Up-going Waves Research Articles

Attenuating Coherent Noise by Wavelet Transform

One conventional method for attenuating coherent noise (such as surface multiples and tube waves) is f-k filtering, which assumes there exist pronounced move-out (i.e. apparent velocity) differences between coherent noise and primaries. Considering the non-stationary nature of seismic signals, the potential of wavelet transform as a multi-resolution tool in attenuating seismic coherent noise such as tube waves is investigated in this paper.Using a synthetic crosswell data set, we compare the performance of wavelet filtering and f-k filtering to attenuate strong and aliased tube waves. It will be shown that the wavelet filtering method is superior to f-k filtering in the sense that little filtering noise (such as the Gibbs phenomena) is introduced and primaries are better preserved. Applications to the Glenn Pool crosswell field data sets also show excellent attenuation of the up-going and down-going tube waves and significant recovery of the weak later arrivals.

Marine seismic wavefield measurement to remove sea‐surface multiples

We propose a new method for removing sea‐surface multiples from marine seismic reflection data in which, in essence, the reflection response of the earth, referred to a plane just above the sea‐floor, is computed as the ratio of the plane‐wave components of the upgoing wave and the downgoing wave. Using source measurements of the wavefield made during data acquisition, three problems associated with earlier work are solved: (i) the method accommodates source arrays, rather than point sources; (ii) the incident field is removed without simultaneously removing part of the scattered field; and (iii) the minimum‐energy criterion to find a wavelet is eliminated.Pressure measurements are made in a horizontal plane in the water. The source can be a conventional array of airguns, but must have both in‐line and cross‐line symmetry, and its wavefield must be measured and be repeatable from shot to shot. The problem is formulated for multiple shots in a two‐dimensional configuration for each receiver, and for multiple receivers in a two‐dimensional configuration for each shot. The scattered field is obtained from the measurements by subtracting the incident field, known from measurements at the source. The scattered field response to a single incident plane wave at a single receiver is obtained by transforming the common‐receiver gather to the frequency–wavenumber domain, and a single component of this response is obtained by Fourier transforming over all receiver coordinates. Each scattered field component is separated into an upgoing wave and a downgoing wave using the zero‐pressure condition at the water‐surface. The upgoing wave may then be expressed as a reflection coefficient multiplied by the incident downgoing wave plus a sum of scattered downgoing plane waves, each multiplied by the corresponding reflection coefficient. Keeping the upgoing scattered wave fixed, and using all possible incident plane waves for a given frequency, yields a set of linear simultaneous equations for the reflection coefficients which are solved for each plane wave and for each frequency. To create the shot records that would have been measured if the sea‐surface had been absent, each reflection coefficient is multiplied by complex amplitude and phase factors, for source and receiver terms, before the five‐dimensional Fourier transformation back to the space–time domain.

Resonant interactions between propagating gravity wave packets

The spatial and temporal evolution of gravity wave packet interactions is studied numerically. It is shown that through the resonant parametric excitation an upgoing gravity wave packet can cause the growth of two secondary waves from noise level up to a significant intensity in several hours. The primary wave packet is apparently deformed as it decays. The energy transfer among the interacting waves is no longer reversible since their amplitudes are localised. Therefore the characteristic time for the interactions is of a particular significance; it represents a time during which the principal energy transfer arises. Beyond the characteristic time the net energy transfer among the interacting waves becomes rather weak, but the local change in the wave energy densities can be considerable. Only a part of the initial energy of the primary wave packet is transferred to the secondary waves during the parametric excitation. The amounts of energy, which each of the two secondary waves extract from the primary wave, are different, exhibiting a parameter preference in the energy transfer. The parametric excitation process can be completed in the propagation time. For the resonant interaction with two gravity wave packets initially having large amplitudes, the evolution rate is faster than that in the parametric excitation. The primary wave packet can lose most of its energy and finally be reduced to a small fluctuation. The viscous dissipation not only decreases the wave energies but also strongly affects the local energy transfer among the interacting gravity wave packets.

