Shawn Biehler

Inversion modeling of gravity with prismatic mass bodies

A combined method for forward and inverse modeling of gravity data is presented. Based on the Fourier transform of Poisson’s equation, the forward modeling is suitable for observation points above, within, and below causative masses with any prescribed density distribution. The inversion is linearized in the spatial domain by superimposing numerous prismatic bodies, each having constant but different density, and fixed geometry. Our inversion algorithm adopts a sampling window to reduce memory storage and computations. Testing, with synthetic and field data, demonstrates that a successful inversion can be obtained from crudely estimated a priori density distributions and uncertainties. Lateral variations in density are well resolved but depth resolution often requires better constrained a priori information. Under various a priori conditions, our modeling indicates that sediment density tends to vary exponentially with depth in the San Jacinto basin, southern California.

The western margin of the Rio Grande Rift in northern New Mexico: An aborted boundary?

The northwestern margin of the Espanola basin, part of the Rio Grande rift in northern New Mexico, is characterized by a zone >17 km wide of oblique-slip faults that offset upper Paleozoic and Mesozoic strata of the eastern Colorado Plateau from Eocene and younger sedimentary rocks of the rift. Along this margin, a reasonably complete section of pre- and synrift Tertiary sediments is exposed. Combined interpretations of seismic reflection, seismic refraction, gravity, and geologic data acquired along a profile perpendicular to this boundary define the geometry of faulting, possible rotation of sedimentary units, and stratigraphy of rift fill. Vertical separation on the westernmost major fault, assumed to be the bounding fault between the rift and the Colorado Plateau, is Although Tertiary units are progressively faulted downward toward the axis of the rift, depth to inferred Precambrian crystalline rocks becomes shallower and the stratigraphic thickness of the intervening Paleozoic and Mesozoic units decreases toward the axis. We interpret pinching out of these units toward the east as erosional thinning on the western flanks of the Laramide-age Sangre de Cristo/Brazos geanticline, which underlay much of the present rift basin. Imprecise age constraints suggest that faulting of the rift margin began 10-7 Ma, but was not active after 7 Ma. Extension was apparently transferred to the Embudo fault zone, which remained active until at least 2.5 Ma and possibly into Quaternary time. The Embudo transfer zone effectively decoupled the Abiquiu embayment from the main Espanola basin. Thus the boundary at Abiquiu preserves an early stage in the formation of the rift boundary. The shift in activity may have resulted from a change in regional stress field, or from increasing magnitude of strain, or both. The change in locus of extension reflects a narrowing of rift basins through time and an integration of main bounding structures between adjacent basins. Although we are uncertain whether the Abiquiu region, which uniquely preserves an early stage of deformation, is representative of other areas of continental extension, our results indicate that the initial formation of rift basins may occur as high-angle, planar normal faults distributed over a broad zone. No evidence from seismic data or from rotation of beds exists to indicate that faults become listric with depth, which is compatible with the small amount of extension (3.5%) inferred at this boundary.

A geophysical model of the Española Basin, Rio Grande rift, New Mexico

A model of the subsurface structure of the eastern part of the Espanola Basin in the northern Rio Grande rift of New Mexico was constructed from geophysical data obtained since 1983 by the Summer of Applied Geophysical Experience (SAGE) field course. Approximately 742 new gravity observations, 1276 ground magnetic stations, 30 km of seismic refraction lines, 19 km of seismic reflection lines, 22 magnetotelluric stations, and several Schlumberger and dipole-dipole resistivity lines were established. Our studies provide new information on one boundary of a major continental rift and on the depositional and structural style of an extensional basin within the rift.Integration of these data sets into a single transect indicates that the Espanola Basin is asymmetrical with approximately 2 to 3 km of sediments and sedimentary rocks near the center, thinning eastward to the Precambrian outcrop of the flanking Sangre de Cristo uplift. Several minor faults with throws of less than 200 m were found, but no major eastern bounding fault was observed. Thus, the Espanola Basin could be an asymmetrical, west-dipping half-graben. However, major fault offset, down toward the basin axis, may occur within Precambrian rocks of the Sangre de Cristo uplift. In either case, the geometry of the basin does not agree well with current models for the structural evolution of continental rifts, which emphasize low-angle detachment faults which create asymmetrical, hinged half-grabens. These models predict that major shoulder uplift should occur adjacent to the side of the graben bounded by a listric master fault rather than adjacent to the hinged side. In contrast, for the Espanola Basin major uplift occurred adjacent to the eastern side, which could be the hinged side of the basin.A thick wedge of older sedimentary rocks with high P-wave velocity (4.4 km/s) and low electrical resistivity (5 Omega .m) was discovered under the younger Tertiary sediments and sedimentary rocks near the center of the basin. This wedge has maximum thickness of 1.2 km at the western end of the profile and thins eastward. The physical properties suggest this layer could be older Tertiary, or possibly a Mesozoic-Paleozoic, section of rocks. If the latter, it has potential economic importance because of the possible presence of a Cretaceous section which is known to produce oil and gas in the Albuquerque Basin to the south and the San Luis Basin to the north. However, based on data from the Yates La Mesa no. 2 well, 10 km south of the transect, this wedge is likely middle Tertiary lacustrine deposits (NMOCD, 1986). The great thickness of lake deposits may represent a major lacustrine facies of the Eocene Galisteo and El Rito formations, exposed around the southern, southwestern, and northwestern margins of the basin.Magnetotelluric data suggest the crystalline basement underlying the central Espanola Basin may be more conductive than near the eastern margin. The entire Espanola Basin is also underlain by a highly conductive layer of about 1 Omega .m at a depth of 15 km. Both the shallow and deep low-resistivity zones may result from hot, saline fluids. Such fluids deep within the crust may reduce the shear strength of the crust significantly and concentrate crustal extension on the west side of the rift.

