Geophone Groups Research Articles

USING HALF‐INTEGER SOURCE OFFSET WITH SPLIT SPREAD CDP SEISMIC DATA

Half‐integer offset is a technique where the source is centered midway between geophone group centers rather than on geophone group centers. With half‐integer offset an even number of traces is dropped in the gap rather than the odd number that is dropped for integer offset. When used with split spread data, half‐integer offset has some powerful advantages over conventional integer offset techniques. They include a statistical edge in velocity analysis, better residual statics correction, and an improvement in the attenuation of coherent shot noise such as ground roll and air‐coupled waves. Statistically, this improvement is three decibels.

Wide Angle Reflection Study of Crustal Structure, Eastern Pennsylvania - Northern New Jersey

A 12 channel, digitally recorded wide-angle reflection profile was conducted in the northeast trending Great Valley of eastern Pennsylvania-northern New Jersey. The wide-angle reflections were inverted to determine the crustal velocity structure of the region:, Specifically, the experiment was designed to determine if the Conrad discontinuity existed in eastern PA - northern NJ and to determine its depth and the thickness of the crust. Six quarry blasts, timed by a geophone located at the quarry, were recorded at three offsets of 73.8 km, 118.3 km, and 146.7 km. The common-receiver array was located at an abandoned railroad track parallel to Paulinskill Lake, Sussex, County, New Jersey. It consisted of 11 geophone groups spaced 213 meters apart. Since the reduced data contained a low S/N ratio for which the computer was unable to pick the P-arrivals, the data were treated subjectively for the sweep of the P-wave arrival across the array at the expected time. The travel time and normal moveout were then calculated for coherent phase signals across the traces. These data were plotted on a t(p) (intercept time-ray parameter) section and best-fit by hand with two elliptical curves. A cusp in the data suggests a 2 layer structure with a velocity discontinuity at 29 km. The upper layer has a near surface compressional wave velocity of 5.8 km. The lower layer is 17 km thick with a velocity of 7.7 km. This velocity transition suggests that the Conrad discontinuity has been observed in the study area.

Estimation of static corrections for shear‐wave profiling using the dispersion properties of Love waves

When processing shear‐wave data it is often difficult to compute the static corrections using the same methods as for P-waves, because of the low velocities involved and of the interference with residual P-waves. A method especially tailored for static corrections of SH-waves is presented, which makes use of the dispersion property of Love waves. Assuming that the weathered zone of thickness H acts as a wave guide for the surface waves, their phase velocity ranges from [Formula: see text] at infinite frequency to [Formula: see text] at zero frequency, where [Formula: see text] and [Formula: see text] are the shear‐wave velocities into and below the weathered zone. For intermediate frequencies the phase velocity of Love waves is a known function of H, [Formula: see text] and [Formula: see text]. If the phase velocity of Love waves is measured on field records for a number of distinct frequencies, it is therefore possible to compute H, [Formula: see text], [Formula: see text], and the static corrections. In contrast with conventional techniques which require first arrivals to be picked manually, the computation of static corrections using the dispersion of Love waves is essentially based on velocity analysis efficiently performed by digital computers. Three field examples were conducted. The first of these was a noise analysis using 256 geophone positions ranging from 3 to 258 m. The two other examples are CDP lines obtained with polarized SH sources. The geophone group extension is 20 m and the distance between groups is 10 m. In all these examples, the upper layer thickness and velocity values resulting from the Love wave dispersion appeared to be in good agreement with those computed from the conventional methods.

A Three-Dimensional Seismic Survey Applied to Field Development in Williston Basin: ABSTRACT