A new method for crosswell reflector imaging

Imaging seismic reflectors with crosshole and VSP tomographic data is only occasionally carried out using Kirchhoff-style migration schemes. We introduce a new kinematic method here for tomographically imaging reflectors. The scheme first picks up traveltimes of each reflected event from common-shot gather tomographic data. It then uses the picked times to image the reflector interfaces with a very important principle: that two reflection points from the same interface that produce reflection pulses on two neighbouring receivers have the same tangent line.The same scheme can be used for surface or crosshole reflection data, but the latter requires some special processing to separate upgoing and downgoing waves. This paper will show how to apply the scheme in tomographic surveys. Traveltimes of each reflection event are initially read from a common-shot gather tomographic seismic section. Then each pair of traveltimes from neighbouring traces is processed in sequence. The two isochronal curves are calculated from the two traveltimes. A common tangent to the two isochronal curves is then found and a reflection segment is defined. In this way, all traveltimes in pairs are processed and a straight or curved interface, corresponding to the locus of tangents, is reconstructed to map the reflection interface.The processing difference between surface seismic reflection data and tomographic (crosshole) seismic reflection data is that the crosshole data contains downgoing waves as well upgoing waves. So the program has to decide if a reflection event comes from above or below the source. Then the program processes the data to obtain the reflector orientation.This imaging method can handle multiple layers and curved interfaces for an arbitrary 2D velocity distribution.

Bayesian parametric separation applied to multicomponent seismic data

The article addresses the parametric estimation of multicomponent seismic waves. The approach of parametric separation based on the maximum likelihood estimator (MLE) is introduced, and the a priori information is obtained by the down-going waves in vertical seismic profile (VSP) data. First, we recall the MLE method. Then the Bayesian approach is introduced, and finally, we show on synthetic seismic data that the estimation of velocities of up-going waves is improved.

Heating of oxygen ions by resonant absorption of Alfvén waves in a multicomponent plasma

Conical distributions of oxygen ions are commonly observed at high altitudes above the auroral zone and in the cusp and cleft regions. The occurrence of these oxygen conics is well correlated with the intensity of waves in the ULF frequency range (of the order of a few hertz). It is shown that the inhomogeneity along the geomagnetic field lines allows a mode coupling between downgoing Alfvén waves (either proton cyclotron or magnetosonic) and upgoing oxygen cyclotron waves. Thus the electromagnetic energy flux carried downward with a large phase velocity can be transferred into a low phase velocity left‐handed mode that resonates with oxygen ions and heats them. We estimate the corresponding rate of absorption of the incident Alfvén waves as a function of the incidence angle and of the parameter ηO+, which is essentially the product of the O+ relative concentration and the characteristic length of inhomogeneity of the magnetic field, normalized to the typical wavelength of the Alfvén waves, at the O+ gyrofrequency. For an incident magnetosonic wave (type II branch) the rate of absorption can be quite large (up to 20%), for ηO+ < 10−2. This peak in the absorption coefficient, however, is obtained for a relatively narrow angular range, when the incidence angle is close to 90°. An incident proton cyclotron wave (type III branch) is more likely to lead to efficient O+ heating because (1) two regimes, either at small or at large incidence angle, lead to an absorption rate up to 20%, (2) these maxima are only slightly dependent on the incidence angle, and (3) large absorption rates are expected over a relatively broad range of values of ηO+ (5 × 10−3 < ηO+ < 10−1), compatible with those values that are expected throughout the magnetosphere.

The effects of ionospheric horizontal electron density gradients on whistler mode signals

Whistler mode group delays observed at Faraday, Antarctica (65° S, 64° W) and Dunedin, New Zealand (46° S, 171° E) show sudden increases of the order of hundreds of milliseconds within 15 minutes. These events (‘discontinuities’) are observed during sunrise or sunset at the duct entry regions, close to the receiver's conjugate point. The sudden increase in group delay can be explained as a tilting of the up-going wave towards the sun by horizontal electron density gradients associated with the passage of the dawn/dusk terminator. The waves become trapped into higher L-shell ducts. The majority of the events are seen during June-August and can be understood in terms of the orientation of the terminator with respect to the field aligned ducts. The position of the source VLF transmitter relative to the duct entry region is found to be important in determining the contribution of ionospheric electron density gradients to the L-shell distribution of the whistler mode signals.

