Carlos L. V. Aiken

Active fragmentation of Adria, the north African promontory, central Mediterranean orogen

Global Positioning System (GPS) velocities indicate that Adria no longer behaves as a rigid tectonic indenter into southern Europe and is divided into northwestern and southeastern velocity domains. Differential motions are recognized in a velocity field determined from the Peri-Tyrrhenian Geodetic Array (PTGA) and International GPS Service (IGS) sites in the circum-Tyrrhenian region of the central Mediterranean and published GPS velocities from the eastern Adriatic coast. In a fixed Eurasian reference frame, PTGA and IGS site velocities in Sicily and southern Italy are as much as 10 mm/yr in a northward direction, similar to GPS velocities along the eastern coast of the Adriatic Sea. In contrast, velocities in northern Italy are small or statistically insignificant and similar to velocities in Sardinia and Corsica outboard of western Adria. The tectonic boundary dividing Adria is seismically active and passes around the southern and eastern margins of the Tyrrhenian Basin, crosses central Italy, extends into the Adriatic Sea, and follows the western margin of the Dinaride tectonic belt north to the Gulf of Venice. The eastern margin of Adria is approximately located and follows the axis of the central Dinaric Alps southeast to the Hellenic arc. Southeastern Adria has a velocity related to northward motion of Africa, whereas northwestern Adria has negligible differential motion in the Eurasian frame and now is part of the Alpine collage of southern Europe.

Active displacement transfer and differential block motion within the central Walker Lane, western Great Basin

Velocities determined for 50 global positioning system sites within the central Walker Lane indicate differential motion among tectonic blocks forming a boundary zone between the Great Basin extensional province and the Sierra Nevada. The velocity field is related to displacement transfer from the Owens Valley and Furnace Creek fault systems of east- ern California to transtensional structures of the Walker Lane and extensional faults of the central Nevada seismic belt. Block boundaries are sharp and appear to be inherited from pre-Tertiary crustal structure. The block geometries exert strong influence on dif- ferential displacements concentrated along boundaries as belts of divergent, transcurrent, and convergent motion. The aggregate velocity accounts for about 25% of the relative motion between the Pacific and North American plates. About 5 mm/yr of the motion is localized along the eastern margin of the Sierra Nevada, whereas about 10 mm/yr is stepped 100 km east along a belt of east-northeast-trending transtensional faults that merge with northwest-trending transcurrent structures of the Walker Lane. About 6 mm/ yr of the velocity field is transferred to north-northeast-trending extensional faults of the central Nevada seismic belt. The heterogeneous distribution of motions is consistent with partitioning of a regional velocity field formed by westward extension and N408W-directed shear.

Phase‐field imaging: The electromagnetic equivalent of seismic migration

An alternative approach to the interpretation of electromagnetic (EM) data is to downward continue the observations to construct a two‐dimensional resistivity image of the medium that produced them. The downward continuation can be done in a straightforward manner by drawing a parallel between the EM and acoustic wave equations. With appropriate approximations and identification of specific terms that are equivalent and with transformation from an initial‐value problem to a boundary‐value problem, EM data can be imaged using any of the methods originally developed for migration of seismic data. The method we chose here is Claerbout’s one‐way, finite‐difference, wave‐equation migration. The algorithm is implemented in the frequency domain. Independent images of relative reflectivity are obtained by processing E and H pseudosections separately; the best estimate of the actual reflector position is the region of coincidence of the two images. The same approach can be used in reverse for modeling the EM respo...

Outcrop fracture characterization using terrestrial laser scanners: Deep-water Jackfork sandstone at Big Rock Quarry, Arkansas

Determination of fracture orientation can be an important aspect of structural analysis in reservoir characterization. The availability of ground-based laser scanner systems opens up new possibilities for the determination of fracture surface orientation in rock outcrops. Scanners are available in low-sample-density, low-accuracy, and fast, high-sample-density, high-accuracy models. These automatic laser scanner systems produce enormous volumes or “clouds” of point data at an instrument-dependent accuracy and resolution, which can be at the millimeter level. This huge volume of data calls for an automated and objective method of analysis. We have developed a surface classification algorithm based on a multipass partitioning of the point cloud. The method makes use of both spatial proximity and the orientation of an initial coarse-grained model of the point cloud. Unsupervised classification of surface sets is demonstrated herein using the new algorithm. Both previously mentioned types of scanners have been used to map the Jackfork sandstone outcrop at Big Rock Quarry in Little Rock, Arkansas. We apply the surface classification algorithm to these data to extract fracture surface orientations from the point cloud. The effectiveness of these new technologies when applied to fracture analysis is clearly demonstrated in this example. It is also shown that the low-density, low-resolution type of scanner is adequate to define general geomorphology but is inadequate for fracture definition. The surface classification algorithm can be used to reliably extract fracture and bedding strike and dip angles from the three-dimensional point locations acquired using centimeter-accurate, high-density laser scanner systems.

