SEISMIC REFRACTION AND SCREENING BY THIN HIGH-VELOCITY LAYERS *

The screening effect of thin, relatively shallow high-velocity layers often presents considerable problems in seismic exploration. Such layers prevent the greater part of the seismic energy from travelling to greater depths and introduce additional refraction arrivals, confusing the seismogram still further. In order to investigate both the screening and refractive properties of high-velocity layers, scale-model experiments have been made over a wide range of layer-thickness/ wavelength ratios (0.05 < d/λ < 2) for suitably chosen material contrasts. The results may be summarised as follows. Refraction arrivals from thin layers in the field may be recognised by their relatively rapid amplitude decay. Furthermore, the “echeloning”-effect observed for refraction first arrivals may be due to the presence of a (thin) layered structure. Since the apparent refraction velocity varies with d/λ when d/λ < 1, differences between vertical well-log velocities and velocities observed along the surface may be expected, making time/depth conversion using surface velocity data inaccurate. Transmission of elastic energy may be expected, if anywhere, only near the shotpoint, at small geophone offset, and for relatively thin screens (d/λ < 0.1). The transmitted signal shape is then independent of the layer thickness. This transmitted energy may be registered either in a reflection set-up with geophones near the shotpoint, or in long-distance refraction work. Three possibilities are offered for overcoming the screening effect of thin high-velocity layers: Use longer-wavelength signals Apply short-spread reflection shooting Apply long-distance refraction shooting The experimental results obtained in scale-model arrangements of such set-ups confirm the potentialities of these methods.     

SEISMIC ATTENUATION IN THE VICINITY OF THE GEOTHERMAL ANOMALY AT URACH OBTAINED FROM NEAR-VERTICAL REFLECTION PROFILES1

The attenuation in the vicinity of the geothermal anomaly at Urach was determined by means of two near-vertical reflection profiles. The attenuation in the sediments and in the upper crust (3-4 km depth) was estimated by interpretation of the first (refracted) arrivals. For calculating the attenuation, the amplitude decay with respect to distance was used. Corrections for the spread factor, i.e. the geometric amplitude divergence was deduced from the traveltime curves. Below the anomaly, higher attenuation values (Q^?1～ 0.008) were observed compared with those in the undisturbed crust (Q^?1～ 0.002). This effect is probably due to the cracks and fissures in the upper part of the crystalline basement. The attenuation in the middle and lower crust was determined using near-vertical reflections from this depth interval. The use of the spectral ratio method leads to higher values of the effective attenuation Q^?1_eff below the heat flow anomaly compared to those of the‘ normal’crust. This zone of high Q^?1_eff coincides with the low velocity body below the heat flow anomaly. Both effects, the higher attenuation and the lower velocities, could be caused by high temperatures, cracks and fissures in the crust.     

Delineation of Subtrappean Mesozoic Sediments in Deccan Syneclise, India, Using Travel-Time Inversion of Seismic Refraction and Wide-Angle Reflection Data

2-D shallow velocity structure is derived by travel-time inversion of the first arrival seismic refraction and wide-angle reflection data along the E–W trending Narayanpur–Nandurbar and N–S Kothar–Sakri profiles, located in the Narmada–Tapti region of the Deccan syneclise. Deccan volcanic (Trap) rocks are exposed along the two profiles. Inversion of seismic data reveals two layered velocity structures above the basement along the two profiles. The first layer with a P-wave velocity of 5.15–5.25 km s^?1 and thickness varying from 0.7–1.5 km represents the Deccan Trap formation along the Narayanpur–Nandurbar profile. The Trap layer velocity ranges from 4.5 to 5.20 km s^?1 and the thickness varies from 0.95 to 1.5 km along the Kothar–Sakri profile. The second layer represents the low velocity Mesozoic sediments with a P-wave velocity of 3.5 km s^?1 and thickness ranging from about 0.70 to 1.6 km and 0.55 to 1.1 km along the E–W and N–S profiles, respectively. Presence of a low-velocity zone (LVZ) below the volcanic rocks in the study area is inferred from the travel-time ‘skip’ and amplitude decay of the first arrival refraction data together with the prominent wide-angle reflection phase immediately after the first arrivals from the Deccan Trap formation. The basement with a P-wave velocity of 5.8–6.05 km s^?1 lies at a depth ranging from 1.5 to 2.45 km along the profiles. The velocity models of the profiles are similar to each other at the intersection point. The results indicate the existence of a Mesozoic basin in the Narmada–Tapti region of the Deccan syneclise.     