VSP Over Deep Coal Using Vibrator and Explosive

A detailed comparison is presented of results for Vertical Seismic Profiling using a surface vibrator source and a buried explosive charge. The geological setting was the southern region of the Sydney basin near Appin where the Bulli coal seam is at a burial depth of approximately 500 m, and where overlying material is sandstone with interbedded shale. Borehole geophones were spaced at 8 m, commencing at 28 m below the surface and continuing down nearly to the Bulli coal seam. The vibrator operated at a location offset 60 m from the borehole, where 50 linear vibrator sweeps of 40 Hz to 360 Hz and 3 s duration were used. The vibrator performance is compared to results of using 0.5 kg of explosive at the same offset location, and buried beneath the weathering layer generally of depth 13 m. The source bandwidth of the buried explosive charge was 40 Hz to beyond 350 Hz, determined by spectral analysis of the unfiltered record from the downhole geophone closest to the surface. Results of VSP sections showing downgoing waves, and an upgoing wave (Bulli coal-seam reflection) are presented, together with processing details. An examination of vibrator and explosive signal bandwidth attenuation versus depth is presented. These results were determined from the unfiltered downhole geophone records of the downgoing waves. Digitally recorded vibrator ground force was obtained using the accepted practice of accelerometers mounted on the vibrator ground contact plate and reaction mass, and was also obtained using an accelerometer buried one metre beneath the ground contact plate. These signals were analysed to establish the quality of the ground excitation; results are presented graphically. VSP sections, both filtered band pass 60?250 Hz, clearly show downgoing waves of good continuity and comparable wavelet width. However, the explosive downgoing wave showed evident broadening of wavelet width at geophone depths greater than 276 m. This observation was supported by spectral analysis of the unfiltered explosive downgoing wave, which showed a marked reduction of the explosive energy bandwidth with depth, compared with the bandwidth of the wavelet energy produced by the vibrator. The vibrator output however, suffered substantial bandwidth attenuation in passing through the weathered layer, which effectively filtered out vibrator frequencies above 250 Hz. The lower energy output of the vibrator compared with the explosive showed up in the Bulli coal-seam reflection, which in the case of the vibrator became buried in noise at downhole geophone locations shallower than 188 m. On the other hand, the explosive reflection was much stronger, and still evident in the record from the geophone located at a depth of 116 m.

Spectral Matrix Filtering Applied to Vsp Processing

The spectral matrix computed from VSP-traces transfer functions contains information about each wave making up the VSP data set. Using a filter based on the eigenvectors of the spectral matrix leads to a decomposition of input traces in eigensections. The eigensections associated with the largest eigenvalues contain the contribution of the correlated seismic events. Signal space is denoted as the sum of these eigensections. Other eigensections represent noise. When the different waves making up the VSP have very different amplitudes, decomposition of input traces into eigensections leads to wave separation without any required knowledge about the apparent velocities of the waves. Limitations of wave separation by the multichannel filtering are a function of the scalar product values of the waves (in frequency domain) and of the relative wave amplitudes. The spectral matrix filtering can always be used to enhance signal-to-noise ratio on VSP data. The eigenvalues of the spectral matrix can be used to estimate the signal-to-noise ratio as a function of frequency. It is possible to qualify the behavior of a VSP tool in a well and to detect some resonant frequencies probably generated by poor coupling. Field data examples are shown. The first example shows data recorded in a vertical well whose converted shear waves are separated from upgoing and downgoing compressional waves using a spectral matrix filter. This field case shows the efficiency of the spectral matrix filter in extracting weak events. The second example shows data recorded in a highly deviated well, where very close apparent velocity events are successfully separated by use of spectral matrix filtering.

Wave splitting and the reflection operator for the wave equation in R3

The problem of wave splitting in a nonhomogeneous medium in R3 is considered. Previous results for wave splitting in a planar stratified medium can be generalized to a general nonhomogeneous medium (with sufficiently smooth velocity). The wave equation is factorized into an up- and down-going wave system using certain integral and integral-differential operators. The equation for the reflection operator (which relates the up-going wave to a down-going wave) is then obtained, and certain properties of the reflection operator are deduced.

Process for separating upgoing and downgoing events on vertical seismic profiles

A process for separating upgoing (45,47) and downgoing (41,43) seismic wave events in a vertical seismic profile, and particularly for vertical seismic profiles wherein the seismic energy source is offset from the borehole in which seismic detec­tors are located in vertical spaced array. The process operates on two detector signals at a time (as S₁ and S₂) and is based on the concept that waves traveling in opposite directions have spatial derivatives of opposite sign. The derivative is approximated by the difference of the two signals which is time integrated to recover the phase. The resulting integrated difference signal I is then amplitude scale corrected and combined by addition or subtraction with a signal So representing the sum of the two detector signals to form a succession of filtered signals which, when recorded in alignment in order of detector depths to form a vertical seismic profile preserves either the upgoing or the downgoing seismic events.