The 4-D microgravity method for waterflood surveillance; a model study for the Prudhoe Bay reservoir, Alaska

Forward and inverse gravity modeling is carried out on a suite of reservoir simulations of a proposed water injection in the Prudhoe Bay reservoir, Alaska. A novel surveillance technique is developed in which surface gravity observations are used to monitor the progress of a gas cap waterflood in the reservoir at 8200-ft (2500-m) depth. This cost‐effective method requires that high‐precision gravity surveys be repeated over periods of years. Differences in the gravity field with time reflect changes in the reservoir fluid densities. Preliminary field tests at Prudhoe Bay indicates survey accuracy of 5–10 μGal can be achieved for gravity data using a modified Lacoste & Romberg “G” type meter or Scintrex CG-3M combined with the NAVSTAR Global Positioning System (GPS). Forward gravity modeling predicts variations in surface measurements of 100 μGal after 5 years of water injection, and 180–250 μGal after 15 years. We use a constrained least‐squares method to invert synthetic gravity data for subsurface densi...

The 4D microgravity method for waterflood surveillance: Part II — Gravity measurements for the Prudhoe Bay reservoir, Alaska

Between 1994 and 2002, a series of experiments was conducted at Prudhoe Bay, Alaska, aimed at the development of an effective 4D (or time-lapse) gravity technique. Theoretical investigations had pointed out the potential for monitoring water injection in the 2500-m -deep reservoir, but it was not clear that gravity measurements of sufficient accuracy could be made in the arctic environment. During the course of these experiments, new techniques and instrumentation were introduced and perfected for both gravity and position measurements. Gravity stations are located using high-precision global positioning system (GPS) techniques without permanent monuments. Robust methods for meter drift control have improved noise resistance in relative gravimeter surveys. Absolute gravity measurements with a field-portable instrument maintain absolute gravity levels among surveys. A 4D gravity-difference noise of 12 μGal standard deviation has been established at Prudhoe Bay for GPS-controlled relative gravimeter surveys...

Creating virtual 3-D outcrop

Because of the high precision of present-day GPS and reflectorless laser technology, geologic information and remotely sensed data (i.e., seismic and GPR grids, wells) can be positioned accurately in 3-D and reconstructed as a virtual image. Hence, we have developed the “virtual outcrop” for applications that require knowledge about the 3-D spatial arrangements of rock types.

GRAVITY ANOMALY MAP OF NORTH AMERICA

The Gravity Anomaly Map of North America is the product of a 12-year international effort to compile, critically edit and merge gravity anomaly data on a continental and global scale. This color‐pixel map, printed on four quadrant sheets at a scale of 1:5 000 000 and including a fifth sheet showing a color index map with data references, is the first such map at this large scale to include several hundreds of thousands of precise surface data of the United States, Canada, Mexico and Central America as well as other high‐quality surface data from neighboring continental and oceanic areas. The map, which shows Bouguer gravity anomalies on land and free‐air gravity anomalies over oceans, is remarkable for its detail. Sixty‐six colors or shadings have been used in a carefully conceived nonlinear scheme to show anomalies at a 5 or 10 mGal interval over a dynamic range from about −300 mGal to +130 mGal.

Models of the Bouguer gravity and geologic structure at Yucca Flat, Nevada

The Bouguer gravity anomaly at Yucca Flat, Nevada has been modeled by two different techniques: the Cordell-Henderson and Parker-Oldenburg methods. The three-dimensional model has incorporated known density and structural information where possible. These models predict the structural relief on the Cenozoic-Paleozoic contact to within 150 m or about 15 percent of the actual depth.The three-dimensional Parker-Oldenburg method has been found to be efficient in an application involving a large (9000 sample) data base. Numerical stability was ensured by the application of a consistent regularization (a low-pass filter tuned to suppress the noise-dominated portion of the data spectrum) of the downward continuation operator. The use of a single regularizing filter for the entire model is not completely satisfactory due to the oversmoothing of shallow regions of the basin.The model is useful in the delineation of the geologic history of the area. Structural features in the model support the hypothesis that regional stress fields rotated significantly during the Tertiary. Major structural elements of the basin are well defined on the Cenozoic-Paleozoic interface. The principal basin-bounding fault is the large-throw Carpetbag fault on the west. This fault was most active during the earliest phases of subsidence. The Yucca fault is seen to be a much smaller feature in the model presented here. The basin is rotated down to the west, with normal hinge faults on the eastern margin.

Digital Geologic Mapping of the Ferron Sandstone, Muddy Creek, Utah, with GPS and Reflectorless Laser Rangefinders

The traditional approach to geologic mapping consists of sketching, taking orientation and thickness measurements with compass and tape, and noting positions of features on topographic maps or photos. These methods are time consuming, often difficult to realize in rough terrain, and poorly constrain lateral variations in sedimentary facies in relatively flat lying strata. We describe a case study that captures the three-dimensional architecture of sandstone bodies and key geological surfaces such as stratigraphic boundaries and faults using digital capture techniques. The Ferron sandstone in Utah is a superbly exposed ancient delta deposit that provides an improtant outcrop analog to fluvio-deltaic subsurface reservoirs. It has been the focus of many traditional outcrop studies, but here we use a methodology (“cybermapping”) based on GPS with offsets from a continuous ranging mode reflectorless laser rangefinder (“laser sketch”) for collection and analysis of basic stratigraphic and structural data in a relatively remote area. We also show hos this data can be analyzed and visualized in three dimensions. The study area was mapped in two days, which included hiking several kilometers into the area. One-the-fly and rapid static post processing of GPS surveying was used for positioning the reflectorness laser rangefinders; 60,000 points were acquired mapping sedimentological and structural features, terrain, and control points. The resultant quantitative 3D model of the geology and terrain allowed robust geometric visualization and analyses.