Reflection Shooting Research Articles

Multifocusing homeomorphic imaging: Part 1. Basic concepts and formulas

The decomposition of a total wave field recorded on a set of seismic traces on parts corresponding to different body waves is one of the fundamental problems of data processing. The central point of this problem is the correlation procedure for a seismic event (wave) on a set of recorded traces. In order to implement this procedure, it is necessary to have a local time correction formula for a family of source–receiver pairs arbitrarily distributed around a chosen central pair. This formula is derived in the work for a 2D seismic medium of arbitrary structure using a new homeomorphic imaging method called multifocusing. The presentation of multifocusing is divided into two parts: the basic ideas and concepts of the method, the time correction formula and associated geometrical relationships form Part 1. The main characteristic of the multifocusing approach is the consideration of the geometry of all possible wave fronts, which could be formed in the vicinity of a chosen central source-receiver pair. Provided that a target wave exists on a chosen central trace, then there is also a corresponding central ray and an infinite family of surrounding wave tubes. The basic idea of the multifocusing technique is based on the association of any pair of traces recorded in the vicinity of the central trace with certain ray tube belonging to the family. This association can be always found. Considering this ray tube, the local time correction formula is obtained, assuming a spherical approximation of two tube cross sections at the end points of central ray. In the case of a central ray with non-zero offset, the formula consists of the following parameters: two velocities near the source and receiver locations, two angles (departure and arrival) and two pairs of dual curvatures of tube cross-sections at the ray end points. The first four parameters are common for all traces, the pairs of dual curvatures are, as a rule, specific for the chosen pair of traces; the formula thus obtained could not be directly used in practice. The essential part of the first paper is devoted to the parameterization of the family of dual curvatures. The exact formulas derived for these curvatures include as parameters, a pair of dual curvatures of two chosen fundamental ray tubes. Different choices for the fundamental ray tubes are considered and important relationships between the dual curvatures and spreading functions for these tubes are established. They are the generalization of the Hubral formula [Hubral, P., 1983. Computing true amplitude reflections in a laterally inhomogeneous earth. Geophysics 48, 1051–1062] and known reciprocity relations. In the case of a zero-offset central ray, most important for reflection shooting, the formulas derived are significantly simplified and involve four parameters only. The results obtained can be used not only in the multifocusing method, but also in migration and forward modeling.

Re-examination on measurement/analysis procedures for “elastic wave reflection shooting”

Re-examination on measurement/analysis procedures for “elastic wave reflection shooting”

Seismic Investigations of The Firn-Ice Structure at Dome C, East Antarctica (Abstract only)

An extensive program of seismic refraction and reflection shooting was undertaken in the vicinity of Dome C station, East Antarctica (latitude 74°39'S, longitude 124°10'E). Refraction experiments (1978–79, 1979–80) were performed at 37 different shot-point-to-center-of-spread distances along four lines of different azimuth using receiver spacings of 2 to 30 m. The principal result is an extremely well-determined velocity-depth relation for the upper few hundred meters of the ice sheet. Availability of good bore-hole control at Dome C allows us to demonstrate the validity of Kohne's (1972) density-velocity formula, which was developed for warmer ice. Also, after a careful search for azimuthal dependence of variations in the time-distance relationship along different refraction lines, deviations from transverse isotropy in the firn and upper ice are not indicated. Our reflection work (1979–80), conducted along three mutually-centered lines (10.2, 10.2, and 10.7 km in length) oriented at 60° angles to one another, used a series of 16 bore holes, each 30 m in depth, and 18 separate 24-channel recording stations. This extensive coverage combined with common reflection-point shooting geometries (to minimize topographic error) and small charge sizes (to increase resolution) yielded the highest quality set of reflection data yet obtained in East Antarctica. Significant crystalline anisotropy in the lower portion of the ice sheet is indicated. A model with 75% of the ice thickness below the firn (approximately 2 400 m) having c-axes distributed throughout a vertical cone with a semi-apex angle of 20° gives good agreement with the field observations.