Falling sphere observations of anisotropic gravity wave motions in the upper stratosphere over Australia

A study of inertial scale gravity wave motions in the region of the atmosphere between 30 and 60 km has been undertaken, using wind and temperature data derived from rocket-borne falling sphere density experiments performed over Woomera, Australia between 1962 nad 1976. The gross features of the wave field compare favorably with those found in similar northern hemispheric studies. Wave propagation is found to be both vertically and horizontally anisotropic. A rotary spectral analysis indicates predominately upgoing wave energy, suggesting that the majority of sources of these waves lie below 30 km. A detailed statistical investigation of the waves, made using the Stokes parameters technique, reveals that phase progression is also highly directional in the horizontal, with a significant zonal component in summer, but with a strong meridional component in winter. Propagation towards the southeast is inferred in summer, with the waves possibly emanating from tropospheric sources in equatorial regions to the north of Australia. The technique also shows that, on average, the waves appear to have mean ellipse eccentricities (=f/ω) around 0.4–0.45. Indirect estimates of a number of important wave parameters are made. In particular,v′ andw′ flux estimates are made over several height intervals. The vertical gradient of density weighted flux implies wave-induced mean flow accelerations of the order 0.1–1 ms−1day−1. This suggests that dissipating gravity waves are a significant source of the momentum residuals that are encountered in studies of satellite data from this region.

Shallow seismic reflection investigations of coal in the Sydney Basin

Surface reflection profiling with the Mini‐SOSIE technique successfully mapped shallow coal seam structure in the western Sydney Basin, New South Wales. Several minor faults and zones of fracturing were detected. In regions of thick Triassic sandstone cover, data quality was poor and unsuitable for geologic interpretation. Synthetic seismograms based on nearby borehole and petrophysical control show excellent agreement with the Mini‐SOSIE sections and illustrate the deleterious filtering effects of coal seams and sequences. To establish a phenomenological basis for seismic wave propagation in shallow coal measures, two vertical seismic profiles (VSPs) which used small explosive charges were recorded with high spatial and temporal sampling. Numerous multiple reflections were observed in the downgoing wave display. The isolated upgoing waves were migrated to yield blurred images of the main coal seams. The subsurface velocity function, also deduced from the VSP, shows broad correlation with the geologic log. The VSP seismograms are not simple because of the combined effects of wave absorption, scattering, and interference. Such problems impede recovery of fine structural detail from seismic data in the shallow environment, particularly when a surface energy source is used.

Evolution of atmospheric spectrum by processes of wave‐wave interaction

Previous work has shown that a primary gravity wave of sufficient amplitude will always decay through resonant wave‐wave interactions with two secondary gravity waves. The interaction among the resonant trio can be reasonably rapid and may be an important process responsible for energy exchange among gravity waves of different wavelengths in the atmosphere. By taking statistical ensemble averages to the interaction equations we obtain an hierarchy of moment equations, the closure condition of which can be effected by making a random phase approximation. The obtained kinetic equation is Boltzmann‐like and describes the time evolution of wave action. The nonlinear kinetic equation is impossible to solve in general except numerically, but the equation is drastically simplified in three limiting cases identified as elastic scattering, parametric subharmonic instability, and induced diffusion processes. Through elastic scattering, an upgoing wave is scattered into a downgoing wave by interacting resonantly with a vertical shear. This process is thus responsible for making the atmospheric spectrum vertically symmetric if it is not so initially. The parametric subharmonic instability process is responsible for transferring energetic large‐scale waves to small‐scale waves at one‐half the frequency. The induced diffusion process is responsible for time evolution of small‐scale waves by a three‐wave process involving two nearly identical waves of small scales interacting resonantly with a large‐scale vertical shear. The rapidity with which these three processes take place in the atmosphere and their implication in the atmospheric spectrum are investigated and discussed.

VERTICAL SEISMIC PROFILING IN COAL*

AbstractThe intellection of seismic wave propagation in coal measures demands direct observation of the wavefield progression. Two vertical seismic profiles with high spatial and temporal sampling, were recently recorded in the Sydney Basin coalfields as part of an experimental coal seismic program.Static corrections and interval velocities were obtained by an automated system to determine first kicks and pulse rise times. Upgoing and downgoing waves were separated in the f—k‐plane using a novel technique of contour slice filtering. The isolated upgoing waves clearly display reflections from the major coal seams within the stratigraphic sequence. The downgoing wave spectra were subjected to attenuation analysis. The deduced specific quality factor Q for Permian coal measure rocks lies in the range 20–70. Similar estimates were obtained in the time domain from measurements of pulse broadening.Synthetic VSP seismograms, computed using an exact recursive formulation, are an indispensable aid to interpretation. They illustrate the filtering effects of coal seams and sequences, and the effects of the contribution of internal and free‐surface multiple reflections in the recorded wavetrains.