A laboratory study of teleseismic Pn waves

In this paper, the influences of mechanical properties of low velocity layers and thickness of high velocity layers in the lower lithosphere on the horizontal propagaties of seismic waves are discussed by means of altrasonic seismic model experiments; It is shown that the structure of the lower lithosphere consisting of soft low velocity layers and thin high velocity layers is favourable for the horizontal propagation of seismic energy, therefore, it is a very possible structure pattern of the lower lithosphere responsible for Pn and later high velocity arrivals observed in long range distances; Such pattern is also consistent with studies of rheological stratification of the lower lithosphere beneath mid-ocean-ridges and continental rifts. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,15, 97–102, 1993. The project is supported by the Chinese joint Seismological Science Foundation.     

Velocity Structure of the West-Bengal Sedimentary Basin, India along the Palashi-Kandi Profile Using a Travel-time Inversion of Wide-angle Seismic data and Gravity Modeling – An Update

The 2-D shallow velocity structure along the north-south Palashi-Kandi profile in the West Bengal sedimentary basin has been updated by travel-time inversion of seismic refraction, wide-angle reflection and gravity data. A six-layer shallow model up to a depth of about 7 km has been derived. The first layer, which has an average velocity of 2.0 kms^?1, represents the alluvium deposit, which rests over the shale formation with average velocity of 3.0 kms^?1. The thin (200 m) Sylhet limestone, observed at a nearby Palashi well, remains hidden in the present data set. Hence a 200-m thin layer with a velocity of 3.7 kms^?1, corresponding to the Sylhet limestone, has been assumed to be present throughout the profile. The fourth layer with a velocity of 4.5–4.7 kms^?1 at a depth of 1.7–2.4 km represents the Rajmahal traps. The ‘skip’ phenomenon and rapid amplitude decay of first arrivals indicate a low-velocity zone (LVZ) in the study area. Using the ‘skip’ phenomena and wide-angle reflection data, identified on seismograms, the LVZ with a velocity of 4.0 kms^?1, indicating the Gondwana sediments, has been delineated below the Rajmahal traps. The next layer with a velocity 5.4–5.6 kms^?1 overlying the crystalline basement (5.8–6.25 kms^?1) may be associated with the Singhbhum group of meta volcanic rock that has been exposed in the western part of the basin. The basement lies at a variable depth of 4.9 to 6.8 km. The overall uncertainties of various velocity and boundary nodes are ± 0.15 kms^?1 and ± 0.5 km, respectively. The elevated basement feature in the north might have acted as a structural barrier for the deposition of Sylhet limestone during the Eocene epoch. The seismically derived shallow structure correctly explains the observed Bouguer gravity anomaly along the profile. The addition of reflections in the present analysis provides a stronger control on the depths and velocities of basement and overlying sedimentary formations, compared to the earlier model derived mainly by the first arrival seismic data.     

Reflected and refracted waves from a linear transition layer

Summary The problem of a transition layer lying between two homogeneous liquid media is discussed. After obtaining the formal solution for a periodic point source lying in the upper layer, the integrand in the expression for the displacement potential of the upper layer is expanded into series of negative powers of exponentials. Some of the terms in the Bromwich expansion are then evaluated along the Sommerfeld loops which give various reflected and refracted waves. The results for the refracted waves are discussed for the two extreme cases, when frequency is extremely low and extremely high. In both the cases it is found that the frequency dependence for refraction arrivals is the same as expected from a sharp boundary, viz., ^–1. And that for high frequency the travel-time of refraction arrivals is the same as expected from geometric ray theory. Both first- order and second-order discontinuities in density and bulk modulus are considered at the boundaries of the transition layer.     

INDUCTIVE SOUNDING OF A STRATIFIED EARTH WITH TRANSITION LAYER RESTING ON DIPPING ANISOTROPIC BEDS *

The generalised three layer boundary value problem with a transition layer sand-witched between an isotropic overburden and dipping anisotropic substratum is discussed assuming that plane electro-magnetic waves are incident normally over the air-earth inter-face. The tangential electric (E_y) and magnetic (H_x) fields and the expression for surface-impedance (E_y/H_x) have been evaluated at the earth's surface. Through numerical analysis it is shown that changes in the values of the parameters m (coefficient of anisotropy), h (thickness of the transition layer), α (angle of inclination of the dipping beds), and b (conductivity ratio between substratum and upper layer) modify the amplitude and phase-variation curves (with skindepth) significantly.     

