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Southern California Areal Mapping Project

 

SCAMP Geophysical Investigations

Three kinds of geophysical investigations currently are being conducted in cooperation with the Southern California Areal Mapping Project (SCAMP):
  • Regional gravity studies
  • Regional aeromagnetic studies
  • Seismic-imaging studies

Geophysical investigations are necessary to understand better the three-dimensional character of geologic materials and geologic structures.

Limitations of surface studies

At the Earth's surface, the geologic setting can be characterized well by geologic-mapping studies and related activities that describe the distribution of geologic materials and geologic structures across the length and breadth of the landscape. The surface studies permit reasonable interpretations about how geologic features extend into the subsurface, and surface geologic maps commonly are accompanied by cross-sectional diagrams that interpret how geologic units and structures might be distributed in the subsurface. However, the cross-sectional interpretations are limited (1) by the observational data available at the surface, (2) by the availability of information from subsurface boreholes, and (3) by the geologist's ability to use nearby geologic relations to constrain the interpolated subsurface position, thickness, and orientation of geologic units and structures along the cross section's transect.

Improving three-dimensional reconstructions

Geophysical studies can greatly improve three-dimensional reconstructions that seek to define the subsurface shape and distribution of geologic materials and structures. As described at a link to geophysical studies at the Fort Irwin Army Facility in the Mojave Desert, geophysical data (gravity measurements, magnetic measurements) can be assembled into maps showing variations in gravity-field strength and magnetic-field strength. Because these variations can be attributed to the density properties or magnetic properties of geologic materials, geophysical maps can be used to interpret the following subsurface geologic features:
  • The thickness of low-density sedimentary material that blankets high-density "basement rock" that forms the floor of the sedimentary basin;
  • The distribution of high-density or highly magnetic rock bodies;
  • The distribution and orientation of faults that form sharp boundaries between subsurface units;
  • The dip orientation of fault planes in the subsurface.

Seismic-imaging investigations use a sound source (small explosion, hammer strike, vibrations) to send sound energy into the subsurface. Because different geologic materials transmit sound energy at different velocities, the boundary zones (geologic contacts, faults) between different geologic materials will be zones where energy waves are refracted or reflected (echoed) back to the surface. There, a string of listening devices (geophones) stretched across the land surface records the echoes. When the numerous echo recordings are processed, synchronized, and analyzed, an image of the layered structure of geologic materials and their orientation along the line of the seismic transect can be developed. This technique also allows geologic structures (faults, folds) to be captured, as well as the depth to high-velocity "basement-rock" materials that lie beneath slow-velocity sedimentary materials.

Current SCAMP geophysical investigations

Currently, three geophysical investigations are ongoing within the SCAMP project area-all of them within or marginal to the San Bernardino Basin:
  • Seismic-imaging studies within the San Bernardino Basin. This study is being conducted by the USGS Central Region Geologic Hazards Team out of Denver, Colorado. This investigation is intended to develop seismic-velocity data to support ground-shaking models for the San Bernardino Basin (e.g., Frankel, 1993, 1994)

Frankel, Arthur, 1993, Three-dimensional simulations of ground motions in the San Bernardino Valley, California, for hypothetical earthquakes on the San Andreas fault: Bulletin of the Seismological Society of America, v. 83, p. 1020-1041.

Frankel, Arthur, 1994, Dense array recordings in the San Bernardino Valley of Landers-Big Bear aftershocks-Basin surface waves, Moho reflections, and three-dimensional simulations: Bulletin of the Seismological Society of America, v. 84, p. 613-624.

California Geological Survey (CGS) also has a geophysics unit that produces geophysics maps for southern California.

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This site last updated September 3, 2004 (ps)

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