Transient response from a planar acoustic interface by a new point‐source decomposition into plane waves

For the wave field of a point source in full space there currently exist two classical decompositions into plane waves. The wave field can be decomposed into either (a) homogeneous and horizontally propagating (vertically attenuated) inhomogeneous plane waves by using the so‐called Sommerfeld‐Weyl integral, or (b) upgoing and downgoing homogeneous plane waves only using the Whittaker integral. Transient representations of both integrals exist. We propose a new decomposition integral that has a greater flexibility than both classical decompositions. Solutions for the point‐source reflection/transmission response from a planar interface, if based on the Sommerfeld‐Weyl integral, have for instance inherently an infinite integration limit. With the new formula, by which the wave field of a transient point source is decomposed into upgoing and downgoing homogeneous as well as horizontally propagating inhomogeneous transient plane waves, the point‐source response is directly obtained in the form of an integral with a finite integration limit. It can also be interpreted in terms of certain plane waves by which the point source is simulated in a new manner. For that matter, solutions based on the new integral readily reveal the “evanescent” or “nonray” character of the point source. The new formula may be considered an extension of the Sommerfeld‐Weyl or Whittaker integral. It can be used to compute reflection/transmission responses in a compact form in situations where the Sommerfeld‐Weyl integral was hitherto considered fundamental.

VSP Fundamentals that Improve CDP Data Interpretation: ABSTRACT

This paper emphasizes the fundamentals of the vertical seismic profile (VSP) that improve the interpreter's confidence and understanding in its use as an exploration tool. The areas covered are VSP acquisition, critical points of the processing sequence, and applications to borehole information in depth and CDP data in time. Correlating reflection character to the stratigraphic section is done typically with synthetic seismograms computed from sonic logs and check shot traveltimes. Problems often make this task difficult, even in areas where the synthetic or surface seismic data are of good quality. The VSP provides an alternative approach and a valuable link between the synthetic and CDP data. A VSP extends check shot surveys from first break traveltime analysis to reflection analysis by recording the complete seismogram at a fine depth interval in the borehole (at least two depths per seismic wavelength). The unique feature of a VSP is the simultaneous measurement of downgoing and upgoing seismic waves as they propagate at depth. A repeatable source such as Vibroseis and the fine depth sampling make it possible to separate the VSP downgoing waves End_Page 465------------------------------ from the upgoing reflections by f-k filtering. The downgoing wave field can be used as signatures to deconvolve the upgoing wave field as the best estimate of the primary reflections in the vicinity of the borehole. VSP processing preserves the polarity and amplitude of reflections, and after deconvolution, a trough corresponds to a positive reflection coefficient. These reflections can be shifted to their two-way traveltime and stacked to produce a VSP extracted trace (VET), which is used to correlate to the CDP data. An important application of a VSP is to correlate the reflection character to depth and the stratigraphy observed in the borehole. The direct downgoing arrival contains the two-way time to depth relationship and the enhanced primary reflections contain the reflection character. Any formation top in the VSP survey can be correlated to time using these features. VSP reflection character indicates the significant features in the sonic log velocities that produce the reflections observed in the surface seismic data. Sonic logs and VSPs attempt to measure the same velocities and reflectivity near the borehole but with significantly different resolution. The sonic log only penetrates a short distance into the formation but provides detailed velocity information vertically. A VSP, however, has poor vertical resolution (50 to 100 ft [15 to 30 m] intervals) but samples a large area around the borehole similar to CDP data. Therefore, the VSP correlates well with CDP data because they have the same resolution. It also correlates well with the synthetic seismogram because they both contain primary reflections without multiples. Finally, multiples in the CDP data are easily identified utilizing the VET and synthetic seismogram. In addition, the depth of origin and periodicity of multiples in the processed VSP can be observed directly because they are generated by downgoing multiples and not by the direct arrival. End_of_Article - Last_Page 466------------

Ionospheric disturbances over Japan due to the 18 May 1980 eruption of Mount St. Helens

The effects of the Mount St. Helens eruption at 1532 UT on 18 May 1980 on the ionosphere over Japan are examined using data on total electron content obtained at three closely-spaced stations and HF (5 and 8 MHz) Doppler recordings together with microbarograph data for the surface pressure perturbation. The results strongly suggest that ionospheric perturbations having a predominant period of about 9 min propagated from north to south approximately along the great circle path with a horizontal velocity of about 300 m s −1. The observed time variations of perturbations can be well explained in terms of Lamb waves propagating through the atmospheric sound channel while launching up-going waves to produce the ionospheric oscillations.

