John A. Dunbar

A case study of stratigraphic interpretation using shear and compressional seismic data

This paper describes the use of surface recorded compressional and horizontal shear wave seismic data to detect lateral changes in the physical properties of a clastic unit. Shear and compressional wave transit times were measured across a selected interval from CDP stacked sections derived from data collected along coincident shear and compressional seismic lines. At each surface position the ratio of the shear to compressional transit time across the target horizon is calculated. It is shown that lateral variations in this ratio, coupled with the behavior of the individual transit time curves, can be used to infer changes in the physical properties of a formation. The horizon selected for this case study was the lower Pennsylvanian Morrow formation which produces gas from channel sand bodies at the Empire Abo field, New Mexico. A detailed geologic section of the producing horizon was mapped along a seismic line oriented so that it crossed productive and nonproductive regions of the field. Shear and comp...

Palinspastic reconstruction of structure maps: an automated finite element approach with heterogeneous strain

Existing methods for the palinspastic reconstruction of structure maps do not adequately account for heterogeneous rock strain and hence cannot accurately treat features such as fault terminations and non-cylindrical folds. We propose a new finite element formulation of the map reconstruction problem that treats such features explicitly. In this approach, a model of the map surface, with internal openings that honor the topology of the fault-gap network, is constructed of triangular finite elements. Both model building and reconstruction algorithms are guided by rules relating fault-gap topology to the kinematics of fault motion and are fully automated. We represent the total strain as the sum of a prescribed component of locally homogeneous simple shear and a minimum amount of heterogeneous residual strain. The region within which a particular orientation of simple shear is treated as homogenous can be as small as an individual element or as large as the entire map. For residual strain calculations, we treat the map surface as a hyperelastic membrane. A globally optimum reconstruction is found that unfolds the map while faithfully honoring assigned strain mechanisms, closes fault gaps without overlap or gap and imparts the least possible residual strain in the restored surface. The amount and distribution of the residual strain serves as a diagnostic tool for identifying mapping errors. The method can be used to reconstruct maps offset by any number of faults that terminate, branch and offset each other in arbitrarily complex ways.

An evaluation of the electrical-resistivity method for water-reservoir salinity studies

An important potential application of the electrical-resistivity method occurs in salinity studies of lakes and water reservoirs. However, uncertainty exists because there is a problem of equivalence in resistivity data interpretation and because the resistivity variation in the water layer in reservoirs is subtle (contrasts of a factor of two) and changes over short intervals of time (typically in hours or days). We carry out numerical modeling and inversion of synthetic data and field resistivity data from Lake Whitney, Texas, U.S.A., to examine how accurately resistivity in the water column in reservoirs can be determined using the electrical-resistivity method. Our objective is to advance the method as a tool in limnologic research for mapping freshwater zones in impacted lakes and water reservoirs. The simulated freshwater target in the synthetic data effectively is realized from the inversion with root-mean-square (rms) error less than 10%. However, the resolutionof the inverted sections decreases w...

Streambank Erosion and Best Management Practice Simulation using SWAT

Streambank erosion can be a significant portion of overall erosion dynamics in the watershed. In the current research the erosion dynamics of Cedar Creek watershed located in North-Central Texas has been studied. Cedar Creek Reservoir is a major water supply reservoir with a storage capacity of over 679,200 ac. feet. It is one among the five major drinking water reservoirs owned and operated by Tarrant Regional Water District that supplies raw water to approximately 1.6 million people. Sediment and nutrient loading into the reservoir has been a major source for algae blooms that threatens the drinking water quality. Hence, as a precursor to developing a TMDL, a pollutant source assessment study was undertaken to identify nutrient and sediment hot spots within the watershed area. Lake sedimentation survey, field/stream channel survey, historical analysis using aerial photographs and modeling the basin with the SWAT model were conducted to study the erosion dynamics in the watershed. The field survey and model study showed that streambank erosion contributes up to 30% of total annual sediment load into the lake. Channel best management practice simulation using the calibrated model show that a comprehensive/holistic approach is needed to reduce streambank erosion.

Binning Method for Mapping Irregularly Distributed Continuous Resistivity Profiling Data onto a Regular Grid for 3-D Inversion

Continuous resistivity profiling (CRP) is an emerging direct current (DC) resistivity method that is increasingly used in shallow fresh water and marine environments to support hydrogeological and near-surface geophysical studies. CRP is popular because it allows efficient data acquisition. However, it is generally not possible to precisely control the spacing and orientation of profiles during CRP data acquisition, resulting in irregularly distributed data. For 2-D profiles, the irregular distribution is handled by extracting relatively straight segments for inversion and using either interpolation or irregular element widths in the inversion process to account for unevenly spaced data along the profiles. Three-dimensional (3-D) inversion of CRP data sets is more challenging because most commercial inversion codes are applicable only to parallel or sub-parallel survey lines, requiring data on a regular grid. In this paper, we develop a technique for binning irregularly distributed CRP data onto a regular grid, suitable for most 3-D inversion codes. We evaluate the method on synthetic data sets, as well as field CRP data sets collected from two water reservoirs on Maui Island, Hawaii. Numerical results of the synthetic data inversion show that 20% and 10% model misfit between a ‘‘control group’’ and binned data can be achieved with data densities of 5 and 15 measurements in each bin, respectively. Also, the 3-D inverted resistivity volumes of the binned field CRP data sets show good agreement with independently collected 2-D profiles, resolving the same subsurface features.

