Roger A. Young

Anomalous cold in the Pangaean tropics

The late Paleozoic archives the greatest glaciation of the Phanerozoic. Whereas high-latitude Gondwanan strata preserve widespread evidence for continental ice, the Permo-Carboniferous tropics have long been considered analogous to today9s: warm and shielded from the high-latitude cold. Here, we report on glacial and periglacial indicators that record episodes of freezing continental temperatures in western equatorial Pangaea. An exhumed glacial valley and associated deposits record direct evidence for glaciation that extended to low paleoelevations in the ancestral Rocky Mountains. Furthermore, the Permo-Carboniferous archives the only known occurrence of widespread tropical loess in Earth9s history; the volume, chemistry, and provenance of this loess(ite) is most consistent with glacial derivation. Together with emerging indicators for cold elsewhere in low-latitude Pangaea, these results suggest that tropical climate was not buffered from the high latitudes and may record glacial-interglacial climate shifts of very large magnitude. Coupled climate–ice sheet model simulations demonstrate that low atmospheric CO 2 and solar luminosity alone cannot account for such cold, and that other factors must be considered in attempting to explain this “best-known” analogue to our present Earth.

Recognizing surface scattering in ground-penetrating radar data

Ground-penetrating radar (GPR) data may show strong noise events as a result of scattering by surface objects on the ground or above the survey line. The relative strength of these events can be large in comparison to reflections from geologic features, because radar signals in the ground attenuate exponentially whereas signals that travel in the air attenuate geometrically. Migration of GPR field data from clastic and carbonate sequences in central Oklahoma distinguishes between scattered events and geologic events because the former are focused at the air-wave velocity, while the latter are focused at the ground-wave velocity. Forward modeling using locations of scatterers derived from migration confirms the presence of scattered events, and common midpoint (CMP) gathers are helpful in identifying surface scattering. Scattered events displayed at a horizontal/vertical scale of 1:1 are easily mistaken for subhorizontal, geologic reflections. Methods of recognizing scattered events and removing them, if possible, are therefore crucial to correct geological interpretation of GPR data.

Implications of thin layers for amplitude variation with offset (AVO) studies

Amplitude variation with offset (AVO), or amplitude variation with angle (AVA), analyses of seismic reflection data are becoming increasingly popular in the exploration industry (Ostrander, 1984; Pichin and Mitchell, 1991; Mazzotti and Mirri, 1991) and also in scientific studies of the earth’s crust (Juhlin, 1990). In the exploration industry, AVO analyses are particularly suitable for the detection and mapping of gas zones since reservoirs often consist of shale with high Poisson’s ratio (high vP/vS) overlying gas bearing sands with low Poisson’s ratio (low vP/vS). If the gas sand has lower impedance than the overlying shale, the magnitude of the reflection coefficient will increase with increasing angle of incidence or offset. Other combinations of rock types will also show a similar increase in magnitude, such as shale over hard limestone, but the sign of the reflection coefficient will be positive in most of these cases. Therefore, if the polarity of the reflection can be determined to be negative and...

Revealing stratigraphy in ground‐penetrating radar data using domain filtering

A common-offset ground-penetrating radar profile was collected from a mid-continent fluvial environment characterized by sand with discontinuous clay layers. Signal was separated from coherent noise by a domain filter that exploits local differences in dip between signal and noise. This filter proved effective in separating reflections from the direct air wave and from system ringing. The processed section locates the base of a Pleistocene river channel incised into bedrock and shows a change in a recent position of the river on the basis of radar character. The water table, which lies at a depth of approximately 2 m, is characterized by a continuous reflection. An earlier reflection is interpreted as a perched lens of water near the ground surface. The predominant frequency of both reflections varies greatly with position along the profile. Shallow cores taken along the profile provide evidence that grain size in the transition zone from saturated to unsaturated sediments may determine the predominant frequency of the associated radar reflection. The domain filter is also successful in separating transition zone reflections from saturated layer boundary reflections that are coincident in arrival time. It is possible, therefore, to determine both grain size in the transition zone and the dip of sediments immediately beneath it even if the reflections from the two are superimposed.

Interactive processing of GPR data

Ground penetrating radar profiles now commonly are processed using widely available software packages designed for CMP seismic surveys. This is a natural outgrowth of the availability of digital GPR data in standard seismic format. There is also a very beneficial spinoff: a wider appreciation of GPR by explorationists using seismic reflection.

