Edmund C. Reiter

Deep structure of the U.S. Atlantic continental margin, offshore South Carolina, from coincident ocean bottom and multichannel seismic data

We present the results of a combined multichannel seismic reflection (MCS) and wide-angle, ocean bottom seismic profile collected in 1988 across the Carolina Trough on the U.S. Atlantic continental margin. Inversion of vertical-incidence and wide-angle travel time data has produced a velocity model of the entire crust across the continent-ocean transition. The margin consists of three structural elements: (1) rifted continental crust, comprising 1–4 km of post-rift sedimentary rocks overlying a 30–34 km thick subsedimentary crust, (2) transitional crust, a 70- to 80-km-wide zone comprising up to 12 km of postrift sedimentary rocks overlying a 10- to 24-km-thick subsedimentary crust, and (3) oceanic crust, comprising 8 km of sedimentary rocks overlying an 8-km-thick crystalline crust. The boundary between rifted continental and transitional crust, marked by the Brunswick magnetic anomaly, represents an abrupt change in physical properties, with strong lateral increases in seismic velocity, density, and magnetic susceptibility. The transitional crust contains mid-crustal seaward-dipping reflections observed on the MCS section and has seismic velocities of 6.5–6.9 km/s in the midcrust and 7.2–7.5 km/s in the lower crust. Modeling of potential field data shows that transitional crust also produces the prominent, margin-parallel gravity anomaly and the Brunswick and East Coast magnetic anomalies. These observations support the interpretation that the transitional crust was formed by magmatism during continental breakup. The prodigious thickness (up to 24 km) of igneous material rivals that interpreted on continental margins of the North Atlantic (e.g., Hatton Bank and Voring Plateau), which formed in the vicinity of the Iceland hotspot. These observations, when combined with other transects across the margin, confirm previous suggestions that the U.S. Atlantic margin is strongly volcanic and further imply that the magmatism was not the result of a long-lived mantle plume.

Imaging with deep-water multiples

Acquisition of on‐bottom hydrophone data recording of a near‐surface source provides an opportunity to treat water column multiples as useful signal. A ray‐equation based Kirchhoff depth migration is used to image primary reflections and deep‐water multiples recorded on an Ocean Bottom Hydrophone (OBH). The image of the subbottom sediments is shown to be improved by inclusion of the deep‐water multiple in the imaging process. Field data, jointly acquired by Woods Hole Oceanographic Institute and University of Texas Institute for Geophysics at Austin and consisting of an OBH (2300 m depth) recording a 10 800 cubic inch air gun array, are used to illustrate the feasibility of the technique. Images are obtained from both the primary reflections and from energy that has undergone an additional passage through the water column. Comparison of these images reveals an excellent correlation of reflectors with the predicted polarity reversal observed in the multiple’s image. Synthetic data are used to examine the d...

A Model for Attenuation and Scattering in the Earth’s Crust

The mechanisms contributing to the attenuation of earthquake ground motion in the distance range of 10 to 200 km are studied with the aid of laboratory data, coda wavesRg attenuation, strong motion attenuation measurements in the northeast United States and Canada, and theoretical models. The frequency range 1–10 Hz has been studied. The relative contributions to attenuation of anelasticity of crustal rocks (constantQ), fluid flow and scattering are evaluated. Scattering is found to be strong with an albedoB0=0.8–0.9 and a scattering extinction length of 17–32 km. The albedo is defined as the ratio of the total extinction length to the scattering extinction length. TheRg results indicate thatQ increases with depth in the upper kilometer or two of the crust, at least in New England. CodaQ appears to be equivalent to intrinsic (anelastic)Q and indicates that thisQ increases with frequency asQ=Qofn, wheren is in the range of 0.2–0.9. The intrinsic attenuation in the crust can be explained by a high constantQ (500≤Qo≤2000) and a frequency dependent mechanism most likely due to fluid effects in rocks and cracks. A fluid-flow attenuation model gives a frequency dependence (Q≃Qof0.5) similar to those determined from the analysis of coda waves of regional seismograms.Q is low near the surface and high in the body of the crust.

Image of the Moho across the continent-ocean transition, U.S. east coast

Strong wide-angle reflections from the Moho were recorded by ocean-bottom seismic instruments during the 1988 Carolina Trough multichannel seismic experiment, in an area where the Moho is difficult to detect with vertical-incidence seismic data. Prestack depth migration of these reflections has enabled the construction of a seismic image of the Moho across the continent-ocean transition of a sedimented passive margin. The Moho rises across the margin at a slope of 10°-12°, from a depth of about 33 km beneath the continental shelf to 20 km beneath the outer rise. This zone of crustal thinning defines a distinct, 60-70-km-wide continent-ocean transition zone. We interpret the Moho in the Carolina Trough as a Jurassic feature, formed by magmatic intrusion and underplating during the rifting of Pangea.

2-D velocity inversion/imaging of large offset seismic data via the tau-p domain

We describe a method for determining a two‐dimensional (2-D) velocity field from refraction data that has been decomposed into some function of slowness. The most common decomposition, intercept time‐slowness or τ-p, is used as an intermediate step in an iterative wave field continuation procedure previously applied to one‐dimensional (1-D) velocity inversions. We extend the 1-D approach to 2-D by performing the downward continuation along numerically computed raypaths. This allows a correction to be made for the change in ray parameter induced by 2-D velocity fields. A best fitting velocity model is chosen as a surface defined by critically reflected and refracted energy that has been downward continued into a three dimensional (3-D) space of velocity, offset, and depth. Synthetic data are used to demonstrate how this approach can compensate for the effects of known lateral inhomogeneities while determining an underlying 1-D velocity field. We also use synthetic data to show how multiple refraction lines...

Joint imaging with primary reflections and deep water multiples

A ray equation based Kirchhoff depth migration is used to image primary reflections and deep water multiples recorded on an ocean bottom hydrophone (OBH). The resulting image of the subbottom sediments is shown to be improved by inclusion of the deep water multiple in the imaging process. Field data acquired jointly by Woods Hole Oceanographic Institute and University of Texas Institute for Geophysics at Austin consisting of an OBH (2300 m depth) recording a 10,800 cubic inch airgun array, are used to illustrate the feasibility of this technique. Images are obtained from both the primary reflections and energy which has undergone an additional path through the water column. Comparison of these images reveal an excellent correlation of reflectors with the predicted polarity reversal observed in the multiples image. Synthetic data are used to examine the difficulties in identifying the true path of the water column multiple. For flat layered media there are two different multiple paths, one which reflects beneath the source and one which reflects over the receiver, which have identical travel times when the seafloor is approximately horizontal. They do not however have the same amplitude and it can be shown that their amplitudes differ sufficiently to allow a reliable image to be extracted from the energy which reflects over the receiver (receiver multiple). The difference in amplitude between this receiver multiple and the primary reflection is mostly due to geometric spreading and attenuation in the water column. This is usually small enough to allow observation of most primary events in the receiver multiple. While conventional seismic imaging techniques utilize only primary reflected energy we have shown that for an on bottom recording geometry energy reflecting from the free surface may also be used to image the subsurface. As a final step the image obtained from the multiple is corrected for the r phase shift from the free surface and added to the image from the primary reflection. The final image shows both extended lateral coverage and increased signal to noise.