DEFORMATION OF SEISMIC WAVELETS BY THIN LAYERS AND LAYERED BOUNDARIES*

Using plane wave theory and assuming a given input wavelet the shape of the reflected (or transmitted) wavelet from a layered boundary is derived. Several types of boundaries are considered, among them the weathered layer and a wedge shaped intermediate layer. Different angles of incidence and all internal multiples are taken into account. The examples shown in the figures can be used for a direct comparison between theoretical and observed shapes of reflected (or transmitted) wavelets from special boundaries.     

SOME EXPERIMENTS CONCERNING THE PRIMARY SEISMIC PULSE*

Recordings were made with three types of detector of the primary compressional (P) and shear (S) wave pulses generated by explosions in boreholes. Charge weights varied from 0.08 kg to 9.5 kg and detector distances varied from about 3 m to about 80 m. Scaling by the simple factor W^1/3 where W is the charge weight, enabled observations from different sized charges to be fitted to a single expression. Experiments were carried out in the Bunter sandstone and the London clay and both fluid and solid tamping were used. This variation in tamping had no significant effect on the P-waves but it may have affected the generation of SV-waves. In both media the P-wave energy carried at 30 m from the shot by frequencies less than 100 Hz decreased rapidly with depth and was usually 1–2 % of the available chemical energy for a shot depth of 15 m. The S-wave energy was much less than this, but was highly directional. The P-wave pulse had the appearance of a damped sinusoid in very good agreement with the predictions of the ‘equivalent radiator’ hypothesis. However, the surface of this radiator should be identified not with the blown cavity but with the surface at which the tensile stresses associated with the stress wave become less than the tensile strength of the rock. The predominant frequency for a 1 kg charge at a depth of 15 m was 24 Hz in the clay and 52 Hz in the sandstone. In these and similar media, therefore, an effort should be made to keep individual charges less than 1 kg in reflection shooting and less than 10 kg in refraction shooting. The value of Q was about 50 in clay and about 25 in the sandstone. These estimates are rather uncertain because of the small distances over which the pulses were observed. The Z-transforms of the sampled pulses indicated that they were all of minimum phase, or very near to it.     

THE ELECTRICAL CURRENT PATTERN INDUCED BY AN OSCILLATING MAGNETIC DIPOLE IN A SEMI INFINITE CONDUCTIVE THIN PLATE*

In a paper by Koefoed and Kegge (1968), which was based on previous work of Wesley (1958), the electrical current pattern has been derived that is induced by an oscillating magnetic dipole in a semi-infinite thin plate of infinitesimal resistivity. In the present paper, the range of validity of the assumptions, on which the work of Wesley is based, is subjected to a theoretical analysis. It is found that the decisive factor for the validity of Wesley's derivation is the quotient of the square of the penetration depth of the electrical current over the product of the thickness of the plate and a distance that is indicative of the size of the current loops in the plate. Wesley's derivation is shown to be valid only when this factor is negligible. It is also shown that in this condition the imaginary component of the anomaly must be negligible. Model experiments are described in which the electrical current pattern is studied also in the range in which the derivation of Wesley is not valid. The procedure used in these model experiments was to measure the tangential component of the magnetic field strength very close to the metal plate that simulated the conductive dyke. In order to express the results of the measurements in terms of the imaginary to real ratio, these results are compared with an interpretation graph for field measurements that was published by Hedström and Parasnis (1958). It is found that the current pattern in the plate is essentially the same as that which follows from Wesley's derivation, provided that the imaginary to real ratio is less than one third. The measurements do not permit to draw conclusions regarding the current pattern in the plate in conditions that correspond to larger values of the imaginary to real ratio.     

THE LAUSANNE CONGRESS

The International Society for Photogrammetry held the Xlth International Congress at Lausanne during July 1968. The authors review the activities of five of the seven I.S.P. Commissions.     

Pole-to-pole moisture conditions for the IGY

Summary A study of the mean atmospheric humidity conditions on a planetary scale during the IGY covering the calendar year 1958 is presented. The fields of mean precipitable water content and of the zonal and meridional transports of water vapor are analyzed for the entire globe. Zonally averaged values of the various quantities at several levels are presented in tabular form and compared whenever possible with previous results or indirectly are analyzed on the basis of information obtained from different sources. The structure of these fields is studied and the corresponding implications for the general circulations of the atmosphere are discussed. Finally the zonal water balance for all the globe is discussed and its implications analyzed.     

PHOTOGRAMMETRIC MEASUREMENTS OF GLACIAL LANDFORMS

Photogrammetric methods using sequential aerial photography can provide the geomorphologist with comparative measurements of rapidly changing glacial landforms. Examples are given of eskers, kame and kettle areas, an ice-dammed lake and coastal features in south-east Iceland.