Seismic Investigations of The Firn-Ice Structure at Dome C, East Antarctica (Abstract only)

An extensive program of seismic refraction and reflection shooting was undertaken in the vicinity of Dome C station, East Antarctica (latitude 74°39'S, longitude 124°10'E). Refraction experiments (1978–79, 1979–80) were performed at 37 different shot-point-to-center-of-spread distances along four lines of different azimuth using receiver spacings of 2 to 30 m. The principal result is an extremely well-determined velocity-depth relation for the upper few hundred meters of the ice sheet. Availability of good bore-hole control at Dome C allows us to demonstrate the validity of Kohne's (1972) density-velocity formula, which was developed for warmer ice. Also, after a careful search for azimuthal dependence of variations in the time-distance relationship along different refraction lines, deviations from transverse isotropy in the firn and upper ice are not indicated. Our reflection work (1979–80), conducted along three mutually-centered lines (10.2, 10.2, and 10.7 km in length) oriented at 60° angles to one another, used a series of 16 bore holes, each 30 m in depth, and 18 separate 24-channel recording stations. This extensive coverage combined with common reflection-point shooting geometries (to minimize topographic error) and small charge sizes (to increase resolution) yielded the highest quality set of reflection data yet obtained in East Antarctica. Significant crystalline anisotropy in the lower portion of the ice sheet is indicated. A model with 75% of the ice thickness below the firn (approximately 2 400 m) having c-axes distributed throughout a vertical cone with a semi-apex angle of 20° gives good agreement with the field observations.

Oil Exploration in Southeast Turkey Thrust Belt: ABSTRACT

The southeast Turkey thrust belt forms the foothills zone of the Late Cretaceous to late Tertiary Alpine orogenic belt, which frames the Arabian craton in southern Turkey and Iran. The thrust belt is characterized by imbricate structures and disharmonic folding. It comprises a northern inner belt in which Late Cretaceous and Tertiary tectonic phases are superimposed, and a southern outer belt in which Late Cretaceous thrusts underlie gently deformed Tertiary sediments. To the south lies the folded foreland. In southeast Turkey, oil has been found in middle Cretaceous carbonate rocks in the frontal overthrusts of the outer thrust belt and in middle and Upper Cretaceous limestones in faulted anticlines of the foreland. Over 300 exploration wells in this area have resulted in the discovery of about 40 oil fields of which 26 lie in the outer thrust belt. Two oil types can be distinguished: (1) low-sulfur, light crude, mainly confined to the thrust belt and thought to have been derived from Silurian source rocks, and (2) heavy, high-sulfur crude, produced from the foreland fields, probably derived from Lower Mesozoic source rocks. Oil prospects in the thrust belt are limited by reservoir deterioration toward the highly deformed inner thrust belt and by the distribution of Silurian source rocks. Exploration tools applied to locate the oil traps in the overthrusts include field gravity, bore-hole gravity, seismic reflection and refraction shooting, and structural trend studies based on subsurface data and theoretical models. End_of_Article - Last_Page 724------------

