John K. Costain

Noise attenuation by Vibroseis whitening (VSW) processing

Reflection seismic data recorded on the Atlantic Coastal Plain to examine suspected faulting of the shallow (200 m) basement were contaminated by severe ground roll noise with an apparent velocity of 587 m/sec, 25 Hz dominant frequency, and 23 m apparent wavelength. A receiver interval of 5 m and a total spread length of 285 m were used to obtain 24‐fold coverage. Although the noise problem was severe, overlapping source and/or receiver arrays on the surface were not used for the recording geometry in order to avoid a decrease in resolution caused by the smoothing effect of overlapping subsurface coverage. Instead, the large‐amplitude surface waves were attenuated by a process called Vibroseis® whitening (VSW). VSW is based on the application of time‐varying amplitude scaling before Vibroseis crosscorrelation. A conventional method of automatic digital gain control was found to be effective for this purpose. This scaling results in a signal‐to‐noise ratio improvement equal to the gain expected from crossc...

Hydroseismicity—A hypothesis for the role of water in the generation of intraplate seismicity

A new hypothesis, termed hydroseismicity, suggests that in crustal volumes with fracture permeability, natural increases in hydraulic head caused by transient increases in the elevation of the water table in recharge areas of groundwater basins can be transmitted to depths of 10–20 km and thereby trigger earthquakes. The flow-path geometry resembles, except for scale, the model familiar to groundwater hydrologists for near-surface flow. Possible trigger mechanisms for hydroseismicity include small increases in fluid pressure at hypocentral depths caused by such transient increases, the dissolution of minerals in water, and the solubility of water in minerals (hydrolytic weakening) that leads to structural weakening. A change in water level of less than 1 m (

Correlations between streamflow and intraplate seismicity in the central Virginia, U.S.A., seismic zone: evidence for possible climatic controls

There is no widely accepted explanation for the origin of intraplate earthquakes. The central Virginia seismic zone, like other seismically active intraplate areas, is a spatially isolated area of persistent, diffuse earthquake activity. We suggested earlier that rainfall plays a key role in the generation of intraplate seismicity (“hydroseismicity”). Observed long-period (10–30 years) changes in streamflow (rainfall) are hypothesized to generate intraplate seismicity by diffusion of pore pressure transients from recharge areas of groundwater basins to depth as deep as the brittle-ductile transition. Streamflow and earthquake strain for a 62-year sample from 1925 to 1987 in the central Virginia seismic zone were cumulated, and a least-squares straight-line fit was subtracted to obtain residuals of streamflow and strain. Residual streamflow was differentiated to obtain the rate of change of residual streamflow. We observed common cyclicities with periods of 10–30 years for residual streamflow and strain. From the one-dimensional diffusion equation, we determined the time response of fluid pressure at depths, ψ, in a hydraulically diffusive crust to an impulsive change in fluid pressure at the surface of a groundwater basin. These responses were convolved with the surface streamflow residuals, or, because of results from reservoir-induced seismicity, with the derivatives of these residuals. Root-mean-square values (rms) of the convolutions were computed for ψ = 5, 10, 15 and 20 km and various values of D. For central Virginia, the number of earthquakes, N, within a crustal slice centered on a depth, ψ, was found to be proportional to the rms value of the convolution, suggesting that the number of intraplate earthquakes generated is directly proportional to the magnitude of the rms changes in fluid pore pressure within the crustal slice. These fluctuations in pore pressure, in concert with stress corrosion and hydrolytic weakening, are hypothesized to trigger intraplate earthquakes in a crust stressed by ridge-push. Crosscorrelation of the residual streamflow convolutions with residual strain, provides a measure of similarity between the streamflow convolution and the strain. Optimum values of crustal diffusivity, D, were considered to be those values for which the Crosscorrelation function looks approximately even. That this occurs only over a reasonable range of diffusivities (near D = 50 km2/year) is consistent with the hypothesized causal relationship between streamflow and intraplate seismicity. Finally, others have shown a clear relationship between the 11-year solar cycle and storm track latitudes in the North Atlantic. Storm tracks over the eastern United States and Canada indicate that north of 50 ° N, the average latitude of storm tracks during December, January, and February is about 2.5 ° farther south at sunspot maximum than at sunspot minimum. A correlation with the 11-year solar cycle was discovered by Labitzke (1987). Significantly, we recognize a clear peak near 11 years in the Fourier spectra of actual (not residual) streamflow in the James River in the central Virginia seismic zone.

