- Home
- A-Z Publications
- Geophysical Prospecting
- Previous Issues
- Volume 21, Issue 2, 1973
Geophysical Prospecting - Volume 21, Issue 2, 1973
Volume 21, Issue 2, 1973
-
-
PATTERNS—WITH A PINCH OF SALT *
Authors P. NEWMAN and J. T. MAHONEYAbstractThe time‐honoured method of attenuating coherent noise in the seismic record is by the use of source and geophone arrays. In theory, and using methods familiar in the synthesis of digital frequency filters, arrays can be designed having virtually any desired response in the wavenumber spectrum.
In practice, arrays cannot be implemented with the same precision that is applied in design. The response actually achieved must be compromized by a number of factors. These include inaccuracies in the effectiveness or positioning of individual array elements, variations in ground coupling, and the effect of local heterogenities in the environment of the array. We have no reliable way of knowing how well a particular array will perform from one location to the next.
Statistical modelling methods have been applied to examine the effects of implementation errors. Experimental results, supported by statistical theory, show that errors are expected to impose a limit upon the rejection capabilities of an array. The expected limiting value of attenuation due to errors in element weights is inversely proportional to the standard deviation of errors and directly proportional to the square root of the number of array elements. Position errors exert a limiting influence which is wavenumber dependent such that attenuation decreases with increasing wavenumber. For arrays of common dimensions, Gaussian random errors of 10% standard deviation in element weights and positions result in an expected attenuation limit of about 30 dB.
It follows that the more ambitious array designs are less tolerant of errors, and must be implemented with greater care and precision in the field. The present work enables us to specify tolerances for any particular array design.
Ultimately, the degree of pattern refinement which may be effectively employed in any area is determined by errors which are associated with the array environment. Complex arrays are expensive to operate. In order to avoid over‐design it would be useful to establish the magnitude of errors to be expected under different terrain conditions.
-
-
-
SEISMIC REFRACTION ON CONTINENTAL SHELVES WITH DETECTORS ON SEA FLOOR *
More LessAbstractSeismic refraction at sea and on continental shelves requires most of all a precise positioning of the shooting and receiving stations and a good signal to noise ratio.
A new system has been developed using detectors anchored on the sea floor and radio transmission of signal for refraction work on continental shelves. This technique satisfies both requirements. The system has been tested in the western Channel in December 1971.
-
-
-
SOME RESULTS OF EXPERIMENTAL GEOPHYSICAL SURVEYS FOR LOCATION OF ANCIENT GOLD WORKINGS, KOLAR, INDIA *
Authors SHIKHAR C. JAIN, R. KUMAR and A. ROYAbstractOn an experimental basis, observations were made with various geophysical methods to locate the ancient gold workings in Kolar Gold Fields, Kolar, India. The results of experimental surveys indicated that two‐electrode resistivity surveys with two spacings followed by limited trenching would be able to locate about 70 percent of the ancient workings, at 55‐60 percent of the cost of conventional trenching.
-
-
-
EXPERIENCES WITH AUTOMATIC MAGNETIC INTERPRETATION USING THE THICK PLATE MODEL *
By S. E. HJELTAbstractVarious experiments are described in designing two‐dimensional magnetic interpretation algorithms using computer curve fitting techniques.
For a single anomaly the position of the anomaly maximum and the half‐width of the anomaly give good initial estimates of the plate position and thickness. A nomogram and formulae for improving these estimates is given. Curves and estimates for the effects of finite depth extent of a plate show the limits, when the lower surface of the plate can be neglected in curve fitting.
The combined anomaly of two parallel plates can be separated into partial anomalies with no common points using the horizontal derivative of the anomaly. The changes of the anomaly maxima and changes in anomaly half‐widths are studied as a function of plate separation. The position of the maxima and the half‐widths can be corrected before applying the one‐plate procedure for obtaining initial estimates of plate positions and thicknesses.
The performance of standard optimization methods of Powell, Davidon, and Marquardt in improving the values of the plate parameters are compared. The Powell method seems to be the most reliable for both single and multi‐plate anomalies. All methods become unacceptably slow when the number of plates is greater than 2 or 3. In these cases feasible interpretation times are obtained using the partial anomalies and sequential parabolic search of the parameter values as tailored specially to the thick plate model.
