Edward W. Collins

Deformation of Permian strata overlying a zone of salt dissolution and collapse in the Texas Panhandle

Brittle deformation of strata overlying salt dissolution zones has been identified in Caprock Canyons State Park, Texas Panhandle. The geometry and distribution of the structures indicate that systematic regional joints that predated dissolution collapse influenced salt dissolution. Collapse has resulted in a sequence of deformations including normal faulting, reverse faulting, folding, and veining. The sequence of structural events suggests horizontal extension prior to major collapse. It is proposed that dissolution fronts are complex features with re-entrants controlled by the locations of joint zones.

Style of Faults and Associated Fractures in Austin Chalk, Northern Extension of the Balcones Fault Zone, Central Texas

ABSTRACT Distributions, geometries, and densities of faults and associated fractures in the Cretaceous Austin Chalk were studied in outcrop within the northernmost extension of the Balcones Fault Zone in Ellis and northern Hill Counties, Texas. Description of the fracture systems may be applicable to hydrocarbon exploration and production from this unit and to locating the proposed Dallas-Fort Worth Area Superconducting Super Collider site in Ellis County. Inactive normal faults with throws of less than 30 m compose this northernmost extension of the fault zone. Most of the major faults strike northeastward to east-northeastward, oblique to the regional strike of bedrock, and dip 45° to 80° northwest and southeast. Smaller faults strike northwestward. Calcite mineralization occurs along most of the fault planes, and slickensides indicate normal to slightly oblique slip. Developed within the fault zone are a series of narrow grabens that are 250 to 600 m wide and as much as 5.5 km long. In plan view these small grabens occur either as (1) depressed strips of bedrock bounded by subparallel hinge faults that increase in displacement to the northeast or as (2) irregularly subsided blocks bounded by double hinge faults that increase in displacement toward the central part of the trough and pinch out toward the northeast and southwest. Areas of high fracture density within the Austin Chalk occur adjacent to the major faults and within gently warped beds located where faults terminate along strike. Narrow zones characterized by different fracture styles and densities occur subparallel to the major structures. These zones vary in width and are composed of (1) complex minor faults (2 m wide), (2) minor faults and joints (15 to 90 m wide), or (3) abundant joints (60 to 140 m wide). Most minor faults have throws of less than 3 m and strike subparallel to an associated major fault or flexure, although some minor faults strike subperpendicular to major structural trends. Minor faults dip between 40° and 70° and are usually partly filled with calcite. Most systematic joints are nearly vertical and also strike subparallel to an associated major fault or flexure. Joints are only rarely filled with calcite, and abutting relationships suggest that minor faults predate joints. Average fracture spacing near faults or within flexures for 0.5- to 1-m-thick chalk beds generally ranges from 0.5 to 1.3 m. Locally joint densities may be as high as 10 joints/2 m. Minor faults and many joints cut through adjacent beds of different thicknesses.

Combining airborne electromagnetic induction and hydrochemistry to quantify salinity contributions to a large basin stream, Colorado River, Texas, USA

We combined multifrequency airborne electromagnetic induction (EM) measurements of apparent ground conductivity with chemical analyses of surface water to delineate natural and oilfield salinity sources that degrade surface water quality by elevating total dissolved solids, chloride and sulphate concentrations along several hundred kilometres of the Colorado River (western Texas, USA). To reduce the cost of airborne geophysical surveying over such large areas, we used a helicopter to tow an EM instrument at low altitude along the stream-axis and measure the apparent electrical conductivity of the ground at multiple frequencies, examined results in the field to identify salinized stream segments and optimal water sampling locations and then flew more detailed surveys over these limited areas rather than over the entire basin as is typical in salinization studies. Minimally processed stream-axis EM data (including apparent conductivities measured at single frequencies and multifrequency ‘spectrograms’ along the stream-axis) helped identify salinized streambed segments, discriminate between surface and subsurface sources of salinity and determine water sampling locations upstream and downstream from each segment. We integrated EM, streamflow and hydrochemical data to calculate salinity loads, identify specific natural and oilfield salinity sources and guide and implement remediation efforts. Stream-axis flight lines offer the advantage of rapidly acquiring high-resolution subsurface conductivity data along long stream segments where traditional gridded flight-line surveys and waterborne measurements are impractical or prohibitively expensive. They also overcome difficulties associated with topographic effects when surveying deeply incised streams. Such surveys provide valuable information on location, extent and type of salinization and can guide water sampling and more intensive ground or airborne measurements.

