- Home
- A-Z Publications
- Geophysical Prospecting
- Previous Issues
- Volume 47, Issue 3, 1999
Geophysical Prospecting - Volume 47, Issue 3, 1999
Volume 47, Issue 3, 1999
-
-
Weighted sum method for calculating ground force: an evaluation by using a portable vibrator system
Authors Michiel Van Der Veen, Jan Brouwer and Klaus HelbigUsing a lightweight portable vibrator, we have evaluated the accuracy of the ‘weighted sum’ method for calculating ground force. Experiments in which the vibrator was suspended elastically have shown that, contrary to expectations based on standard theory, the amplitude of the weighted sum ground force was significantly above zero at high frequencies (> 500 Hz). Complementary investigations with load cells confirmed these results. If not accounted for, these deviations may introduce significant ‘vibroseis‐correlation noise’ in processed records. Furthermore, we have demonstrated that ground force and base‐plate velocity can be used to estimate the radiation impedance, which describes the interaction of (vibratory) sources with the ground. Using the mechanical characteristics of the system (i.e. maximum displacement, maximum velocity and maximum acceleration of the base‐plate) and the radiation impedance, the behaviour of the portable vibrator on typical Dutch soil types was evaluated. We found that for the same sweep, more high‐frequency energy could be generated on hard grounds (e.g. concrete) characterized by a higher radiation impedance than on softer grounds (e.g. clay or sand). Knowledge of this behaviour may provide important information for use in data interpretation.
-
-
-
Comparison of six different methods for calculating traveltimes
Authors Leidenfrost, Ettrich, Gajewski and KosloffWe study six different methods for the calculation of seismic traveltimes. All methods yield traveltimes at all points of a regular grid.
The methods examined comprise three different variants of finite‐difference (FD) eikonal solvers, the graph method, wavefront construction and a combined FD and Runge–Kutta method.
The main points of investigation are computational time, accuracy and memory requirements. We took measures to obtain a high level of both general validity and clear understanding of the results. We used a profiling program to be able to measure the time that the actual core algorithm needs, thus avoiding any overhead of highly system‐dependent in‐/output operations.
The comparison shows that no single method is the most appropriate but that the choice depends on the task to be fulfilled. The FD eikonal solver that uses expanding squares proves to be best suited for models which are not too complicated because it offers the best compromise between speed and accuracy, whereas wavefront construction should be applied to complex media because of its superior reliability which then justifies the much higher computational times.
-
-
-
Modelling of local velocity anomalies: a cookbook
Authors Corinne Juliard and Pierre D. ThoreThe determination of small‐scale velocity anomalies (from tens to a few hundreds of metres) is a major problem in seismic exploration. The impact of such anomalies on a structural interpretation can be dramatic and conventional techniques such as tomographic inversion or migration velocity analysis are powerless to resolve the ambiguity between structural and velocity origins of anomalies. We propose an alternative approach based on stochastic modelling of numerous anomalies until a set of models is found which can explain the real data. This technique attempts to include as much a priori geological information as possible. It aims at providing the interpreter with a set of velocity anomalies which could possibly be responsible for the structural response. The interpreter can then choose one or several preferred models and pursue a more sophisticated analysis. The class of retained models are all equivalent in terms of data and therefore represent the uncertainty in the model space.
The procedure emulates the real processing sequence using a simplified scheme. Essentially, the technique consists of five steps:
1 Interpretation of a structural anomaly in terms of a velocity anomaly with its possible variations in terms of position, size and amplitude.
2 Drawing a model by choosing the parameters of the anomaly within the acceptable range.
3 Modelling the traveltimes in this model and producing the imaging of the reflected interface.
4 Comparing the synthetic data with the real data and keeping the model if it lies within the data uncertainty range.
5 Iterate from step 2.
In order to avoid the high computational cost inherent in using statistical determinations, simplistic assumptions have been made:
• The anomaly is embedded in a homogeneous medium: we assume that the refraction and the time shift due to the anomaly have a first‐order effect compared with ray bending in the intermediate layers.
• We model only the zero‐offset rays and therefore we restrict ourselves to structural problems.
• We simulate time migration and so address only models of limited structural complexity.
These approximations are justified in a synthetic model which includes strong lateral velocity variations, by comparing the result of a full processing sequence (prestack modelling, stack and depth migration) with the simplified processing. This model is then used in a blind test on the inversion scheme.