Offshore-seismic surveying technique

First Page

Seismic refraction measurements of internal friction in Antarctic ice

Seismic refraction measurements were carried out in 1970–1971 near Byrd Station, Antarctica, with a specific emphasis on the determination of seismic wave attenuation. The primary difficulty in interpreting field results is the extreme sensitivity of the observed attenuation to small variations in the seismic velocity gradient and its first derivative. Detailed velocity analysis, based on multiple surface-reflected arrivals, has made possible an estimate of the average internal friction for compressional waves between 100- and 500-m depth (temperature, −28°C): QP−1 = 0.0014 at 136 Hz, with upper and lower standard error estimates of +0.0005 and −0.0008, respectively. This value has been compared with laboratory measurements by Kuroiwa (1964) on cold slightly impure polycrystalline ice. The comparison required extrapolation of Kuroiwa's results to lower frequency, a correction for the difference between shear and compressional wave absorption, and an estimate of the amount of salt contained in ice frozen from a known solution. The agreement between field and laboratory results is good for the ionic impurity concentration measured in the ice near Byrd Station. All field measurements of QP−1 by reflection shooting in Antarctica and Greenland, although they cannot be assigned to a particular temperature, are also in satisfactory agreement, as are some previous determinations by refraction shooting in Greenland, although some from the high interior part of the Greenland ice sheet are seriously discrepant, probably because of inadequate knowledge of the seismic velocity gradient. We conclude that with the possible (but unlikely) exception of the central Greenland ice sheet, cold polar ice sheets exhibit an internal friction in the frequency range 50–200 Hz that is completely explained by the two fundamental mechanisms in polycrystalline ice: molecular relaxation (dominant at relatively cold temperatures) and grain boundary viscosity (dominant near the melting point). There is no evidence of any additional mechanism associated with internal structures such as bubbles or microcracks. Appendices 1 and 2 are available with entire article on microfiche. Order from American Geophysical Union, 1909 K Street, N.W., Washington, D.C. 20006. Document J76-003; $1.00. Payment must accompany order.

Comparison of Electromagnetic and Seismic-Gravity Ice Thickness Measurements in East Antarctica

AbstractThe errors involved in ice thickness determinations in Antarctica by seismic reflection shooting, gravity observations and radio-echo sounding are briefly discussed. Relative accuracies of 3%, 7-10% and 1.5% have been suggested. Double checks of ice depths from radar sounding in east Antarctica indicate an internal consistency of measurement for this technique of <1%. Comparison of carefully executed seismic shooting and routine radio-echo sounding results against absolute ice thickness values from two deep core drilling sites show no significant differences between these two remote methods (i.e. both are better than 1.5%).Over 60 comparisons are examined between radar ice thicknesses and over-snow measurements obtained on eight independent traverses in east Antarctica. Three traverses exhibit consistently unacceptable results-U.S. Victoria Land Traverse II (southern leg), Commonwealth Transanlarctic Expedition and the U.S.S.R. Vostok to South Pole Traverse—which probably result from misinterpretation of “noisy” seismograms. The remaining comparisons indicate mean differences, including some navigational uncertainty, of ≈3%, <8% and 5% between radio-echo and (1) seismic, (2) gravity, and (3) gravity tied to seismic determinations, respectively.

Comparison of Electromagnetic and Seismic-Gravity Ice Thickness Measurements in East Antarctica

AbstractThe errors involved in ice thickness determinations in Antarctica by seismic reflection shooting, gravity observations and radio-echo sounding are briefly discussed. Relative accuracies of 3%, 7-10% and 1.5% have been suggested. Double checks of ice depths from radar sounding in east Antarctica indicate an internal consistency of measurement for this technique of <1%. Comparison of carefully executed seismic shooting and routine radio-echo sounding results against absolute ice thickness values from two deep core drilling sites show no significant differences between these two remote methods (i.e. both are better than 1.5%).Over 60 comparisons are examined between radar ice thicknesses and over-snow measurements obtained on eight independent traverses in east Antarctica. Three traverses exhibit consistently unacceptable results-U.S. Victoria Land Traverse II (southern leg), Commonwealth Transanlarctic Expedition and the U.S.S.R. Vostok to South Pole Traverse—which probably result from misinterpretation of “noisy” seismograms. The remaining comparisons indicate mean differences, including some navigational uncertainty, of ≈3%, <8% and 5% between radio-echo and (1) seismic, (2) gravity, and (3) gravity tied to seismic determinations, respectively.

