Wide-angle Reflection Profiles Research Articles

Field Observations of Electromagnetic Pulse Propagation in Dielectric Slabs

The propagation of electromagnetic pulses in naturally occurring dielectric surface layers has been examined. Pulse duration used in field experiments reported here has been on the order of nanoseconds with pulse bandwidths in the high VHF to low UHF band. The layers were sheets of fresh water ice and granite at thicknesses ranging between 0.4 and 4 m. Both transverse electric (TE) and transverse magnetic (TM) modes were attempted but only the TE propagation could be interpreted. Analog recordings of wide-angle reflection and refraction (WARR) profiles were taken and recorded in a continuous graphic display. The displays allowed easy identification of phase fronts thereby facilitating study of the dispersion of the pulses. The phase and group velocities of the wave-group packets agree well with the velocities predicted from dispersion curves derived from the modal waveguide equation. In one case the Airy phase of wave-packet propagation occurred. The best measure of the dielectric constant of the layer was the frequency of the air wave.

Crustal structure and variability of the Himalayan border of Tibet

Reversed critical wide angle reflection profiles of the crust–mantle boundary south of the Yarlung Zangbo suture in Tibet reveal a deep (70 km) Moho extending north from the High Himalayas, whilst to the south the Moho is 15 km higher. Even further south, reflections from 35 km depth may correspond to the Moho of peninsular India.

Cooperative seismic surveys across the Superior–Churchill boundary zone in southern Canada

Three seismic surveying techniques have been employed in a study of the Superior–Churchill boundary zone in southwestern Manitoba and southeastern Saskatchewan. Two reversed refraction – wide angle reflection profiles, one north–south within the Superior tectonic province and one east–west traversing part of the Superior tectonic province, the boundary zone, and part of the Churchill tectonic province, were used to obtain information on the gross velocity structure of the crust over a large region. Preliminary results from these surveys suggest that the crust beneath the north–south profile is typical of previously published crustal models of the western Superior Province, while the crust beneath the east–west profile is similar to that reported for the Churchill Province in eastern Alberta and western Saskatchewan. Generally, the upper and middle crustal sections in the two tectonic provinces are quite similar, while the lower crust in the Churchill Province has a distinct ~7 km/s layer that is not observed in this part of the Superior Province. In addition, there is a marked thickening of the crust within the boundary zone from ~41 km in the Superior Province to ~46 km in the Churchill Province.A 72 km length of fourfold common reflection point coverage was collected in order to determine the fine structure of the crust over a relatively small region. Reliable stacking velocities that may be used for future processing of the common reflection point data were obtained from an expanding spread reflection survey. Various data processing techniques, including common reflection point stacking, linear and nonlinear velocity filtering, and velocity spectral analysis, have been successful in enhancing reflections from the middle and lower parts of the crust. From the preliminary results of the two reflection surveys, it may be concluded that those parts of the crust which are shown as relatively simple layers in the refraction derived models, may be quite complex when viewed on a smaller scale.

Acoustic anisotropy in marine sediments and sedimentary rocks

Marine sediments under overburden pressure commonly develop elastic anisotropy, probably of the transverse type with a single vertical axis of symmetry. Surface measurements of sound interval velocity in such materials, as by wide angle reflection profiling, provide a value between the vertical and horizontal velocities, thus introducing errors when measured velocity is used in determining section thickness. Empirical regression equations from laboratory measurements allow estimation of in situ vertical or horizontal velocity when one or the other is known in marine calcareous (Vv, km/s = 0.18 + 0.89Vh) and siltclay (Vv, km/s = 0.25 + 0.83 Vh) sediments and sedimentary rocks. These equations allow determination of the maximum error caused by anisotropy in section thickness determinations.

Mantle reflections in the Bay of Biscay

This paper describes the results of two experiments designed to obtain vertical mantle reflections in the Bay of Biscay. The first experiment consisted of a wide angle reflection and refraction profile to detail the structure of the area, while the second was a vertical reflection profile in the same location. The horizon interpreted as being the Mohorovicic discontinuity is shown and the reasons for this interpretation outlined.

Geological results of a wide angle reflection survey using sonobuoys in the southern Chukchi Sea

The results of six sonobuoy wide angle reflection profiles in the southern Chukchi Sea show that from 200 to 700 m of unconsolidated sediments with acoustic velocities less than 2 km/sec overlie 1–1.5 km of sandstones and shales with velocities in the range of 2.1–2.8 km/sec. A higher velocity formation of approximately 4 km/sec was found at two of the six stations. These results show that extensive deposition, as well as marine erosion, has played an important role in making the Chukchi Sea floor the flat feature that it is today.