2-D Crustal Velocity Structure Along Hirapur-Mandla Profile in Central India: An Update

The 2-D crustal velocity model along the Hirapur-Mandla DSS profile across the Narmada-Son lineament in central India (Murty et al., 1998) has been updated based on the analysis of some short and discontinuous seismic wide-angle reflection phases. Three layers, with seismic velocities of 6.5–6.7, 6.35–6.40 and 6.8 km s^–1, and upper boundaries located approximately at 8, 17 and 22 km depth respectively, have been identified between the basement (velocity 5.9 km s^–1) and the uppermost mantle (velocity 7.8 km s^–1). The layer with 6.5–6.7 km s^–1 velocity is thin and is less than 2-km deep between the Narmada north (at Katangi) and south (at Jabalpur) faults. The upper crust shows a horst feature between these faults, which indicates that the Narmada zone acts as a ridge between two pockets of mafic intrusion in the upper crust. The Moho boundary, at 40–44 km depth and the intra-crustal layers exhibit an upwarp suggesting that the Narmada faults have deep origins, involving deep-seated tectonics. A smaller intrusive thickness between the Narmada faults, as compared to those beyond these faults, suggests that the intrusive activities on the two sides are independent. This further suggests that the two Narmada faults may have been active at different geological times. The seismic model is constrained by 2-D gravity modeling. The gravity highs on either side of the Narmada zone are due to the effect of the high velocity/high density mafic intrusion at upper crustal level.     

Basement structure and yalu river fault belt in Dandong region

Some geophysical surveying works in the northeast part of Dandong, such as shallow shock refracted wave, electrical prospecting, electrical sounding and wave velocity measuring, are introduced in this paper, and the dynamic parameters are calculated. The results show that the basement structure in surveying region is very complex, the overburden thickness of the quaternary period, velocity distribution and dynamic parameters are of regional characteristics. The depth of basement is deep in the north and shallow in the west, the difference between north and west region is about 5–10 m. The south part of Yalu river fault belt is composed of F₁, F₂, F₃, F₄, F₅ fault, their strike direction is NE, we can determine that the F₂ fault is the main one in Yalu river fault belt, and the south part of Yalu river fault belt has no activity since Holocene Epoch. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,15, 282–288, 1993.     

我国西北地区地壳中的高速夹层

在我国西北地区的柴达木盆地东部和甘肃地区,在距离炮点40互100公里处,能够接收到不少能量较强的地壳深界面反射波。另外还发现一种与一般反射波性质不同的波,其视速度特大,视速度随距离的变化不大,而且有较明显的终点;其吋距曲线与一般深界面反射波的时距曲线相交。根据它的特征可以判断地壳中存在具有速度梯度的高速夹层.求得的夹层参数为: 甘肃地区柴达木盆地东部覆盖层厚度 18.8公里 30.5公里覆盖层平均速度 5.5公里/秒 5.3公里/秒夹层厚度 6.0公里 3.2公里夹层速度 7.5-8.5公里/秒 7.5-8.0公里/秒夹层的上下界面均为强反射面,可以产生多次反射波。分別利用相邻两个反射波可以求得各层参数,并能避免射线折射的影响。甘肃地区和柴达木盆地东部的地壳厚度分別为51和52公里。地壳中有高速夹层的存在,可以更好地说明P~*速度分散的原因,而且也能够解释Lg波的传播机制。     

REFLECTIONS AND REFRACTIONS FROM CURVED INTERFACES: MODEL STUDY *

Reflections and refractions from curved interfaces were studied on two dimensional scale models. Time of arrival, amplitude and character of reflected, converted, and refracted waves were mainly used for this study. Some reflected refractions, refracted reflections and diffractions were also considered. It was possible to separate PS and SP waves and to study their amplitude and character separately. From the amplitude study of refracted arrivals it was concluded that the refracted ray path penetrates into the high velocity layer rather than propagating along the interface. Although most of the results are interesting from the theoretical point of view, a few applications to exploration problems are suggested.     