STREAMBED INDUCTION LOGS: AN AIRBORNE APPROACH TO IDENTIFYING SALINITY SOURCES AND QUANTIFYING SALINITY LOADS

We delineated natural and oil-field salinity sources that degrade water quality in the upper Colorado River (west Texas) and Petronila Creek (Texas coast) by combining multifrequency airborne EM measurements of apparent ground conductivity with chemical analyses of surface water at key stream locations. To reduce the cost of high-resolution airborne surveying over such large areas, we first flew along the stream axes and then examined preliminary results in the field to identify likely salinized stream segments. We then flew more detailed surveys over these areas rather than over the entire basin. Stream-axis EM data also helped identify water-sampling locations upstream and downstream from each salinized segment. We used these data to calculate salinity loads, discriminate among possible natural and oil-field salinity sources, and more effectively implement best-management practices to optimize remediation efforts. We acquired stream-axis airborne EM data along the upper Colorado River using a Geophex GEM-2A instrument operating at five frequencies between 450 Hz and 39 kHz. Increases in chloride, sulfate, and total dissolved solids loading in the upper Colorado River basin occur along eleven segments of elevated apparent conductivity identified from airborne EM data. Each segment encompasses areas of baseflow salinity contributions to the stream from natural dissolution of evaporite minerals in the Permian basin, from oil-field produced water, or both. Analyses of surface water confirm increases salinity loading associated with each segment. Airborne EM data acquired on the coast along Petronila Creek revealed three stream segments with elevated ground conductivity. Increases in chloride, sulfate, and total dissolved solids loading are attributed to shallow baseflow contributions. Using airborne EM and hydrochemistry data, we interpret the dominant salinization mechanism to be historic discharge of produced water into unlined drainage ditches and pits, infiltration into sandy Pleistocene channel deposits, lateral migration as far as several kilometers, and discharge into the stream.

APPLYING AIRBORNE ELECTROMAGNETIC INDUCTION IN GROUND- WATER SALINIZATION AND RESOURCE STUDIES, WEST TEXAS

In 2001, two high-resolution airbome geophysical surveys were flown in West Texas using Fugros MEGATEM II system to acquire time-domain EM (TDEM) and magnetic field data. One survey, flown at 100-m line spacing on the Edwards Plateau, identified and assessed groundwater salinization in an oil-field area. The second survey, flown at 400-m line spacing over a West Texas basin, identified favorable areas for groundwater exploration.

Fracture Zones between Overlapping En Echelon Fault Strands: Outcrop Analogs within the Balcones Fault Zone, Central Texas

ABSTRACT This study describes two types of fault overlap within the Balcones Fault Zone: a relay ramp between overlapping master faults dipping in the same direction and a structural bridge between overlapping faults dipping in opposite directions. Cretaceous limestone outcrops within these structural zones have been described for this study, and the outcrops reveal a variety of fracture characteristics, including fracture type, geometry, spacing, and connectivity, that are important in understanding the framework of fractured strata. Areas between overlapping normal faults contain abundant fractures and therefore are potential targets for hydrocarbons in fractured reservoirs, as well as potential areas for preferential ground-water recharge and flow in fractured aquifers. The fault overlap areas are up to 0.6 mi (1 km) wide, and the en echelon master faults may overlap by as much as 1.2 mi (2 km). Strata within these fault overlap zones are cut by joints and by abundant small-displacement normal faults, commonly having throws of less than 1.6 ft (0.5 m). These deformed areas consist of a mosaic of intermingled fracture sets that have multiple strikes; thus, fracture connectivity is locally high. Fracture spacing is variable within fault overlap areas. Some individual beds and multiple-bed packages are more fractured than other beds of similar thickness and composition. Also, fractures do not have uniform spacing within any given unit. Spacing of single small faults and fracture swarms is commonly between 6.5 and 150 ft (2 and 46 m) along traverses perpendicular and oblique to the master overlapping faults. Swarms of small faults, commonly as much as 20 ft (6 m) wide, may contain as many as 15 faults. Joint swarms that are as much as 40 ft (12 m) wide have fracture spacings of 2 to 5 ft (0.6 to 1.5 m).

Identifying Ground-water Resources and Intrabasinal Faults in the Hueco Bolson, West Texas, using Airborne Electromagnetic Induction and Magnetic-field Data

ABSTRACT We conducted a high-resolution airborne geophysical survey at the eastern margin of the basin-and-range province of the southwestern U.S. to examine the hydrostratigraphy of the Hueco Bolson and identify possible ground-water resources in this arid area. The survey, flown in 2001 over a 372-km2 area east of El Paso, Texas, acquired time-domain electromagnetic induction (TDEM) and magnetic field data. These data were used to map conductivity trends to depths of at least 200 m that are related to lateral and vertical changes in lithology, water content, water chemistry (EM data), basin geometry, and the location of intrabasinal faults (magnetic data). Pre-survey, ground-based TDEM soundings established the achievable exploration depths and demonstrated that the relatively deep groundwater (80 to 120 m) significantly influenced the transient signal and was within the exploration depth of the airborne system. Airborne EM and magnetic field data identified intrabasinal faults that influence basin-fill...