IMPROVED MULTIPLE ATTENUATION IN COMMON DEPTH POINT STACKING

Since the introduction of the common depth point method of seismic reflection shooting, we have seen a continued increase in the multiplicity of subsurface coverage, to the point where nowadays a large proportion of offshore shooting uses a 48 fold 48 trace configuration. Of the many benefits obtained from this multiplicity of coverage, the attenuation of multiple reflections during the common depth point stacking process is one of the most important.Examinations of theoretical response curves for multiple attenuation in common depth point stacking shows that although increased multiplicity does give improved multiple attenuation, this improvement occurs at higher and higher frequencies and residual moveouts (of the multiples) as the multiplicity continues to increase. For multiplicities greater than 12, the improvement is at relatively high frequencies and residual moveouts, while there is no significant improvement for the lower frequencies of multiples with smaller residual moveouts, which unfortunately are those most likely to remain visible after the stacking process.The simple process of zeroing, or muting, certain selected traces (mostly the shorter offset traces) before stacking can give an average 6 to 9 decibels improvement over a wide range of the low frequency and residual moveout part of the stack response, with 9-15 decibels improvement over parts of this range. The cost of this improvement is an increase in random noise level of 1-2 decibels. With digital processing methods, it is easy to zero the necessary traces over selected portions of the seismic section if so desired.The process does not require a detailed knowledge of the multiple residual moveouts, but can be used on a routine basis in areas where strong multiples are a problem, and a high stacking multiplicity is being used.

SHAPED CHARGE—A POWERFUL EXPLOSIVE SURFACE SEISMIC SOURCE *

AbstractThe directional effect of shaped charge is a well‐known feature used for a long time in military weapons, oil well guns, and steel industry. This charge was successfully applied as a seismic energy source by Petrobrás during the past three years under different surface and geological conditions.Preliminary amplitude measurements taken with fixed gain shallow refraction instruments showed a consistent difference between conventional and shaped charges. Lately, a similar difference has been noted in deep reflection energy recorded with digital binary gain instruments as well as in deep oil well velocity surveys.Direct comparisons along more than 50 km of multiple coverage field reflection shooting are in agreement with these results and have proved the practical advantage of this source as compared to conventional dynamite.This source has been used since 1971 in routine seismic operation in the Amazon jungle with 300 gram unit charges distributed in small and large shot arrays increasing substantially the coverage and halving the cost at a higher record quality. A large amount of production seismic field work has been carried out in several other areas attesting the successful application of the shaped charges.

Seismic Refraction and Reflection Measurements at “Byrd” Station, Antarctica

AbstractSeismic refraction and reflection shooting was carried out along three profiles about 10 km long, angled 60° to one another, near “Byrd” station, Antarctica, during the 1970–71 field season. No dependence of velocity upon azimuth was found, but velocities at 200 or 300 m depth were slightly greater than at a site 30 km away where measurements were made in 1958. The difference can probably be attributed to different ice fabrics arising from a 50% difference in snow accumulation rates at the two sites. The velocity depth and density–velocity functions at the two sites are also significantly different, but close agreement was found at each site between the depths to significant changes in the velocity gradient and the depths of fundamental change in the densification process. Such agreement may permit density–depth curves, and consequently accumulation rates, to be measured by seismic refraction shooting alone.The reflection shooting on a common reflection-point profile led to a good determination of mean velocity through the ice as a function of angle of incidence. The results agree closely with similar measurements at the 1958 site, and with an anisotropic model based on glaciological and sonic logging observations in the deep drill hole. The mean vertical velocity of 3.90–3.93 km/s through the solid ice is about 2% higher than has commonly been used for determinations of ice thickness from seismic reflection shooting.

Seismic Refraction and Reflection Measurements at “Byrd” Station, Antarctica

Abstract Seismic refraction and reflection shooting was carried out along three profiles about 10 km long, angled 60° to one another, near “Byrd” station, Antarctica, during the 1970–71 field season. No dependence of velocity upon azimuth was found, but velocities at 200 or 300 m depth were slightly greater than at a site 30 km away where measurements were made in 1958. The difference can probably be attributed to different ice fabrics arising from a 50% difference in snow accumulation rates at the two sites. The velocity depth and density–velocity functions at the two sites are also significantly different, but close agreement was found at each site between the depths to significant changes in the velocity gradient and the depths of fundamental change in the densification process. Such agreement may permit density–depth curves, and consequently accumulation rates, to be measured by seismic refraction shooting alone. The reflection shooting on a common reflection-point profile led to a good determination of mean velocity through the ice as a function of angle of incidence. The results agree closely with similar measurements at the 1958 site, and with an anisotropic model based on glaciological and sonic logging observations in the deep drill hole. The mean vertical velocity of 3.90–3.93 km/s through the solid ice is about 2% higher than has commonly been used for determinations of ice thickness from seismic reflection shooting.

