The magnetic method, perhaps the oldest of geophysical exploration techniques, blossomed after the advent of airborne surveys in World War II. With improvements in instrumentation, navigation, and platform compensation, it is now possible to map the entire crustal section at a variety of scales, from strongly magnetic basement at regional scale to weakly magnetic sedimentary contacts at local scale. Methods of data filtering, display, and interpretation have also advanced, especially with the availability of low-cost, high-performance personal computers and color raster graphics. The magnetic method is the primary exploration tool in the search for minerals. In other arenas, the magnetic method has evolved from its sole use for mapping basement structure to include a wide range of new applications, such as locating intrasedimentary faults, defining subtle lithologic contacts, mapping salt domes in weakly magnetic sediments, and better defining targets through 3D inversion. These new applications have increased the method’s utility in all realms of exploration — in the search for minerals, oil and gas, geothermal resources, and groundwater, and for a variety of other purposes such as natural hazards assessment, mapping impact structures, and engineering and environmental studies.

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The historical development of the magnetic method in exploration

Marine and terrestrial magnetic surveys have been integrated to study the tectonic structure of the convergent margin of Chile between 32°–34°S. Three magnetic domains have been identified: oceanic, continental margin, and subaerial. The oceanic domain has seafloor spreading anomalies (16 (∼37 Ma) to 18 (∼39.5 Ma)) disturbed by anomalies of the Juan Fernandez hot spot chain. In the continental margin, the most prominent fabric are E-W anomalies in the upper slope corresponding to onshore E-W anomalies of large intrusive bodies. Onshore, a N-S lineament of short-wavelength anomalies defines the roots of a Cretaceous volcanic arc. A resembling lineament offshore indicates a submerged older volcanic arc and that continental basement extends to ∼50 km landward of the trench. Absolute Cenozoic plate motion for Nazca and South American plates and dating of the Juan Fernandez chain provide a kinematic model of ridge-continent collision. The reconstruction indicates rapid southward migration of the collision point along ∼1400 km of the margin from 20 to 11 Ma (∼20 cm yr−1). From 11 Ma to present the collison point has migrated at a slower rate along ∼375 km of the margin (3.5 cm yr−1). The predicted location of the subducted portion of the Juan Fernandez chain coincides with the south edge of the southward migrating flat slab segment of the subducted lithosphere and with a cluster of deep earthquakes indicating a causal relationship. In the last ∼10 Myr the ridge has separated a sediment starved trench to the north where subduction erosion may dominate from a sediment filled trench to the south where recent sediment accretion dominates. These observations indicate that subduction of the Juan Fernandez chain plays a major role in arc-forearc tectonics.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2000JB900337

Magnetic anomaly interpretation across the southern central Andes (32°–34°S): The role of the Juan Fernández Ridge in the late Tertiary evolution of the margin

The Mesozoic opening history of the southern ocean between South America, Africa and Antarctica is one of the largest gaps in knowledge on the evolution of this region. Competing geodynamic models were published during the last two decades to explain the geophysical and geological observations. Here we report on aeromagnetic data collected along the East Antarctic coast during five seasons. These data provide new constraints on the timing and geometry of the early Gondwana break-up. In the Riiser-Larsen Sea/Mozambique Basin, the first oceanic crust between Africa and Antarctica formed around 155 Ma. In the west the Weddell Rift propagated from west to east with a velocity of 63 km/Myr between chrons M19N and M17N. At chron M14N South America and Africa finally were split off the Antarctic continent. Stretching in the area of the South Atlantic started at the latest from 155 Myr onwards. The different spreading velocities and directions of South America and Africa created at chron M9N the first oceanic crust in the South Atlantic. A new model indicates that the Karoo and Dronning Maud Land magmatism occurred well before any new ocean floor was created and, therefore, the first formation of new oceanic crust cannot directly be related to a plume interaction.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2002JB001802

Timing and geometry of early Gondwana breakup

The gravity method was the first geophysical technique to be used in oil and gas exploration. Despite being eclipsed by seismology, it has continued to be an important and sometimes crucial constraint in a number of exploration areas. In oil exploration the gravity method is particularly applicable in salt provinces, overthrust and foothills belts, underexplored basins, and targets of interest that underlie high-velocity zones. The gravity method is used frequently in mining applications to map subsurface geology and to directly calculate ore reserves for some massive sulfide orebodies. There is also a modest increase in the use of gravity techniques in specialized investigations for shallow targets. Gravimeters have undergone continuous improvement during the past 25 years, particularly in their ability to function in a dynamic environment. This and the advent of

Historical development of the gravity method in exploration

Potential‐field geophysical data observed at scattered discrete points in three dimensions can be interpolated (gridded, for example, onto a level surface) by relating the point data to a continuous function of equivalent discrete point sources. The function used here is the inverse‐distance Newtonian potential. The sources, located beneath some of the data points at a depth proportional to distance to the nearest neighboring data point, are determined iteratively. Areas of no data are filled by minimum curvature. For two‐dimensional (2-D) data (all data points at the same elevation), grids calculated by minimum curvature and by equivalent sources are similar, but the equivalent‐source method can be tuned to reduce aliasing. Gravity data in an area of high topographic relief in southwest U.S.A. were gridded by minimum curvature (a 2-D algorithm) and also by equivalent sources (3-D). The minimum‐curvature grid shows strong correlation with topography, as expected, because variation in gravity effect due to...

A scattered equivalent-source method for interpolation and gridding of potential-field data in three dimensions

An equivalent source algorithm is described for continuing either one‐ or two‐dimensional potential fields between arbitrary surfaces. In the two‐dimensional case, the dipole surface is approximated as a set of plane faces with constant moments over each face. In the one‐dimensional case, the plane faces of the dipole surface reduce to straight line segments. Application of the algorithm to model and field examples of aeromagnetic data shows the method to be effective and accurate even when the terrain has strong topographic relief and is composed of highly magnetic volcanic rocks.

R. O. Hansen

Papers

Continuation of potential fields between arbitrary surfaces