3D Ground Penetrating Radar Imaging of a Shallow Aquifer at Hill Air Force Base, Utah

A 3D Ground Penetrating Radar (GPR) survey at 50 MHz center frequency was conducted at Hill Air Force Base, Utah, to define the topography of a clay aquitard. Conventional processing augmented by multichannel domain filtering showed a strong reflection from a depth of 6.1–9.1 m despite attenuation by an artificial clay cap approximately .6 m thick. This reflection correlates closely with the top of the aquitard as seen in lithology logs at 3 wells crossed by common offset radar profiles from the 3D dataset. Lateral and vertical resolution along the boundary are approximately .6 m and .3 m, respectively. The boundary shows abrupt topographic variation of 1.5 m over horizontal distances of 6.1 m or less and is probably due to vigorous erosion by streams during lowstands of ancient Lake Bonneville. This irregular topography may provide depressions for accumulation of hydrocarbons and chlorinated organic pollutants. A ridge running the length of the survey area may channel movement of ground water and of hydr...

Near-surface, SH-wave surveys in unconsolidated, alluvial sediments

The past decade of hydrocarbon exploration has been marked by sweeping technological innovations that have greatly advanced methods for exploration and development of oil and gas reserves. An example of major importance is the use of shear waves in marine oil and gas exploration to image reflectors beneath gas chimneys. This technology grew from infancy to maturity in the 1990s, is now incorporated into commercial processing packages, and is being used with success in a number of situations. Recent SEG Annual Meetings and the Special Section of this issue of TLE have had many documented case histories about the use of converted ( P-SV ) waves. The SH -wave (another type of shear wave), however, has been of less interest to the energy industry during the past decade. Near-surface applications of SH -waves, in contrast, have received increasing attention. The present article briefly reviews shear-wave technology advances made in the energy industry over the past decade that prepared the way for the present near-surface application of SH -waves. The article concludes with a near-surface case study using combined P - and SH -wave interpretation in an unconsolidated, alluvial setting. A shear wave having particle motion in a horizontal direction is called an SH -wave. It stands in contrast to an SV -wave, for which particle motion is in the vertical plane. SH -waves travel the entire path from source to receiver as shear waves, but recorded SV -waves commonly start as P -waves at the source and are converted to SV -waves by reflection along the way. SH -waves had their energy industry heyday in the late 1980s and early 1990s after an extensive field study directed by Conoco (the Conoco Group Shoot) and supported by many oil companies (Domenico and Danbom, 1987). That study spurred interest in the imaging potential of …

A hybrid laser-tracking/GPS location method allowing GPR acquisition in rugged terrain

Ground-penetrating radar images of geologic structures provide information about the crucial third dimension, which is inaccessible to geologic mapping. In order to correlate images with geologic mapping, photomosaics, and behind-outcrop borehole logs, radar profiles must be accurately mapped. Moreover, accurate profile elevations are required during processing to avoid introducing into the radar image false structures associated with topographic relief. In recent years, radar station locations and elevations have been obtained very efficiently from a self-tracking laser theodolite operated at the same time the radar data are being acquired. The required physical link between radar and laser systems, however, may make such a system unusable in areas of rugged terrain.

3-D dip filtering and coherence applied to GPR data; a study

Three‐dimensional seismic data are now used routinely in hydrocarbon exploration and reservoir exploitation. Poststack processing of 3-D seismic data often includes the application of 2-D filters to the stacked data one line at a time (so‐called 2-D by 2-D filtering). In this application, 2-D filtered data are sorted into cross line ensembles before a second pass of 2-D filtering. Alternatively, local three‐dimensional filters may be applied to a volume of seismic data, true 3-D filtering.

Spectral decomposition of 3D ground-penetrating radar data from an alluvial environment

This study uses spectral decomposition of GPR data to recognize tuning in a wedge of unconsolidated alluvial sediments. It also compares the spectral decomposition analysis to the results of more traditional attributes such as instantaneous frequency. The study area is on the northern bank of the Canadian River, south of Norman, Oklahoma, USA. (Figure 1). The alluvium ranges in thickness from approximately 10 to 15 m and is mostly sand, with varying amounts of silt and clay and rare occurrences of gravel. Underlying the alluvium is the Hennessey Shale.