Seismic reflection evidence for the evolution of a transcurrent fault system: The Norumbega fault zone, Maine

A seismic reflection profile in east-central Maine reveals a steeply dipping fault zone that terminates in diffraction hyperbolae that are associated with an offset of the Moho. When combined with data from geologic mapping, the seismic data imply Mesozoic or post-Mesozoic dip-slip reactivation of the Norumbega fault zone, hitherto interpreted as a locus of mid- to late Paleozoic dextral offset. This finding highlights the value of seismic reflection data in determining the history of fault displacement and warns of the complexities that arise in attempts to seismically characterize faults in polygenic tectonic regimes.

Climatic changes, streamflow, and long-term forecasting of intraplate seismicity

Abstract Regions of intraplate seismicity in the eastern United States are spatially isolated areas of persistent, diffuse earthquake activity. There is no widely accepted explanation for the origin of these earthquakes. We have suggested that climate plays a key role in triggering such intraplate seismicity. Long-term increases and decreases in rainfall cause periodic regional and temporal variations in the elevation of the water table that, by pore pressure diffusion, result in small changes in fluid pressure at any given depth in the crust. In a fractured, hydraulically permeable crust, the depth of penetration of this pore pressure diffusion can be as deep as the brittle-ductile transition (15–18 km). In a seismogenic crust, stress corrosion and fatigue of rock asperities might be more important than purely mechanical effects due to small changes in hydrostatic fluid pressure; however, because any chemical effects are quasi-static, the temporal characteristics of the triggering process might ultimately be determined by the mechanical process, resulting in a “hydraulically induced” seismicity trigger that acts somewhere along paths of pore pressure diffusion. We called this model of intraplate earthquake generation “hydroseismicity”. Because streamflow is related to the regional and temporal morphology of the water table, we searched for and report here several attempts to find links between climatic changes, streamflow, and intraplate seismicity. Our results fall broadly into four categories: (1) temporal correlations of streamflow with the earthquake strain factor; (2) spectral analyses of flow and seismicity and the identification of common spectral peaks; (3) numerical modeling to estimate fluctuations in pore pressure at hypocentral depths; and (4) climatic driving mechanisms (e.g., sunspot cycles) that might substantiate a climate-earthquake link. We observe a statistically significant peak in the Fourier spectrum of surface streamflow for the seismic zones bisected by the Mississippi River, Illinois, and James River, Virginia, in the period range of 11–13 years that might be associated with sunspot activity. In addition, there is positive correlation between periods of above average values of the standard deviation of streamflow time series and periods of seismicity in the central Virginia seismic zone. Many aspects of the weather appear to be modulated by a 20-year cycle. We observe a similar periodicity (18–20 years) in seismicity in the central Virginia seismic zone. A good agreement is observed when a streamflow time series is superimposed on the record of the earthquake strain factor if a value of 50 km 2 /year is assumed for crustal hydraulic diffusivity. In the central Virginia seismic zone, it is found that the number of earthquakes versus depth, ψ, is directly proportional to pressure fluctuations at the depth ψ. In addition, the fractal dimension determined from downward-continued streamflow is approximately the same as the fractal dimension of intraplate seismicity. Furthermore, using the Gutenberg-Richter relation and assuming that the earthquake data sets in the New Madrid and central Virginia seismic zones are complete for all magnitudes m ⩾ 2, the ratio of the number of earthquakes occurring per year in the New Madrid zone to the central Virginia zone is about 40. The ratio of the standard deviations of downward-continued Mississippi River streamflow (at Thebes, Illinois) to the James River streamflow is also about 40. One interpretation of this common ratio is that the number of intraplate earthquakes generated in a seismogenic crust is directly proportional to the standard deviation of vertical variations in the elevation of the water table. If the hydroseismicity hypothesis is correct, then long-term variations in streamflow can be used to forecast long-term statistical variations in intraplate seismic activity.