Characterizing a Shallow Groundwater System beneath Irrigated Sugarcane with Electrical Resistivity and Radon (222Rn), Puunene, Hawaii

ABSTRACT In this study, we use a combination of electrical resistivity profiling and radon (222Rn) measurements to characterize a shallow groundwater system beneath the last remaining, large-scale sugarcane plantation on Maui, Hawaii. Hawaiian Commercial & Sugar Company has continuously operated a sugarcane plantation on the western flank of Haleakala Volcano since 1878. The sugarcane is irrigated with a combination of surface water brought through tunnels from the wetter, eastern flank of Haleakala Volcano and groundwater from wells within the plantation. To assess the flow of irrigation water through the shallow subsurface, we collected a representative topo-sequence of four 2-D resistivity profiles that sample different topographic and hydrologic settings within the plantation. The profiles show a down-slope-thickening (0 to 20 m), surficial low-resistivity (10-200 Ohm-m) layer extending from the upslope limit of irrigated sugarcane to the lowest elevations of the plantation. At a canal crossing, the l...

Color‐encoding multifrequency acoustic data for near‐bottom studies

In this paper we describe color encoding methods for combining acoustic data in widely separate spectral bands into composite displays. These new display methods better image the near‐bottom is interval in fine‐grained depositional environments. In fine‐grained depositional environments the water bottom is typically not a discrete surface but is a transition zone spanning 1–3 m. This near‐bottom interval is not completely imaged by either fathometers or subbottom profilers alone. Multiple acoustic frequencies spanning both the fathometer and subbottom bands are needed for applications in which the details of the near bottom are critical. Such applications include surveys of water reservoirs, scour around bridge piers, and predredging surveys in harbors. To make effective use of the multifrequency data, we combine three or more signal frequencies into composite false color displays by adapting multispectral photography methods used in optical remote sensing. One approach is to encode amplitudes of traces r...

Student Response to Quantitative Aspects of Instruction in an Introductory Geology Course

Because most universities and colleges require all students to take a laboratory science course, many geology classes are composed of large sections of nonscience majors. Pedagogic strategies based on small classes of highly motivated students are not effective in this application. It is particularly difficult to convey to these students the quantitative and discovery aspects of science and to demonstrate the importance of a basic knowledge of science and its methods to them as nonscientists. Student responses to quantitative initiatives taken in a large, introductory class of nonscience majors at Baylor University indicate that an overwhelming number of such students can be convinced of the importance of geological processes to their lives and that a majority of them can be interested in the application of elementary mathematics to the earth sciences if the application has a profound result with which they can identify. Hands-on, discovery-type, field exercises in which small laboratory sections acquire, process, and interpret geophysical data are effective in generating interest in the scientific process. Introductory courses that explore a relatively narrow breadth of material in some depth offer opportunities for discovery-based learning not available to the typical introductory survey course, which usually covers a wide breadth of material in very little depth.

Mapping an Eighteenth-Century Wichita Village Site

Abstract Originally located in a Cross Timbers/short grass prairie ecotone on the Brazos River in central Texas, the Stone site (41ML38), the archaeological site of the late eighteenth-century Wichita (Tawakoni) village of Quiscat’s band, was investigated by Baylor University between 1999 and 2002. A 10 hectare geophysical survey (EM31 ground conductivity meter using a 1m grid interval) produced a village plan showing 75 circular anomalies (10-15 m in diameter), corresponding to the locations of round, grass-thatched lodges, arranged in three concentric ring clusters with outlying parallel rows. Archaeological excavation of one anomaly recovered post-molds arranged in a pattern that corresponded exactly in format (with three concentric rings: fireplace, 12 support posts, and outer walls) and size (9 m diameter) to the pattern derived from analysis of early twentiethcentury ethnographic photographs from Anadarko. The Wichita house served as a model for the layout of the village, and the pattern of house construction persisted from the 1770s–1905.

Application of a Simple Headcut Advance Model for Gullys

Gully erosion begins in streambanks and uplands as a consequence of adjustments in driving forces on the landscape imposed by changes in land use or climate. The deleterious effects of gullies worldwide have led to many site-specific studies of gully form and function. In the continental United States, gully erosion in agricultural land has destroyed valuable farmland yet, prediction of gully processes remains problematic on a national scale. This research has proposed a simple method to predict gully headcut advance. When combined with SWAT hydrologic flow routines, the model predicted gully headcut advance with reasonable accuracy on a daily time step for time periods exceeding two decades. The model was tested in two distinct land resource areas of the United States with differing climate, soils, cover and drainage. The inputs for the headcut model have been kept simple as the model will be applied over large areas. Model inputs consist of headcut height, headcut resistance (based on soil erodibility and a root-cover factor), and daily flow. The model is compared to an annual time step model used in assessment of headcut advance and appears to offer a better way to assess gully headcut advance.