DETECTION AND RESOLUTION OF THIN LAYERS: A MODEL SEISMIC STUDY1

The detection and resolution of a thin layer closely situated above a high-impedance basement are predominantly determined by both the frequency content of the incident seismic wavelet and the existence of the nearby high-impedance bedrock. The separation of the thin layer and the basement arrivals is investigated depending on the low-frequency content of the wavelet. The high-frequency content of the wavelet is kept constant. The initial wavelet spectrum with low frequencies has a rectangular shape. All wavelets used have zero-phase characteristics. Numerical and analogue seismic modelling techniques are used. The study is based on the geology of the Pachangchi Sandstone in West Taiwan. Firstly the resolution of a thin layer between two half-spaces is examined by applying the Ricker and De Voogd-Den Rooijen criteria. The lack of low-frequency components of the incident seismic wavelet reduces the shortest true two-way traveltime by about 20%. In addition, low-frequency components of the wavelet diminish the deviation between true and apparent two-way traveltime by about 65% for layer thicknesses in the transition from a thick to a thin layer. The second step deals with the influence of a high-impedance basement just below a thin layer on the detection and resolution of that thin layer. Reflected signal energies and apparent two-way traveltimes are considered. The reflected signal energy depends on the low-frequency content of the incident wavelet, the layer's thickness and the distance between the basement and the layer. This applies only to layers with thicknesses less than or equal to one-third of the mean wavelength in the layer, and a distance to basement in the range of one to one-half of the mean wavelength in the rock material between layer and basement. The minimum thin-layer thickness resolvable decreases with increasing distance to the basement; i.e. for a layer thickness of one-third of the mean wavelength in the layer the relative error of the two-way traveltime increases from 5% to 30%, if the distance is reduced from one to one-half of the mean wavelength in the material between the basement and the thin layer. Finally, a combination of vertical seismic profiling and downward-continuation techniques is presented as a preprocessing procedure to prepare realistic data for the detection and resolution investigation.     

Styles of volcano-induced deformation: numerical models of substratum flexure, spreading and extrusion

The gravitational deformation of volcanoes is largely controlled by ductile layers of substrata. Using numerical finite-element modelling we investigate the role of ductile layer thickness and viscosity on such deformation. To characterise the deformation we introduce two dimensionless ratios; Π_a (volcano radius/ductile layer thickness) and Π_b (viscosity of ductile substratum/failure strength of volcano). We find that the volcanic edifice spreads laterally when underlain by thin ductile layers (Π_a>1), while thicker ductile layers lead to inward flexure (Π_a<1). The deformation style is related to the switch from predominantly horizontal to vertical flow in the ductile layer with increasing thickness (increasing Π_a). Structures produced by lateral spreading include concentric thrust belts around the volcano base and radial normal faulting in the cone itself. In contrast, flexure on thick ductile substrata leads to concentric normal faults around the base and compression in the cone. In addition, we show that lower viscosities in the ductile layer (low Π_b) lead to faster rates of movement, and also affect the deformation style. Considering a thin ductile layer, if viscosity is high compared to the failure strength of the volcano (high Π_b) then deformation is coupled and spreading is produced. However, if the viscosity is low (low Π_b) substratum is effectively decoupled from the volcano and extrudes from underneath it. In this latter case evidence is likely to be found for basement compression, but corresponding spreading features in the volcano will be absent, as the cone is subject to a compressive stress regime similar to that produced by flexure. At volcanoes where basement extrusion is operating, high volcano stresses and outward substratum movement may combine to produce catastrophic sector collapse. An analysis of deformation features at a volcano can provide information about the type of basement below it, a useful tool for remote sensing and planetary geology. Also, knowledge of substratum geology can be used to predict styles of deformation operating at volcanoes, where features have not yet become well developed, or are obscured.     

The P-wave velocity structure of Deception Island,Antarctica, from two-dimensional seismic tomography

Deception Island is a volcanic island with a flooded caldera that has a complex geological setting in Bransfield Strait, Antarctica. We use P-wave arrivals recorded on land and seafloor seismometers from airgun shots within the caldera and around the island to invert for the P-wave velocity structure along two orthogonal profiles. The results show that there is a sharp increase in velocity to the north of the caldera which coincides with a regional normal fault that defines the northwestern boundary of the Bransfield Strait backarc basin. There is a low-velocity region beneath the caldera extending from the seafloor to > 4 km depth with a maximum negative anomaly of 1 km/s. Refracted arrivals are consistent with a 1.2-km-thick layer of low-velocity sediments and pyroclastites infilling the caldera. Synthetic inversions show that this layer accounts for only a small portion of the velocity anomaly, implying that there is a significant region of low velocities at greater depths. Further synthetic inversions and melt fraction calculations are consistent with, but do not require, the presence of an extensive magma chamber beneath the caldera that extends downwards from ≤ 2 km depth.     

NUMERICAL VLF MODELING*

The VLF response of laterally inhomogeneous and anisotropic models is calculated numerically using the finite element method. Some results are presented for a slab model in terms both of the polarization parameters, i.e., the tilt angle and ellipticity of the magnetic polarization ellipse, and the amplitude ratio |H_z/H_x|. On the basis of both the ellipticity and the tilt angle, it is possible to discriminate between a poor conductor and a good one. The direction of the dip can be determined from the anomaly profiles of all diagnostic parameters. The effect of the conductive overburden is most noticeable on the ellipticity profile: one observes attenuation for a poor conductor and “negative attenuation” for a good conductor. The anomaly profiles for anisotropic cases are consistent with the ones of the isotropic cases.     