-
-
-
A comparison between airguns and explosives as wide‐angle seismic sources
More LessThe relative merits of a 48‐gun, 9324 cu. in. (153 litre) airgun array and a 200 kg explosive source are considered for the purposes of long‐range (0–400 km) refraction seismic work, with particular reference to traveltime modelling. Theoretical source calculations indicate that in the frequency range 2.5–12.0 Hz, the airgun source will produce an RMS pressure ∼ 8% of that produced by the explosive source and an initial burst pressure ∼17% of that produced by the explosive source. Observed data support these calculations at short ranges and illustrate the greater attenuation of the airgun signal with range due to its lack of very low frequency (< 5 Hz) content. At short offsets, the airgun array provides a preferable seismic source to the explosives, due to densely spaced shots and a consistent waveform resulting in excellent trace‐to‐trace coherence. With increasing offsets, it may be necessary to stack the airgun data to enhance its signal‐to‐noise ratio: here we use a 4‐fold stack. Large explosive shots, although more powerful, produce a less consistent waveform and are more widely spaced due to operational constraints. The offset at which airguns provide a preferable source is dependent on the ambient noise. This practical comparison of real sources demonstrates that, even without advanced processing, a well‐tuned airgun array may provide a preferable source to explosives at offsets up to 160 km, under favourable experimental conditions.
-
-
-
The problem of velocity inversion in refraction seismics: some observations from modelling results
More LessThe applicability of seismic refraction profiling for the detection of velocity inversion, which is also known as a low‐velocity layer (LVL), is investigated with the aid of synthetic seismogram computations for a range of models. Our computational models focus on the inherent ambiguities in the interpretation of first‐arrival time delays or ‘skips’ in terms of LVL model parameters. The present modelling results reveal that neither the measure nor even the existence of a shadow zone and/or a time shift (skip) in first arrivals is necessarily indicative of an LVL. Besides attenuation effects, the cap‐layer velocity gradient is a critical parameter, determining the termination point of the cap‐layer diving wave and thus the time skip.
We suggest that shallow LVLs can be delineated more reliably by traveltime and amplitude modelling of coherent phases reflected from their top and bottom boundaries, often clearly observed in the pre‐ and near‐critical ranges in seismogram sections of refraction profiling experiments with a close receiver spacing. We demonstrate the applicability of this approach for a field data set of a refraction profile in the West Bengal Basin, India. The inferred LVL corresponds to the Gondwana sediments underlying the higher‐velocity layer of the Rajmahal Traps. This interpretation is consistent with the data from a nearby well in the region.
-
-
-
Attenuation of P‐ and S‐waves in limestones
Authors Solomon Assefa, Clive McCann and Jeremy SothcottUltrasonic compressional‐ and shear‐wave attenuation measurements have been made on 40, centimetre‐sized samples of water‐ and oil‐saturated oolitic limestones at 50 MPa effective hydrostatic pressure (confining pressure minus pore‐fluid pressure) at frequencies of about 0.85 MHz and 0.7 MHz respectively, using the pulse‐echo method. The mineralogy, porosity, permeability and the distribution of the pore types of each sample were determined using a combination of optical and scanning electron microscopy, a helium porosimeter and a nitrogen permeameter. The limestones contain a complex porosity system consisting of interparticle macropores (dimensions up to 300 microns) and micropores (dimensions 5–10 microns) within the ooids, the calcite cement and the mud matrix. Ultrasonic attenuation reaches a maximum value in those limestones in which the dual porosity system is most fully developed, indicating that the squirt‐flow mechanism, which has previously been shown to occur in shaley sandstones, also operates in the limestones. It is argued that the larger‐scale dual porosity systems present in limestones in situ could similarly cause seismic attenuation at the frequencies of field seismic surveys through the operation of the squirt‐flow mechanism.
-
-
-
Mise‐à‐la‐masse interpretation using a perfect conductor in a piecewise uniform earth
More LessA useful analysis of the mise‐à‐la‐masse problem can be made by considering a perfectly conducting orebody in a piecewise uniform conducting earth. While the use of a perfect conductor is clearly an idealization of the true geological conditions it provides several advantages for the present purpose.