Seismic-wave velocities in anisotropic ice: A comparison of measured and calculated values in and around the deep drill hole at Byrd Station, Antarctica

Ultrasonic (28 kHz) velocity logging was carried out to a depth of 1550 meters in the 2164-meter drill hole at Byrd station, Antarctica, in 1969–1970. Smoothed traveltimes from the logger chart were calibrated by taking spot measurements with a calibrated oscilloscope. As a result, velocities are accurate within about ±10 m/sec throughout most of the log. Velocities at depths of <400 meters show an excellent correlation with density. At all depths the correlation with velocities calculated from fabric diagrams is also good, although the calculated velocities show substantially more scatter than those observed. An explanation for a previously unexplained shear-wave arrival observed in refraction shooting has been found. Extrapolating velocities to the bottom of the ice on the basis of fabrics leads to a mean vertical velocity below the compaction zone of 3915 m/sec, which is in satisfactory agreement with estimates from wide-angle reflection shooting at the surface but is about 2½% higher than those commonly used in calculating ice thickness from reflection shooting. A very slow downward increase in the velocity calculated for horizontally propagating waves (0.02 sec−1) has been found to be consistent with both the effect of pressure on the solid ice and the results of detailed examination of refraction velocities. The velocities to a depth of 1000 meters appear to be very closely similar at two sites about 20 km apart near old and new Byrd stations.

MULTICOVER MEASUREMENTS IN REFRACTION SHOOTING*

AbstractMulticover measurements in refraction shooting are comparable to long‐spread reflection shooting. Of course, spread length, distance of traces and offset may be larger than in reflection shooting and depend on the, depth, the velocity and the dipping of the refractors to be detected. For later processing an equation for the refractor velocity is derived in case of flat and steep dipping refractors. The depth and the angle of dip will be computed from delay times. An outlook to digital processing is given.

SOME EXPERIMENTS CONCERNING THE PRIMARY SEISMIC PULSE*

ABSTRACTRecordings 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 W1/3 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.