Travel-time residuals and three-dimensional velocity structure of Italy

Jeffreys-Bullen P and PKP travel-time residuals observed at more than 50 seismic stations distributed along Italy and surrounding areas in the time interval 1962–1979, indicate the complex velocity pattern of this region. Strong lateral velocity inhomogeneities and low velocity zones are required to explain the observed pattern of residuals. In particular, late arrivals of about 1 sec are observed in the Apenninic mountain range, requiring both greater crustal thickness and low velocity layers, coherent with seismic refraction data and surface wave dispersion measurements. The seismic stations located in the Western and Eastern Alps indicate the presence of high velocities. In the Western Alps the strong azimuthal variation of residuals and the high values of early arrivals have a close relationship to the Ivrea body, an intrusive crustal complex characterized by a velocity as high as 7–7.2 km/sec.A travel-time inversion performed with theAki et al. (1977) block model, confirms the peculiar characteristics and the sharp variations in the lithosphere of the whole Italian region, with values of velocity perturbations between many adjacent blocks, ranging in size from 50 to 100 km, and independent from the earth parametrization chosen, reaching values up to 10% in the lithospheric part and 5% in the asthenosphere. 3-D inversion requires also high velocity along the Tyrrhenian coastal margin, equivalent to an uprise of major crustal and lithospheric discontinuities along this part of the Italian peninsula. Moreover low velocity material must be present in the northern part of the Adriatic foreland, in the lithosphere-asthenosphere system, closely related to the stress and seismicity pattern, and the lateral bending of the lithosphere in the same region.     

Interpretation of first arrival travel times in seismic refraction work

The necessary condition for the seismic refraction method to succeed is that the refracted first arrivals from each layer in a multilayered earth system should be detected on a seismogram as first arrivals, and this is possible only when velocities of all underlying layers are successively greater. The usual procedure to interpret the refraction travel times is to fit such a data set with several intersecting straight lines by employing a visual technique which may lead to errors of subjective judgment, as the velocity model depends on the selection of various line segments through the data. To remove the visual fit we propose here a layer stripping method based on minimum intercept time, apparent velocity, rms residual, and maximum data points by least-squares fitting to yield several intersecting straight lines. Once data are segmented out, the conventional equations can be used to determine the velocity structure.     

Crustal structure of Tushka Region, Abu-Simbel, Egypt, inferred from spectral ratios of P waves of local earthquakes

The crustal structure of North Abu-Simbel area was studied using spectral ratios of short-period P waves. Three-component short period seismograms from the Masmas seismic station of the Egyptian National Seismic Network Stations were used. The Thomson-Haskell matrix formulation was applied for linearly elastic, homogeneous crustal layers. The obtained model suggests that the crust under the study region consists of a thin (0.8 km) superficial top layer with a P-wave velocity of 3.8±0.7 km/s and three distinct layers with a mean P-wave velocity of 6.6 km/s, overlaying the upper mantle with a P-wave velocity of 8.3 km/s (fixed). The results were obtained for 14 different earthquakes. The P-wave velocities of the three layers are: 5.8±0.6 km/s, 6.5±0.4 km/s and 7.2±0.3 km/s. The total depth to the Moho interface is 32±2 km. The crustal velocity model estimated using observations is relatively simple, being characterized by smooth velocity variations through the middle and lower crust and normal crustal thickness. The resultant crustal model is consistent with the model obtained from previous deep seismic soundings along the northern part of Aswan lake zone.     

Tomographic joint inversion of first arrivals in a real case fromSaudi Arabia

A joint inversion of both first and refracted arrivals is applied on a seismic line, acquired onshore, in order to obtain a well‐resolved velocity field for the computation of static corrections. The use of different arrivals in the inversion involves exploiting the information derived from the different raypaths associated with each wave type, thus enhancing the reliability of the inversion. The data was gathered by Saudi Aramco in an area of the Arabian Peninsula characterized by strong lateral variations, both in topography and shallow velocity, and where therefore a well‐defined near‐surface velocity field is important. In addition to velocity, the depth distribution of the quality factor Q is computed from the tomographic inversion of the seismic‐signal frequency shift. Thus, the Q‐factor field is used to perform an inverse Q‐data filtering and improve the resolution of the final stacked section.