The Medicine Lake field of Sheridan County, Montana, was discovered in March 1979. In October 1981, a mini-3-D seismic survey covering 2.5 mi2 (6.2 km2) was acquired over this field in order to facilitate development drilling by delineating the field's reservoirs and obtaining a more accurate image of the subsurface structure. A multiline system, consisting of 240 geophone groups distributed on 8 lines, was used. The energy source was shothole dynamite using 5 lbs (2.3 kg) charges at 250 ft (46 m). The shotpoints were arranged in a cross pattern with extra shotpoints included to provide necessary control on the weathered zone. The average subsurface coverage was 600%, with CDP bins 165 ft (50 m) square. Prior to the actual shooting, a computer simulation of the resulting fold was performed to verify the field geometry. The entire survey was recorded in one day with no movement of the geophones, thus minimizing costs. The data volume was processed in preserved amplitude through 3-D migration and 1-D inversion. The subsurface image was substantially improved by the 3-D migration process. The advantages of this enhanced focusing ability are particularly important when attempting to delineate the lateral extent of reservoirs and detect lithologic variations. The Medicine Lake field is located on a structural high, although there are stratigraphic implications for several of the producing zones. The interpretation of the data therefore focused on both structural and stratigraphic features. The Medicine Lake structure is prominently displayed on the Winnipeg event, showing a closure in excess of 180 ft (55 m). Several reflectors near the base of the Red River interval terminated against the Winnipeg event, indicating that this structure was a high in Red River time. Discontinuities in the Cambrian and Precambrian reflectors suggest that the Medicine Lake structure is a result of basement faulting. The objective of the stratigraphic interpretation was to outline zones of possible porosity, particularly in the Madison and Red River intervals. The horizontal and vertical inverted sections were particularly useful for ascertaining the location and lateral extent of those anomalous zones. The results correlate well with known production, and should aid in the location of future development wells. End_of_Article - Last_Page 541------------

A GRAPHICAL METHOD FOR COMPUTING GEOPHONE GROUP RESPONSE

The theory for multiple seismometers or shotholes has been dealt with extensively in the literature (see, for instance, Parr and Mayne, 1955; Savit et al, 1958; White, 1958). In deciding upon a particular pattern, a seismic crew, after having determined the relevant noise characteristics of the area, may follow one of the two paths: (1) the crew may decide on a particular response and may then seek analytically, in a rather involved manner, an optimum geophone group whose response satisfactorily approximates the desired response or (2) the crew may compute the responses of a variety of geophone groups to the predominant noise pulse and accept the one that gives the best overall attenuation. Most seismic crews would choose the second alternative as being simpler and more practical, if less elegant than the first one. Besides, a calculated optimum group almost invariably would require fractional weights for the individual elements that are not practically realizable. Our note describes a very simple graphical procedure for fast computation of array responses that is applicable to a standing sine wave or a pulse and to linear, tapered, equally‐spaced, or unequally‐spaced arrays. The method is so obvious that it is probably in use already in some organizations; although no published evidence to that effect seems to be available.

Sub-bottom reflection measurements in the Tyrrhenian and Ionian Seas

Introduction. During the summer of 1956 several series of point-source sub-bottom reflection shots were recorded aboard R.V. Vema in portions of the Tyrrhenian and Ionian seas along the tracks shown in Figure 1. All measurements were made with the ship underway and towing a 400-meter-long reflection string containing 6 groups of geophones. For a sound source 9-lb charges of tetrytol were exploded at the surface. Bottom and sub-bottom reflection arrivals received at the detector array were fed to a 3-channel AVC amplifier through a step attenuator and bandpass filter and recorded on a conventional oscillograph camera. Each seismogram was read and the arrivals plotted in crosssectional form as the delay time sequence following the first bottom reflection R1. The generalized topography of the sea floor along the track was obtained from precision depth recorder (PDE) data.

STEADY STATE POLAR SENSITIVITY CURVES

The resultant amplitude, “A,” of the combined outputs of a group of geophones is derived as a function of the difference in time of arrival, ΔT, at extreme geophones in the group—the number of geophones and the period of the waves being parameters. “A” plotted as a function of ΔT is shown to have principal and secondary maxima whose amplitudes, separation, and sharpness are discussed. The relative response of the geophone group to waves from every direction is considered and illustrated by polar sensitivity curves. The changes in the polar sensitivity curves effected by changes in the number of geophones, the geophone spread, or the wave length, are considered. The effect of introducing artificially controlled time differences between geophone outputs, so as to vary the direction of maximum response (so‐called variable compounding) as is done in the Rieber Sonograph, is considered. By this means the resultant response of the group of geophones can be focused, so as to emphasize waves arriving from any specific direction. While the results are strictly applicable only to the case of steady state, sinusoidal waves, they may apply qualitatively to all the waves encountered in seismic exploration. The versatility of variable compounding is pointed out.