Experiments with three different error norms, the classical least squares, weighted least squares and minimax, show that the first norm gives the best overall performance in automatic interpretation. The behaviour of the classical least squares norm as a function of the plate parameters is also briefly described.
-
-
-
THE AUTOMATIC FITTING OF A RESISTIVITY SOUNDING BY A GEOMETRICAL PROGRESSION OF DEPTHS *
By D. MARSDENAbstractThe problem of interpretation of geoelectrical resistivity soundings has been studied and a compromise method of interpretation developed. A simple depth sequence of increasing bed thickness is taken and resistivities assigned to the various layers so that a good fit is obtained to the observed apparent resistivity curve. An initial estimate of the resistivities is made by a direct method but the model is then altered to give an improved fit by the indirect method of curve matching. The whole method is computerized and input data to the computer program consists of spacings and apparent resistivities. The method is extremely rapid, about one third of the computation time is devoted to obtaining the resistivity transform by the method of convolution. A normal type of solution, incorporating additional information if necessary, can also be obtained.
-
-
-
ACOUSTIC WAVES ALONG OILFILLED STREAMER CABLES *
More LessAbstractThere are different types of acoustic noise induced in streamer cables. One type consists of pulsed waves preferentially generated by vertical motions of the towing ship or of the tail buoy. The pulsed waves travel along the streamer with low attenuation and a velocity about 15% less than the velocity of water. Frequencies observed lie between 5 and 15 Hz.
These pulsed waves are well known and for the last few years acoustic isolator sections have been used to reduce their generation. But under bad weather conditions and especially in cold water these waves become stronger. Recent investigations demonstrated that the pulsed waves are able to produce a high noise level even in new streamers with acceleration cancelling hydrophones. We point out that the material of the hoses has a dominant effect on the amplitude picked up by the hydrophones.
Acoustic theory is applied to understand the phenomenon of these waves.
-
-
-
ELECTROMETRIC OBSERVATIONS OF ANTIFILTRATIONAL CEMENTATION CURTAINS *
Authors V. A. BOGOSLOVSKY and A. A. OGILVYAbstractCreation of antifiltrational cementation curtains needed to prevent increased seepage from water reservoirs is a rather labour‐consuming and costly measure. High cost of such curtains necessitates careful checking of the quality of cementation as well as the strength and stability of the solidified cement which is being formed in the pores and fissures of injected rocks.
The traditional methods of control are rather labour‐consuming and do not allow observations during process of cementation. The advantage of electrometric observations of cementation curtains lies in the possibility of making measurements on the surface from the moment of pumping slurry into a borehole up to the termination of cement hardening.
The laboratory investigations carried out by the authors demonstrate an increase in the specific electrical resistivity of a cement in the course of hardening as well as the existence of a functional dependence between the electrical and strength parameters which allows to use the resistivity method for checking the cement strength.
Methods of electrometric observations of cementation curtains are dependent on the particular features of cementation and the curtain dimensions. The observations are usually conducted in a network of profiles parallel to the curtain axis. The cement slurry spreading in the space is studied on the maps of ρa and geoelectrical sections obtained at different distances from the feed line. Zones of the strongest absorption of cement slurry are indicated by resistivity minima. The electrometric observations should be compared with the data on cement expenditure in the boreholes.
The cement strengthening is monitored by comparing the results of control observations consecutively conducted in 1‐2 months following the cementation, with utilization of the ρa maps and graphs. If the specific resistivities of the skeleton of the enclosing rocks and the filler of cavities are known, one can approximately evaluate the changes in the cement strength in absolute units (kg/cm2).
By way of example, the authors give the results of electrometric observations conducted to evaluate the quality of an antifiltrational curtain on one of the high‐mountain water reservoirs in Armenia.
-
-
-
A NEW PARAMETER FOR THE INTERPRETATION OF INDUCED POLARIZATION FIELD PROSPECTING (time‐domain) *
By D. PATELLAAbstractIn a previous paper it has been shown that we can relate the transient IP electric field Ep, existing in a rock after a step wave of polarizing current, with the steady‐state current density Jss during the current step wave as follows: Ep=ρ'Jss.