The electric field associated with the above model can be expressed in terms of a surface integral of the normal potential gradient over the boundary of the conductor, where the normal gradient satisfies a well‐posed Fredholm integral equation of the first kind. This integral equation formulation remains unchanged when the conductor is arbitrarily located in the conducting earth, including the important case when it crosses surfaces of conductivity discontinuity. Moreover, it is readily specialized to the important case of a thin, perfectly conductive lamina.
Consideration of the boundary value problem relevant to a conductive body fed by a stationary current source suggests that under certain circumstances, equivalent mise‐à‐la‐masse responses will result from any perfect conductor confined by the equipotential surfaces of the original problem. This type of equivalence can only be reduced by extending the potential measurements into or on to the conductor itself.
This ambiguity in the interpretation of mise‐à‐la‐masse surveys suggests a simple if approximate integral solution to the mise‐à‐la‐masse problem. The solution is suitable for modelling the responses of perfect conductors and could possibly be used as the basis of a direct inversion scheme for mise‐à‐la‐masse data.
-
-
-
Resistivity anomaly imaging by probability tomography
Authors Paolo Mauriello and Domenico PatellaProbability tomography is a new concept reflecting the inherently uncertain nature of any geophysical interpretation. The rationale of the new procedure is based on the fact that a measurable anomalous field, representing the response of a buried feature to a physical stimulation, can be approximated by a set of partial anomaly source contributions. These may be given a multiplicity of configurations to generate cumulative responses, which are all compatible with the observed data within the accuracy of measurement. The purpose of the new imaging procedure is the design of an occurrence probability space of elementary anomaly sources, located anywhere inside an explored underground volume. In geoelectrics, the decomposition is made within a regular resistivity lattice, using the Frechet derivatives of the electric potential weighted by resistivity difference coefficients. The typical tomography is a diffuse image of the resistivity difference probability pattern, that is quite different from the usual modelled geometry derived from standard inversion.
-
Volumes & issues
-
Volume 72 (2023 - 2024)
-
Volume 71 (2022 - 2023)
-
Volume 70 (2021 - 2022)
-
Volume 69 (2021)
-
Volume 68 (2020)
-
Volume 67 (2019)
-
Volume 66 (2018)
-
Volume 65 (2017)
-
Volume 64 (2015 - 2016)
-
Volume 63 (2015)
-
Volume 62 (2014)
-
Volume 61 (2013)
-
Volume 60 (2012)
-
Volume 59 (2011)
-
Volume 58 (2010)
-
Volume 57 (2009)
-
Volume 56 (2008)
-
Volume 55 (2007)
-
Volume 54 (2006)
-
Volume 53 (2005)
-
Volume 52 (2004)
-
Volume 51 (2003)
-
Volume 50 (2002)
-
Volume 49 (2001)
-
Volume 48 (2000)
-
Volume 47 (1999)
-
Volume 46 (1998)
-
Volume 45 (1997)
-
Volume 44 (1996)
-
Volume 43 (1995)
-
Volume 42 (1994)
-
Volume 41 (1993)
-
Volume 40 (1992)
-
Volume 39 (1991)
-
Volume 38 (1990)
-
Volume 37 (1989)
-
Volume 36 (1988)
-
Volume 35 (1987)
-
Volume 34 (1986)
-
Volume 33 (1985)
-
Volume 32 (1984)
-
Volume 31 (1983)
-
Volume 30 (1982)
-
Volume 29 (1981)
-
Volume 28 (1980)
-
Volume 27 (1979)
-
Volume 26 (1978)
-
Volume 25 (1977)
-
Volume 24 (1976)
-
Volume 23 (1975)
-
Volume 22 (1974)
-
Volume 21 (1973)
-
Volume 20 (1972)
-
Volume 19 (1971)
-
Volume 18 (1970)
-
Volume 17 (1969)
-
Volume 16 (1968)
-
Volume 15 (1967)
-
Volume 14 (1966)
-
Volume 13 (1965)
-
Volume 12 (1964)
-
Volume 11 (1963)
-
Volume 10 (1962)
-
Volume 9 (1961)
-
Volume 8 (1960)
-
Volume 7 (1959)
-
Volume 6 (1958)
-
Volume 5 (1957)
-
Volume 4 (1956)
-
Volume 3 (1955)
-
Volume 2 (1954)
-
Volume 1 (1953)