Jacob Verhoef

Magnetic interpretation using the 3-D analytic signal

A new method for magnetic interpretation has been developed based on the generalization of the analytic signal concept to three dimensions. The absolute value of the analytic signal is defined as the square root of the squared sum of the vertical and the two horizontal derivatives of the magnetic field. This signal exhibits maxima over magnetization contrasts, independent of the ambient magnetic field and source magnetization directions. Locations of these maxima thus determine the outlines of magnetic sources. Under the assumption that the anomalies are caused by vertical contacts, the analytic signal is used to estimate depth using a simple amplitude half-width rule. Two examples are shown of the application of the method. In the first example, the analytic signal highlights a circular feature beneath Lake Huron that has been identified as a possible impact crater. The second example illustrates the continuation of terranes across the Cabot Strait between Cape Breton and Newfoundland in eastern Canada.

Magnetic anomalies in the Canary Basin and the Mesozoic evolution of the central North Atlantic

The data from a recent magnetic compilation by Verhoefet al. (1991) off west Africa were used in combination with data in the western Atlantic to review the Mesozoic plate kinematic evolution of the central North Atlantic. The magnetic profile data were analyzed to identify the M-series sea floor spreading anomalies on the African plate. Oceanic fracture zones were identified from magnetic anomalies and seismic and gravity measurements. The identified sea floor spreading anomalies on the African plate were combined with those on the North American plate to calculate reconstruction poles for this part of the central Atlantic. The total separation poles derived in this paper describe a smooth curve, suggesting that the motion of the pole through time was continuous. Although the new sea floor spreading history differs only slightly from the one presented by Klitgord and Schouten (1986), it predicts smoother flowlines. On the other hand, the sea floor spreading history as depicted by the flowlines for the eastern central Atlantic deviates substantially from that of Sundvik and Larson (1988). A revised spreading history is also presented for the Cretaceous Magnetic Quiet Zone, where large changes in spreading direction occurred, that can not be resolved when fitting magnetic isochrons only, but which are evident from fracture zone traces and directions of sea floor spreading topography.

New database documents the magnetic character of the Arctic and North Atlantic

Magnetic observations of the Arctic and North Atlantic Oceans and adjacent land areas have been compiled in a new database that offers an unprecedented look at the magnetic anomalies over the continents and oceans of the study area. The most visible result of this compilation is the shaded relief map shown in Figure 1, which offers one of the most complete and coherent perspectives to date of the regions magnetic character. When combined with other types of observations, this information promises to shed new light on the breakup of the continents and the creation of the seafloor. Another product of the compilation is a grid of magnetic anomalies with wavelengths shorter than 400 km. Defined at regular intervals of 5 km, these grid values are well suited to quantitative tectonic investigations and to the automated production of accurate maps. Upon conclusion of the project in the first quarter of 1996, a digital version of the final grid will be released into the public domain for free and unrestricted use by investigators, along with full documentation and regional magnetic anomaly maps.

Magnetic imprints of continental accretion in the U.S.S.R

The evidence for these events has been studied extensively by Soviet investigators, who in recent years have applied the concepts of plate tectonics to the geological framework and evolution of the continental U.S.S.R. and summarized their findings in a series of tectonic reconstructions. The results of these studies have been synthesized, and their English publication “e.g., Khain, 1987; Zonenshain et al, 1990a,b” has provided Western investigators with a modern and comprehensive overview of the geological structure and history of the Soviet Union. Complementing these recent publications is a new digital grid of about 16 million points describing the magnetic anomaly field of the U.S.S.R. This data set was produced from a series of 18 maps, scale 1:2,500,000, published by the U.S.S.R. Ministry of Geology “Makarova, 1974” and digitized recently by the U.S. Naval Oceanographic Office in Bay St. Louis, Miss.

A compilation of magnetic observations from the Arctic and North Atlantic Oceans and adjacent land areas

Progress made in a new compilation of magnetic observations from the Arctic & North Atlantic Oceans and adjacent l and a reas , c a r r i e d o u t a t t h e A t l a n t i c Geoscience Centre of the Geological Survey of Canada, a l lows the p roduc t ion of a new preliminary map (see Figure). P rocedures app l i ed to p rocess the magnetic observations and incorporate them in to a da ta base a re b r ie f ly d i scussed . The new magnetic data base w i l l s e r v e a s a t o o l t o s t u d y p l a t e tec ton ic p rocesses in the nor the rn hemisphere.

Reply by the authors to N. L. Mohan

The purpose of our paper (Roest et al., 1992) was to present the generalization of the analytic signal (Nabighian, 1972) from two to three dimensions and illustrate its use in magnetic interpretation. The comments by Dr. Mohan can be separated into three categories.