This relation may be interpreted as a generalized Ohm's law, valid in linear cases, in which ρ’(fictitious resistivity) is defined as the product of the true resistivity ρ with the chargeability m.
Supposing Ep=—grad Up and applying the divergence condition div Jss= o, one can, for a layered earth, obtain a general expression for the depolarization potential Up as a solution of Laplace's equation ∇2Up= o.
Since the mathematical procedure for the solution of this last equation is identical to that used in resistivity problems, we propose now the introduction of an apparent fictitious resistivity ρ'a (defined as the product of the apparent resistivity ρa with the apparent chargeability ma) as a new parameter for the interpretations of IP soundings carried out over layered structures with a common electrode array.
The most general expression of ρ'a as a function of the electrode distance turns out to be mathematically identical to the general expression of ρ'a. Therefore it is possible to interpret a ρ'a field curve using the same standard graphs for resistivity prospecting with the usual method of complete curve matching.
In this manner a great deal of work is saved since there is no need to construct proper ma graphs for the interpretation of IP soundings, as it has been done up to now.
Finally some field examples are reported.
-
-
-
THREE‐DIMENSIONAL SEISMIC MODELING FOR ARBITRARY VELOCITY DISTRIBUTIONS *
More LessAbstractThe growing awareness of the significance of three‐dimensional interpretation of seismic reflection data carries with it a need for better understanding of the role of the velocity configuration in the production of reflection patterns. Modeling the reflection responses of a simple dipping plane reflector through velocity models of various degrees of complexity demonstrates the importance of overlying velocities in determining the reflection pattern.
Modeling is accomplished using a raytracing technique which determines total travel time of the normal incidence raypath through an arbitrary iso‐velocity layer model.
-
-
-
ELECTROMAGNETIC RESPONSE OF TWO‐DIMENSIONAL INHOMOGENEITIES IN A DISSIPATIVE HALF‐SPACE FOR TURAM INTERPRETATION *
Authors A. DEY and H. F. MORRISONAbstractAn integral equation technique based on the integral formulation of the electric field of a harmonically excited line‐source in the presence of a two‐dimensional scattering in‐homogeneity is outlined. In this treatment the inhomogeneity is assumed to have an arbitrary cross‐sectional shape and to be located in a layered, dissipative half‐space. The scattering responses in terms of the horizontal and the vertical magnetic field components observed on the ground surface are studied for a variety of geologic models involving overburden layers, vertical or inclined veins and multiple inhomogeneities. The horizontal component of the total magnetic field appears to be the most diagnostic response parameter. A thin vein shaped target with a cross sectional area of 0.05 δ2× 0.50 δ2 (δ2 being the skin‐depth in the lower half‐space) located 0.50 δ2 away from the line‐source and under a highly conductive overburden layer (σ1/σ2= 300) of thickness 0.02 δ2 is easily detected with a moderate resolution. The scattering responses are also sensitive to changes in depths of burial, inclination and conductivity contrast with the surrounding half‐space of the target inhomogeneity. An interpretation scheme as well as a number of characteristic detectability parameters are developed for the conventional Turam method used in mineral exploration.
-
-
-
COMPUTATION OF THE TIME‐DISTANCE CURVE FOR A DIPPING REFRACTOR AND VELOCITY INCREASING WITH DEPTH IN THE OVERBURDEN *
By J. D. LASKIAbstractThe paper presents the method and algorithm for computing the time‐distance curve for head waves in the case of a dipping refractor and velocity increasing with depth in the overburden. The method is illustrated by an example.
-
-
-
A STUDY ON THE DIRECT INTERPRETATION OF DIPOLE SOUNDING RESISTIVITY MEASUREMENTS OVER LAYERED EARTH *
Authors BY U. C. DAS and D. P. GHOSHAbstractDipole sounding resistivity measurements over layered earth can be interpreted directly by adapting the procedure given by Koefoed (1968) for Schlumberger system. To carry out the first step of the interpretation leading to the determination of the raised kernel function, partial resistivity functions for the dipole method are derived and given in the form of standard curves. The second step involving the derivation of layering parameters from the kernel being independent of the electrode configurations remains unaltered. The applicability and limitations of the method are discussed.
-
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)