS profiles at the accelerograph station sites. The measured VS profiles correspond well with the modelled profiles and they significantly enhance the ground motion model derivation. The similarity between the theoretical transfer function from the VS profile and the observed amplification from vertical array stations is also excellent

Ground-motion prediction models for induced earthquakes in the Groningen gas field, the Netherlands

Small-magnitude earthquakes induced by gas production in the Groningen field in the Netherlands have prompted the development of seismic risk models that serve both to estimate the impact of these events and to explore the efficacy of different risk mitigation strategies. A core element of the risk modelling is ground-motion prediction models (GMPM) derived from an extensive database of recordings obtained from a dense network of accelerographs installed in the field. For the verification of damage claims, an empirical GMPM for peak ground velocity (PGV) has been developed, which predicts horizontal PGV as a function of local magnitude, ML; hypocentral distance, Rhyp; and the time-averaged shear-wave velocity over the upper 30 m, VS30. For modelling the risk due to potential induced and triggered earthquakes of larger magnitude, a GMPM for response spectral accelerations has been developed from regressions on the outputs from finite-rupture simulations of motions at a deeply buried rock horizon. The GMPM for rock motions is coupled with a zonation map defining frequency-dependent non-linear amplification factors to obtain estimates of surface motions in the region of thick deposits of soft soils. The GMPM for spectral accelerations is formulated within a logic-tree framework to capture the epistemic uncertainty associated with extrapolation from recordings of events of ML ≤ 3.6 to much larger magnitudes

Ground-motion prediction models for induced earthquakes in the Groningen gas field, the Netherlands

Small-magnitude earthquakes induced by gas production in the Groningen field in the Netherlands have prompted the development of seismic risk models that serve both to estimate the impact of these events and to explore the efficacy of different risk mitigation strategies. A core element of the risk modelling is ground-motion prediction models (GMPM) derived from an extensive database of recordings obtained from a dense network of accelerographs installed in the field. For the verification of damage claims, an empirical GMPM for peak ground velocity (PGV) has been developed, which predicts horizontal PGV as a function of local magnitude, ML; hypocentral distance, Rhyp; and the time-averaged shear-wave velocity over the upper 30 m, VS30. For modelling the risk due to potential induced and triggered earthquakes of larger magnitude, a GMPM for response spectral accelerations has been developed from regressions on the outputs from finite-rupture simulations of motions at a deeply buried rock horizon. The GMPM for rock motions is coupled with a zonation map defining frequency-dependent non-linear amplification factors to obtain estimates of surface motions in the region of thick deposits of soft soils. The GMPM for spectral accelerations is formulated within a logic-tree framework to capture the epistemic uncertainty associated with extrapolation from recordings of events of ML ≤ 3.6 to much larger magnitudes

Geo-Electrical Detection of Impermeable Membranes in the Subsurface

In this paper, we have investigate looking ahead capabilities of electrical resistivity tomography (ERT) for a soil penetrating tool with electrodes. In our case study, the desired detection resolution (10 to 20 cm at a depth of at least 6 m) was much higher than what can be achieved from classical surface ERT measurements. Therefore, we designed a logging-type tool, that can be pushed into the ground. Our target was a buried PVC membrane which acts as an electrical insulator. In this phase, we performed numerical simulations and laboratory measurements. The methodology is based on a two-step approach. First, we calculate the background resistivity of the tool’s path is determined by inversion of near-looking electrode configurations. Next, the theoretical response (kernel) of the far-looking configurations is calculated for different membrane positions. The root mean square (RMS) error between the kernel and the measurements is minimized to detect the membrane. If the membrane is within sensing reach, the RMS has a minimum for the kernel corresponding to the true membrane position. If no minimum in RMS is found, the membrane is not within sensing reach and the tool can be pushed closer to the membrane. The laboratory tests comprised measurements in a tank filled with either water or saturated sand or saturated sand with clay slabs and chunks. The laboratory results were successful in pinpointing the position of the membrane with an accuracy of 10 to 20 cm, depending on the dimension of the tool and the distance from the membrane