Gravity investigation in the southeastern Mojave Desert, California

A gravity investigation consisting of some 900 stations was conducted over an area of approximately 3,500 km2 between the Eagle Mountains and the Colorado River in the southeastern Mojave Desert to define the major anomalies. The data were reduced to the complete Bouguer anomaly and presented on a contour map. The density contrasts that exist between the basin sedimentary rocks and the underlying basement rock produce gravity anomalies that can be readily delineated by gravity surveys. Several gravity anomalies are associated with regional structural trends, particularly faults. Two-dimensional model analyses along profiles across five major anomalies are presented to show the distribution and depth of sedimentary deposits. Subsurface geology in the area is poorly known. Analysis of the gravity anomalies, however, indicates that the basement beneath the sedimentary basins is highly faulted, consisting of complex structures and rapidly varying rock types similar to those in the mountain ranges of the area. In the center of the several subdivisions of the Chuckwalla Valley, depths to basement range from less than 200 m to more than 2 km. The Palen Valley is shallow in the northern section yet reaches a probable depth of over 1.5 km in its southern portion. The Palo Verde Valley east of the Chuckwalla Valley is divided into two separate structural styles. A northwest-trending sedimentary basin in the north has a maximum depth of at least 2 km, while basement to the south is generally more flat and less deep. The structural and geological information inferred from the gravity data and resultant anomalies provides important input toward a better understanding of the geology in the area.

A seismic refraction and reflection study across the central San Jacinto Basin, Southern California

The San Jacinto Basin is a northwest‐trending, pull‐apart basin in the San Jacinto fault zone of the San Andreas fault system in southern California. About 24 km long and 2 to 4 km wide, the basin sits on a graben bounded by two strands of the San Jacinto fault zone: the Claremont Fault on the northeast and the Casa Loma Fault on the southwest. We present a case study of shallow structure (less than 1 km) in the central basin. A 2.75-km refraction line running from the northeast to southwest across the regional structural trend reveals a groundwater barrier (Offset I). Another line, bent southward and continued for 1.65-km, shows a crystalline basement offset (Offset III) near an inferred trace of the Casa Loma Fault. Although a basement refractor was not observed along the 2.75-km line, a mismatch between the estimate of its minimum depth and the basement depth determined for the 1.65-km line suggests that an offset in the basement (greater than 260 m) exists around the junction of the two refraction lin...

SAGE: Learning geophysics by immersion

Combine the bold landscape of northern New Mexico (Figure 1) with a unique educational program that blends teaching and research as a partnership among universities, industry, and federal laboratories and you have SAGE (Summer of Applied Geophysical Experience). SAGE was conceived from a vision that something special would come from pooling the resources and talents of diverse groups. It enables undergraduate and graduate students from large and small schools alike to share the excitement of hands-on, modern field geophysical research and learning. Much more than a summer geophysics field camp, SAGE is an immersion in geophysics, an educational experience that many students say is the most satisfying in their lives. Students participate in every phase of the field program: They collect data with modern equipment; they process, model, and interpret the data with workstations and PCs; and they present their results in both oral and written form. The program is not just about using equipment; its about unde...

Faculty receives Excellence in Geophysical Education Award

“The second AGU Excellence in Geophysical Education Award was presented to the faculty of the Summer of Applied Geophysical Experience (SAGE): Scott Baldridge, Shawn Biehler, Larry Braile, John Ferguson, Bernard Gilpin, and George Jiracek. The persistence and commitment of this group has provided the geophysical community with a superb educational program for over 16 years, reaching nearly 400 students, including undergraduates, graduates, and professionals. The award was presented at the AGU Fall Meeting Honors Ceremony, which was held on December 8, 1998, in San Francisco, California.

SAGE celebrates 25 years of learning geophysics by doing geophysics

The increasing world demand and record-high costs for energy and mineral resources, along with the attendant environmental and climate concerns, have escalated the need for trained geophysicists to unprecedented levels. This is not only a national need; its a critical global need. As Earth scientists and educators we must seriously ask if our geophysics pipeline can adequately address this crisis. One program that has helped to answer this question in the affirmative for 25 years is SAGE (Summer of Applied Geophysical Experience). SAGE continues to develop with new faculty, new collaborations, and additional ways to support student participation during and after SAGE.