Moderate-temperature geothermal resource potential of the northern Atlantic Coastal Plain

Analysis of geothermal gradients determined in 65 exploratory holes in the Atlantic Coastal Plain indicates that in many areas temperatures exceed 40 °C at the base of the sediments. Water of such temperature, if it is available in sufficient quantity, represents a viable geothermal resource. Preliminary results of a test at Crisfield, Maryland, are encouraging in that brackish water at a temperature of 56 °C was produced from an aquifer at 1.2 km depth. Further testing of the temperatures and transmissivity of the deep aquifers beneath the Coastal Plain is required before the geothermal resource potential can be evaluated.

Paleozoic and Grenvillian Structures in the southern Appalachians: Extended interpretation of seismic reflection data

Interpretive reprocessing of seismic reflection data has elucidated Paleozoic and Grenvillian structures in the southern Appalachians. The seismic data include a 7500-km² grid of ADCOH, Seisdata, and COCORP reflection profiles that traverse the Blue Ridge and Inner Picdmont geologic provinces of North Carolina, South Carolina, and Georgia. Surface geology and potential field data were used to constrain the interpretation. The reprocessed seismic reflection data have delineated the internal and external geometry of the crystalline Blue Ridge-Inner Piedmont allochthon, including the locations of the Blue Ridge master decollement, Haycsville fault, and Brevard fault zone. On the basis of the reprocessed data, all of the major faults within the allochthonous upper crust sole in the Blue Ridge master decollement. Reflections extending to the southeast from beneath the surface location of the Hayesville fault to the Blue Ridge thrust might be the seismic signature of a high strain zone. This implies that internal deformation of the Blue Ridge allochthon associated with the Alleghanian orogeny might have occurred farther to the west than has been previously documented from field studies. Relative amplitude seismic data enabled the discrimination between Blue Ridge-Inner Piedmont crystalline rocks and underlying lower Paleozoic shelf strata, thereby delineating the Blue Ridge thrust. The interpreted geometry constrains the top of the shelf sequence beneath the Blue Ridge to depths of less than 3 km. This relatively shallow depth of the shelf strata together with the presence of duplex structures and bright spots that are imaged within the sequence might imply favorable conditions for hydrocarbon exploration beneath the Blue Ridge. Midcrustal reflections from within the upper-to-Iower crust are interpreted to originate from preserved Grenvillian structures that were reactivated at the basement surface during Late Proterozoic-Early Cambrian extension. Reflection continuity is occasionally disrupted by interpreted post-Grenvillian, pre-Early Cambrian low-density intrusions. Topography at the basement surface, possibly caused by the intrusions, is interpreted to have controlled the formation of some of the structures within the overlying allochthon, including Blue Ridge and Brevard fault zone ramps. Correlation of seismic time-structure contour maps with available gravity data and two-dimensional gravity modeling suggest that anomalies in the gravity field can be attributed to low-density sources within the autochthonous crust. Discontinuous reflection packages from depths of 36–42 km are interpreted to originate from the Mohoroviĉiĉ discontinuity. The reflectors trend about N15°E with a true dip of approximately 15°NW.

Use of integrated energy spectra for thin-layer recognition

The detection and resolution of thin beds are important problems in reflection seismology. A thin bed is defined as one for which the two‐way traveltime thickness is less than the tuning thickness for the incident wavelet. Reservoir thickness and shape are of critical importance in estimating hydrocarbon reserves. Previous studies of thin‐bed resolution have focused attention on the time domain, i.e., on the properties of the seismic trace and wavelet Widess (1973) defined a thin bed as one whose thickness is less than λ/8, where λ is the dominant wavelength of the seismic wavelet in the thin bed. He based this criterion on the observation that for thin beds of thickness less than or equal to λ/8, the reflected wavelet is essentially the shape of the derivative of the incident wavelet.