Characterization of Fractures in Limestones, Northern Segment of the Edwards Aquifer and Balcones Fault Zone, Central Texas

ABSTRACT Fracture distributions, orientations, and densities in Comanche Peak, Edwards, and Georgetown limestones (Edwards aquifer strata) were determined in conjunction with geological mapping near the San Gabriel River from Lake Georgetown to Weir, Texas, to increase our understanding of the geology of the Balcones Fault Zone and to provide data useful in identification of potential recharge areas and assessment of local ground-water flow. Cretaceous Comanche Peak, Edwards, Georgetown, Del Rio, Buda, Eagle Ford, and Austin strata dip gently (1) eastward and are overlain in some places by terrace deposits and alluvium. Several major normal faults, downthrown to the east, strike northward across the area. Gentle flexures, possibly related to faulting, parallel the faults. Minor normal faults and joints are most abundant in areas adjacent to major faults and flexures. These fractured-strata zones probably parallel the length of the faults or flexure axes and may be as wide as 1.6 km. Most minor faults strike 340 - 040°, have displacements less than 2 m, and dip from 40°-80° both eastward and westward. Slickensides on minor fault planes indicate slight oblique slip that could be related to rotation of fault blocks. Most joints strike 340 -020 and 260 - 300°, and fracture densities range from 4 joints/1 m to 1 joint/5 m in 1- to 2-m-thick beds. Fractures in the Comanche Peak, Edwards, and Georgetown limestones exhibit both similar and dissimilar characteristics important to ground-water flow. Apertures of fractures in the Comanche Peak and Georgetown limestones are generally less than 1 mm. whereas apertures in Edwards limestones can be several centimeters wide. Near-vertical joints and minor faults in Comanche Peak and Georgetown strata appear to be common only near major faults or flexures. Major joint sets in all three units have similar strikes, and many of the minor fault planes are filled with calcite. Joints do not have mineral fillings, and abutting relationships suggest minor faults formed before joints. Faults and joints in limestones of the northern part of the Edwards aquifer probably influence ground water in the same way they do in southern parts of the aquifer. The nonuniform distribution of fractures suggests that the hydrologic characteristics of the aquifer also are nonuniform. Highly fractured areas adjacent to faults should be more permeable than areas farther from faults. For example, springs in Georgetown City Park discharge Edwards aquifer water that may migrate upward along Fractures associated with a major fault 1 km west of the springs.

Airborne Geophysics, Remote Sensing, UAV (Drone)-based Surveys and Mining Geophysics

Knowledge of occurrence and extent of quick clay is vital regional for hazard zonation and in detail for infrastructure projects. Quick clay poses a serious geohazard in Scandinavia and Canada (amongst others) as it practically liquefies at failure and thus leads to serious, retrogressive slides. To this end, geotechnical drillings and samples are analyzed, to indicate sensitive clay in an area. As quick clay has a higher resistivity than marine clays, geophysical methods looking at resistivity distribution provide a valuable tool in addition to geotechnical assessment. So far, this has mostly focused on electrical resistivity tomography (ERT) for detecting these subtle changes. Supporting a recent road development project close to Oslo AEM was suggested in order to link drill sites and fill the data gaps between them. Quick clay is not easily identified in the AEM data, but some possible occurrences agree well with the results from drillings. Especially where the sediment layer is thick, variations in electrical resistivity within this layer can be resolved. These subtle changes can be linked to quick-clay extend by comparison with borehole data and ground-based geophysical methods (ERT and IP). We discuss results from the combination of these methods and outline the possibilities and limitations of quick-clay mapping using AEM.

Airborne lidar and near-surface geophysics: a new approach to discriminating Quaternary depositional units on the Texas coastal plain

Depositional units preserved on coastal plains worldwide are an important repository of information about large-scale climate change that has occurred during more than 20 Quaternary glacial-interglacial cycles. In general, the lateral and vertical complexity of these depositional units and their response to climatic and sea-level change are poorly understood, making it difficult to place historical and anticipated future climate and sea-level change in a natural geologic context. Mapping Quaternary siliciclastic depositional units on low-relief coastal plains historically has been based on aerial photographs. Accuracy and detail have been hindered, however, by lack of exposure and low relief. High-resolution airborne lidar surveys, along with surface and borehole geophysical measurements, are being used to identify lateral and vertical boundaries of stratigraphic units on the Texas Coastal Plain within upper Quaternary strata. Ground and borehole conductivity measurements discriminate sandy barrier island and fluvial and deltaic channel deposits from muddy floodplain, delta-plain, and estuarine deposits. Borehole conductivity and natural gamma logs similarly distinguish distinct depositional units in the subsurface and identify erosional unconformities and soil horizons that likely separate units deposited during different glacialinterglacial cycles. High-resolution digital elevation models obtained from airborne lidar surveys reveal previously unrecognized topographic detail that greatly aids mapping of subtle surface features such as sandy channels, interchannel deposits, and accretionary features on barrier islands that formed during the last interglacial period. An optimal mapping approach for coastal-plain environments employs (1) an initial lidar survey to produce a detailed elevation model; (2) selective surface sampling and geophysical measurements based on preliminary mapping derived from lidar data and aerial imagery; and (3) borehole sampling, logging, and analysis at key sites selected after lidar and surface measurements are complete.