Piotr Tucholka

Paleointensity of the geomagnetic field during the last 80,000 years

High-resolution records of the relative paleointensity of the geomagnetic field have been obtained from five marine cores. Three duplicate records were used to estimate the regional coherency of the data within a single area (Tyrrhrenian Sea) while the two others document the field variations in the eastern Mediterranean and the southern Indian Ocean. Careful investigations of distinct rock magnetic parameters have established the downcore uniformity of the sediments in terms of magnetic mineralogy and grain sizes. The time-depth control was provided by oxygen isotopes, and small-scale variations in the deposition rates were constrained by means of tephrachronology. The synthetic curve calculated from the Mediterranean records provides a continuous record of the intensity variations during the last 80,000 years (80 kyr), which correlates well with the sparse volcanic data available for the period 0–40 kyr. The fact that identical behavior is seen in both data sets and that they also compare quite well with results from a core collected in the Pacific Ocean establishes the truly dipolar character of these variations. The dipole field moment is characterized by large-scale changes as shown by the existence of pronounced drops (at 39 and 60 kyr) alternating with periods of higher intensity. The record suggests a periodic nature for these intensity variations; however, the period studied is not sufficiently long to state this conclusively. These results demonstrate the potential of sediments for such studies and constitute a first step towards obtaining a global paleointensity record over a long period of time.

The Blake geomagnetic event: transition geometry, dynamical characteristics and geomagnetic significance.

Abstract Two records of the Blake geomagnetic event have been obtained from marine cores in the Mediterranean. The upper bound for duration of the event, calculated from accurate oxygen isotope stratigraphy and/or tephrochronology, is ∼ 4000 years. One record is very detailed with 70 transitional directions and allows some characteristics of the event to be precisely retrieved. The directional changes do not occur at constant speed, rather a stop-and-go behaviour is observed as in detailed records of reversals. The virtual geomagnetic pole (VGP) path, confined to two bands of longitude over the Americas and antipodal to them, is virtually identical to the only other path of this event obtained from lacustrine sediments in the western USA, suggesting that a dipolar component may be present in the transitional field during the Blake event. The two bands of longitude coincide with those of many recent reversals, suggesting that the physical processes leading to reversals or events are basically the same and that the same mechanism is statistically observed over long periods of time.

Magnetic mineral variations of South Caspian Sea sediments at laminae scale

Abstract The 998-cm long sediment core was collected from the South basin of the Caspian Sea. Sediments from the lower, Late Pleistocene part of the core (998-90 cm) are marked by laminations, which rapidly disappear under oxidising conditions. TEM/EDS analyses of magnetic extracts, done at laminae scale, show the presence of iron monosulphide (greigite Fe 3 S 4 ) in grey and black laminae. There are also small amounts of magnetite. Magnetic hysteresis parameters Mrs/Ms and Bcr/Bc, presented on a Day plot, cluster near SD/PSD boundary, with smaller particles for black laminae. This tendency is due to a higher content of greigite, relative to coarser grained detrital magnetite in black laminae, and to a lower relative content of greigite in grey laminae. Analyses of Ca/Mg carbonates, grain size of the bulk sediment, feldspars/quartz ratios and water content show that these parameters do not vary at laminae scale. These results suggest that the laminae are of early diagenetic rather than detrital origin, reflecting rhythmic, subtle variations in the geochemical environment during laminae formation.

Ground-penetrating radar prospection over a gallery network resulting from neolithic flint mine (Borownia, Poland)

A Ground-penetrating Radar has been used to map a 100 m2 area suspected to be above a network of galleries remaining from flint prospection during the neolithic period. Twenty profiles have been acquired. Their interpretation, gives evidence for banded flint layers dipping to the North, and underground remaining galleries.

Hydrodynamic Parameters of a Sandy Soil Determined by Ground-Penetrating Radar Monitoring of Porchet Infiltrations

This study presents how Mualem-van Genuchten (M-vG) hydraulic parameters of a sandy soil can be obtained from ground-penetrating radar data acquired during water infiltration down a tube placed in a vertical hole. Water content profiles were simulated at various infiltration time steps using SWMS-2D based on initial values of the M-vG parameters and were converted to dielectric permittivity profiles using the Complex Refractive Index Model. The GprMax2D suite of programs was used to generate radargrams showing the electromagnetic reflections on the simulated wetting bulb. The shuffled complex evolution algorithm was used to estimate the M-vG parameters from GPR reflections measured during a constant head borehole infiltration. The method is presented on synthetic examples and on field experiments carried out in a sandy soil. The estimated parameters were compared with values obtained in laboratory on soil samples and with disk infiltrometer measurements.

Inverting surface GPR data to estimate wetting and drainage water retention curves in laboratory

In this study we present a laboratory experiment where we monitor volumetric water content in a 80-cm sand column using a Ground Penetrating Radar (GPR). Antennae were placed at the top of the sand column. The water table was set at different levels during drainage and wetting processes in order to exhibit hysteresis on the sand water retention function. We used the Mualem-van Genuchten model to fit retention curve data and model the hysteresis using different α parameters for drying and wetting processes.

Evaluating GroundPenetrating Radar use for water infiltration monitoring

Ground-Penetrating Radar (GPR) was used to monitor water infiltration in sand. Water was injected down an 81 cm long tubed hole with a piezometer recording the depth of water and a tap valve used to adjust it to 15 cm ± 2 cm above the bottom of the tube. During the 20 minutes of infiltration two GPR antennas (transmitter and receiver) were recording a trace every second from an offset position on the surface. The signal, enhanced by differential correction allows to trace the evolution of top and bottom limits of the water bulb in space and time. Comparison with hydrodynamic model of the infiltration process proves that the GPR reflections trace the wetting front and not the saturation bulb. An exact, quantified estimation of the evolution of the top border of the wetting zone is provided. For the bottom border, further work on the estimation of the velocity of radar wave in the infiltration bulb will be needed.