F.W. Jones

An analogue model study of electromagnetic induction for island-continent ocean channels

Abstract The behaviour of time-varying electromagnetic fields near an island situated in a shallow ocean with a nearby continent is investigated using a scaled analogue model. To study the effect of the proximity of the continent, various island-continent distances are treated. The presence of the continent tends to augment enhancements of the field components at the island coastlines for all channel widths studied, while the island affects the enhancements of the fields over the continental coastline only for very narrow channel widths (half the island width or less), and does not affect the horizontal to vertical magnetic field ratio at the coastline at all. To examine the effect of the shape of the island, square and circular island models are used. For the frequencies studied, the island shape has little effect on the fields over the continental coastline, but over the island, the spatial variation of the fields is considerably less for the circular island than for the square island.

Induction in laterally non-uniform conductors: Theory and numerical models

Abstract The study of electromagnetic induction in laterally non-uniform conductors is briefly reviewed. The two-dimensional perturbation problem is considered and the two polarization cases which arise from Maxwells equations are discussed. Techniques for the solution of the equations for laterally non-uniform conductors are discussed with emphasis on numerical methods.

A comparison of numerical, analogue model, and field-station vertical magnetic-fields for the Vancouver Island region

Abstract Vertical magnetic fields for a three-dimensional numerical model, for a laboratory-analogue model, and from field stations for the Vancouver Island region of British Columbia, Canada, are compared. The numerical results are obtained using a three-dimensional finite-difference numerical technique employing a 25 × 25 × 25 mesh of grid points for a simplified mathematical model of the Vancouver Island region. The calculations are carried out for a source frequency of 0.004 Hz. The analogue model results for four traverses over the Island model and the field station values (obtained from transfer function analysis) for ten locations are those discussed previously by Nienaber et al. (1979a, b). General agreement exists between the numerical, analogue, and field station data, and comparison of results between these methods is important in three-dimensional electromagnetic induction studies of complex geomagnetic induction problems.

Numerical and analogue model results for electromagnetic induction for an island situated near a coastline

Abstract The behaviour of time-varying electromagnetic fields near an island situated in a shallow ocean is investigated using both a three-dimensional finite-difference numerical method and a scaled analogue model method. The effect of a coastline located at some distance from the island is included in the study. The numerical model results and the scale model results are compared for various traverses across the island. The results indicate a high degree of compatibility between the two methods for studying problems involving three-dimensional conductivity structures.

A magneto-telluric deep crustal study in south-central Portugal

SummaryA geothermal anomaly reported to exist in southern Portugal and with heat flow density values as high as 160 mW/m2 was studied by the magneto-telluric method. The results of the magneto-telluric survey indicate that the study area is divided into several high electrical resistivity, deep-rooted blocks separated by low electrical resistivity zones. These latter zones coincide with the main trends of the faults that cross the region. The high heat flow density values reported for the area are incompatible with the high electrical resistivities observed in the magneto-telluric survey. Furthermore, heat production values calculated for some of the rocks that crop out in the area of the geothermal anomaly cannot explain the high heat flow density valuss. Since no hydrothermal activity is known for the region and recent volcanism is absent, it is suggested that previous heat flow density calculations for the area covered by the magnetotelluric survey are overestimated.

An analogue model study of electromagnetic induction for island-continent ocean channels for the H polarization☆

Abstract Using an electromagnetic analogue model, the behavior of time-varying electromagnetic fields for an island near a continental coastline is examined for the H-polarization case, in which the electric field of the inducing source is perpendicular to the continental coastline. A study of the effect of the shape of the island, using square- and circular-island models, indicates that over the island, anomalies in the field components are confined to a smaller area for the circular island than for the square island. A study of the effect of ocean-channel width between the island and continent shows that, as the channel width decreases, anomalies in the magnetic field components over the island decrease. The anomaly in the electric field at the continental coastline first increases and then decreases with decreasing channel width.

The calculation of perturbation and induction arrows for a three-dimensional conductivity model and dipole source fields

A numerical method is used to calculate the electromagnetic fields associated with a three-dimensional conductivity anomaly. The source field is due to horizontal magnetic dipoles placed at two different positions with respect to the conductivity anomaly. The transfer functions and related perturbation and induction arrows associated with the fields are calculated and compared with the arrows obtained from a uniform source calculation. The results show the source effect on the induction arrows and indicate that the perturbation arrows provide a method of outlining the spatial extent of the anomaly. The transfer function calculations are made for both exact and approximate normal fields. In the transfer function calculation the anomalous fields are correlated with a normal field as suggested by Schmucker (1970) and Cochrane and Hyndman (1970).

Preliminary results of a magneto-telluric survey over a geothermal anomaly in Portugal

Abstract During summer 1990 a magneto-telluric (MT) survey was conducted over a geothermal anomaly discovered a few years ago in south central Portugal. This anomaly, known as the Alentejo Geothermal Anomaly (AGA), exhibits heat flow density values (greater than 160 mW m2) at its centre. The area covered by the MT survey was about 2500 km2, and 34 MT sites were occupied. The preliminary results indicate that the central area of the AGA coincides with a region of high electrical resistivity which continues to depths greater than 10 000 m. As no intense hydrothermal manifestations are known in the study area, it is suggested that the heat flow density anomaly is the result of high heat production in some of the geological formations that outcrop in the region. Two major faults cross the area, intersecting at approximately a right angle. Low-resistivity zones appear to be associated with the fault zones, and a possible interpretation is that these conductive zones may be related to fluids contained in fractures surrounding the faults.

The geothermal regime of the northern Yukon and Mackenzie delta regions of northwest Canada — studies of two regional profiles

Temperature data from deep petroleum exploration wells and thermal conductivity estimates based on net rock analysis data have been used to make terrestrial heat flow estimates along two profiles across the sedimentary strata of the Mackenzie Delta, northern Yukon, and offshore Beaufort Sea regions.Both profiles exhibit low heat flow values that range from 34 mWm−2 to 58 mWm−2, and little change occurs over large distances in the continental part of the area. Low heat flow values (<40 mWm−2) occur in the Beaufort-Mackenzie Basin and Rapid Depression, both of which are areas of thick successions of Cretacecus and Tertiary clastic sedimentary strata. High heat flow values of almost 80 mWm−2 occur to the south in the Taiga Nahoni Foldbelt and values as high as 60 mWm−2 are indicated along the Aklavik Arch Complex, northeast of Aklavik.The regional variations of effective thermal conductivity are insufficient to account for the heat flow variations along the profiles, and so these may indicate deep radiogenic or other heat sources.

The perturbation of alternating geomagnetic fields by discontinuities with high conductivity contrasts

Abstract The perturbation of alternating geomagnetic fields by conductivity discontinuities is considered. A numerical method is used to solve the two-dimensional induction problem. Models in which the conductivity contrast between conductive regions is great, such as between oceanic and continental regions, are considered. The perturbation field distributions for models with higher conductivity contrasts are compared with lower conductivity contrast models using a method recently developed by Jones (1972). Total field solutions are obtained for several models by using a very large grid as well as a folding-in technique. Results from the two methods for the various models are compared. It is found that the folding-in technique offers an alternative method for handling higher conductivity contrasts.