Estimating the sources of error in reflection shooting

Estimating the sources of error in reflection shooting

HISTOIRE GEOPHYSIQUE DU CHAMP DE LACQ*

ABSTRACTWhereas the first success of petroleum exploration in France (the gas deposit in the St. Marcet anticline in the St. Gaudens region) was essentially based on geological surveys, the second (the Lacq field in the Pau region) was the fruit of reflection shooting. In fact, the Lacq anticline cannot be detected by surface geology because of the Quaternary and Tertiary cover. Besides, electric and telluric prospecting methods could not be used over this zone as an electrified railway line passes through it.As stated then, it was the reflection shooting method that revealed the Lacq structure in 1948, during a reconnaissance survey conducted by Compagnie Générate de Géophysique using a very simple technique (only one geophone per trace).By the seismic technique of 1948, quite good reflections were obtained from a level adjacent to the top Cretaceous, but only sporadic reflections were picked up from a deep level and then only on the flanks of the structure, except for the southern one where results were nil.Towards the end of 1949, the first well, La 1, using a heavy rig designed to reach deep layers, led to the unexpected discovery of the upper oil deposit at a depth of 600 m in the Upper Cretaceous (lower Senonian), while the third well, La 3, led to the discovery by blow‐out of the lower gas accumulation at a depth of 3,500 m in beds later identified as lowermost Cretaceous and, more particularly, Upper Jurassic (Purbeckian).As no great geophysical effort was called for, the upper oil deposit was rapidly developed, its depth and dimensions being modest (6 km2).As far as the deeper gas deposit is concerned, the main objective up to 1954 was to gain a picture of the central part of the anticline. Despite the use of a more detailed seismic technique, it was difficult to plot the top of the structure at the level of the uppermost Jurassic. From 1952 to 1957 the wells La 101, La 102, La 103, La 104, La 105, La 106 and La 113 were drilled, which made it possible to evaluate the gas reserves.Then began the wide‐scale and systematic exploration of the structure, by drilling on the one hand and by reflection shooting on the other. Profiles were shot perpendicular to the axis, but long enough (20 to 30 km) to cover the entire anticline (seismic surveys of 1956–57–58).The eastern pericline and the northern flank were quite easily plotted by seismic survey at the level of the top Jurassic, whereas the southern flank, which has a distinctly steeper slope than the northern one, could not be traced very far–so that the problem of the relationship between the uncomplicated structure of Lacq and the country to the south, with its complex, deep tectonics, still remains an open question.The western pericline, however, remained a subject of concern. Its relationship to the anticline of Ste. Suzanne (outcropping Jurassic) was a mystery.In 1956–57 the well SV 101 confirmed the hypothesis of an overthrust which, without affecting the anticline of Lacq, borders it to the south and west. The thrust is not very large in the south (Lagor wells) but assumes considerable proportions in the west: the Ste. Suzanne anticline actually forms part of a thrust from the south, about 5, 000 to 6, 000 m thick, as the exploration of the region by reflection shooting and drilling (SSE 101, OR 102) has shown. This exploration had been undertaken in search of a new structure under the thrust that might form an extension of the Lacq structure.In 1959 a detailed gravimetric survey (2 to 3 st./km2) was carried out in the Lacq region because the old survey map (1948–1 station/8 km2) had proved progressively inadequate for interpreting the seismic data. It was found that the structure of Lacq had only a small anomaly in comparison with its dimensions. This surprising phenomenon is still difficult to explain. Are the surfaces of equal specific gravity independent of the stratigraphic planes, or has the effect of the anticlinal roof of the Cretaceous and the Jurassic been balanced by that of a saliferous and intumescent Triassic right in the core of the structure?The small anomaly which it was possible to detect may be explained by a thick sequence of reef limestones in the centre of the anticline on the Lower Cretaceous, as indicated by drilling results. These severely fissured limestones are undoubtedly partly to blame for the bad seismic results obtained on the central part of the structure as far as the deep horizon, adjacent to the top Jurassic, is concerned. They are also responsible for the considerable anomalies in the velocity of seismic wave propagation.

MULTIPLE EVENTS IN REFRACTION SHOOTING*

ABSTRACTA description and classification is given for those later events in seismograms of refraction–and wide angle reflection shooting which travel nearly parallel and at constant time intervals behind the first arrivals and which are frequently observed in field surveys. Multiple events with the velocity V1 of the uppermost layer may be caused by multiple P‐reflections and certain velocity conditions or by PS‐conversions. In addition to these types multiple events with the velocity V2 or Vn may be caused by reflected refractions or by reflected diving waves. The different types will be distinguished mainly by means of travel time–and amplitude‐considerations.

SYNTHETIC SEISMOGRAMS APPLIED TO THE SEISMIC INVESTIGATION OF A COAL BASIN*

ABSTRACTIn the present study an attempt is made to relate the general pattern of reflection seismograms obtained over a coal basin to the acoustical properties peculiar to the Coal Measures. The Carboniferous is characterized by a very rapid succession of distinct lithological units, resulting in a surprising variability in their physical properties. Coal seams in particular are conspicuous by their low density and velocity and they produce acoustical contrasts of 35–50 % with respect to country rock. The resulting heavy shielding action prevents the seismic energy from penetrating deeply into this type of formation.With the aid of synthetic seismograms, arranged into synthetic profiles, it is shown that only a minor part of the Coal Measures reflections is of primary origin. The majority has a secondary character brought about by reverberations inside the weathering zone. Thus the conclusion is reached that reflection shooting allows of mapping only the structure of the top section of the Carboniferous, which is sufficient for the purposes of the coalmining industry.