Effect of varying fluid composition on mass and energy transport in the Earth's crust

The ability of fluids flowing through a given volume of crustal rock to affect mass and energy transfer is controlled by the physical and chemical nature of the fluid. Most numerical simulations of fluid-flow and fluid-rock interaction approximate the mass and energy transfer characteristics of crustal fluids using the physical, thermodynamic and transport properties of H2O. Results of fluid inclusion studies indicate that H2O is, in fact, the dominant component of fluids from most shallow to intermediate depth crustal environments, with other components usually present in varying amounts. In many crustal environments, properties of the fluids are adequately represented by those of the system H2O-CO2-NaCl. In the pure H2O system, mass and energy transport properties reach extrema and become very sensitive to small variations in temperature or pressure in a restricted region of P-T space near the critical point. This “critical region” migrates to higher and lower temperatures as NaCl or CO2, respectively, are added to water, and mass and energy transport properties become less sensitive to temperature or pressure variations when either NaCl or CO2 are added to water. The effect of fluid composition on mass transfer is illustrated by the system SiO2-H2O-NaCl. The SiO2-H2O sub-system is characterized by a solubility maximum, or region of retrograde solubility, at temperatures and pressures slightly above the critical point of water. Addition of NaCl increases the solubility of silica and shifts the quartz solubility maximum to higher temperatures and pressures relative to pure H2O.

Crustal structures and the eastern extent of lower Paleozoic shelf strata within the central Appalachians: A seismic reflection interpretation

Reprocessing of line PR3 proprietary seismic reflection data has delineated Grenvillian, Paleozoic, and Mesozoic structures within the Appalachian foreland, Blue Ridge, and Piedmont of the central Appalachians in Virginia and West Virginia. The eastern portion of PR3 can be correlated along strike with the western portion of line I-64, reprocessed earlier at Virginia Tech. The combined seismic reflection data image the crust from the eastern Valley and Ridge, Blue Ridge, Piedmont, and Atlantic Coastal Plain provinces. Within the Piedmont, large (as much as 10 km wide) reflective structures imaged on both lines PR3 and I-64 are interpreted to be thrust sheets that might be composed of deformed Catoctin, Evington Group, and possibly younger metamorphosed rocks. A concealed extension of the Green Springs mafic mass intrudes a thrust sheet imaged along the PR3 profile. The Blue Ridge-Piedmont allochthon was transported north-west along the Blue Ridge thrust, which ramps upward ∼12 km east of the surface exposure of the Mountain Run Fault. Westward along line PR3, the Blue Ridge thrust maintains an undulating geometry; the maximum thickness of the Blue Ridge allochthon is interpreted to be ∼4.5 km. The Blue Ridge allochthon is generally acoustically transparent and overlies lower Paleozoic shelf strata. The maximum thickness of these strata is ∼8 km. Shelf strata are interpreted to extend in the subsurface 5 km east of the surface exposure of the Mountain Run Fault, the northeastward extension of the Brevard Fault Zone, where they are truncated by the Blue Ridge thrust at a depth of 10.5 km (3.5 s). Various folds and blind thrusts are imaged beneath the Appalachian foreland; however, the foreland has not experienced the same degree of deformation as observed in the eastern provinces. A basement uplift ∼45 km wide is imaged beneath the Valley and Ridge province and is interpreted as having formed prior to Late Cambrian time. Farther west, reflections imaged beneath the Glady Fork anticline in the Appalachian Plateau are interpreted as a positive flower structure associated with wrench fault tectonics. Relatively few deep (>9 km) crustal reflections are imaged along line PR3. The majority of reflections that do exist at these depths are observed beneath the Piedmont and eastern Blue Ridge. The high reflectivity associated with the Grenvillian basement in these areas might be the result of the Paleozoic orogenies and extension related to Late Proterozoic and Mesozoic rifting.