An examination of tube wave noise in vertical seismic profiling data

Tube waves act as noise that camouflages upgoing and downgoing body wave events which are the fundamental seismic data measured in vertical seismic profiling (VSP). In two onshore vertical seismic profiles, the principal source of tube waves is shown to be surface ground roll that sweeps across the well head. Secondary tube wave sources revealed in these VSP data are the downhole geophone tool itself and the bottom of the borehole. Body wave signals are also shown to create tube waves when they arrive at significant impedance contrasts in the borehole such as changes in casing diameter. Computer simulated vertical geophone arrays are used to reduce these tube waves, but such arrays attenuate and filter body wave events unless static time shifts are made so that the body wave signal occurs at the same two‐way time at each geophone station. Consequently, actual downhole vertical geophone arrays are not an effective means by which tube waves can be eliminated. Power spectra comparisons of tube wave and compressional body wave events demonstrate that band‐pass filters designed to eliminate tube waves also suppress body wave signals. A simple but effective field technique for reducing tube waves is shown to be proper source offset. Using velocity filters to retrieve upgoing compressional events from VSP data heavily contaminated with tube wave noise yields in one example an agreement with surface measured reflections that is superior to that obtained from synthetic seismograms calculated from log data recorded in the same well.

Logs From P and S Vertical Seismic Profiles

Vertical seismic profiles differ from ordinary well velocity surveys, or check shots, in two aspects: the records are longer and the depth intervals between measurements are much shorter. Accurate picking of first breaks makes the plotting of P and S velocity logs possible. From the velocity logs, a vs/vp log can be calculated. Introduction A vertical seismic profile (VSP) is the seismogram obtainable by lowering a multitrace seismic array in a well, activating a seismic source near or on the surface, and recording the seismic sensors during a time equal to at least twice the travel time between the surface and the deepest mirrors.Probes presently in use are not more than a few feet long, and the VSP seismogram is obtained by clamping the probe against the walls at regularly spaced depths and activating the seismic source for each position of the probe.Fig. 1 shows the three kinds of seismic signals detected at a geophone (receiver) R placed in a well when a source S located on the surface is activated:direct wave causing the first breaks,up-going reflections, anddown-going reflections resulting from the reflection of up-going waves by mirrors located above R.Fig. 2 shows how these events line up on a VSP seismogram. The left-hand sketch shows ray paths corresponding to two locations of the probe R1 and R2, with R1 above R2. Down-going waves, first breaks, and down-going reflections reach R1 before R2. Their lineups have parallel slopes within a given velocity interval. Up-going reflections reach R2 before R1. The transit time is the same as in the case of down-going events, but the slope of the lineup on the seismogram has the opposite sign.Though field operations are basically the same, there are two main differences between VSP's and ordinary well velocity surveys, or check shots:the recording time is much longer in the case of VSP'S, andthe depth interval between successive locations of the probe is shorter in the case of VSP's 15 to 70 ft (5 to 20 m) vs. 165 to 650 ft (50 to 200 m), or even more in the case of ordinary check shots.VSP's generally are recorded in view of a study of the reflection phenomena at a given location. Good VSP's make possible the direct observation of attenuation, multiple reflections, correlation of P and S events, etc. Also, the short spacing between recording stations makes it possible to obtain transit time logs based on the accurate picking of first breaks. When both P and S events are recorded, elasticity moduli can be estimated, and a much more accurate knowledge of the lithology is made possible. It is the purpose of this paper to illustrate, through the study of an example, how interesting logs can be obtained from P and S VSP's recorded in the same well. VSP Displays and Records Figs. 3 and 4, respectively, correspond to P and S VSP's recorded in the same well. The stratigraphic series listed in Table 1, and indicated on the left side of each figure, is typical of the central part of the Paris basin in France. Depths indicated in Table 1 are exact depths in the considered well. JPT P. 1843＾