Common Midpoint Data Research Articles

The San Gabriel Mountains bright reflective zone: possible evidence of young mid-crustal thrust faulting in southern California

During the Los Angeles Region Seismic Experiment (LARSE), a reflection/refraction survey was conducted along a line extending northeastward from Seal Beach, California, to the Mojave Desert, crossing the Los Angeles basin and San Gabriel Mountains. Shots and receivers were spaced most densely through the San Gabriel Mountains for the purpose of obtaining a combined reflection and refraction image of the crust in that area. A stack of common-midpoint (CMP) data reveals a bright reflective zone, 1-s thick, that dominates the stack and extends throughout most of the mid-crust of the San Gabriel Mountains. The top of this zone ranges in depth from 6 s (∼18-km depth) in the southern San Gabriel Mountains to 7.5 s (∼23-km depth) in the northern San Gabriel Mountains. The zone bends downward beneath the surface traces of the San Gabriel and San Andreas faults. It is brightest between these two faults, where it is given the name San Gabriel Mountains ‘bright spot’ (SGMBS), and becomes more poorly defined south of the San Gabriel fault and north of the San Andreas fault. The polarity of the seismic signal at the top of this zone is clearly negative, and our analysis suggests it represents a negative velocity step. The magnitude of the velocity step is approximately 1.7 km/s. In at least one location, an event with positive polarity can be observed 0.2 s beneath the top of this zone, indicating a thickness of the order of 500 m for the low-velocity zone at this location. Several factors combine to make the preferred interpretation of this bright reflective zone a young fault zone, possibly a ‘master’ decollement. (1) It represents a significant velocity reduction. If the rocks in this zone contain fluids, such a reduction could be caused by a differential change in fluid pressure between the caprock and the rocks in the SGMBS; near-lithostatic fluid pressure is required in the SGMBS. Such differential changes are believed to occur in the neighborhood of active fault zones, where ‘fault-value’ action has been postulated. Less likely alternative explanations for this velocity reduction include the presence of magma and a change in composition to serpentinite or metagraywacke. (2) It occurs at or near the brittle-ductile transition, at least in the southern San Gabriel Mountains, a possible zone of concentrated shear. (3) A thin reflection rising from its top in the southern San Gabriel Mountains projects to the hypocenter of the 1987 M 5.9 Whittier Narrows earthquake, a blind thrust-fault earthquake with one focal plane subparallel to the reflection. Alternatively, one could argue that the bends or disruptions in the reflective zone seen at the San Gabriel and San Andreas faults are actually offsets and that the reflective zone is therefore an older feature, possibly an older fault zone.

Shallow structure of the Yadong‐Gulu rift, southern Tibet, from refraction analysis of Project INDEPTH common midpoint data

Refracted arrivals on International Deep Profiling of Tibet and the Himalaya (INDEPTH) common midpoint (CMP) data provide insight into the shallow structure of the Yadong‐Gulu rift in southern Tibet. This is the first such information available for the Neogene‐Quaternary rifts on the Tibetan Plateau. Geologic cross sections across three separate valleys within the rift have been constructed from these data and available surface geological information. In each case, the valleys are floored by asymmetric, low‐velocity sedimentary fills ranging from ∼100 m to ∼1.5 km in thickness. Beneath all three, the basement appears to dip gently to the east (∼ 5°). Seismic reflection data from one of these valleys (Nyinzhong) show a ∼30° east dipping reflector within the basement beneath the valley that projects upward to the normal‐sense Nyainqentanglha detachment exposed in the range immediately to the west. Field investigations by INDEPTH personnel have additionally shown the existence of previously unrecognized normal‐sense shear zones bordering the other two valleys. The newly discovered shear zones are 200 to >500 m thick and in both instances are characterized by moderately to steeply west dipping mylonites with top‐to‐west shear sense. Metamorphic minerals within the shear zones indicate at least amphibolite grade metamorphism with increasing metamorphic grade down section. The seismic data, coupled with the new geologic information and geomorphologic data from Landsat imagery, suggest that the structure of the northern Yadong‐Gulu rift is comparable to other well‐documented rift systems in which steep range‐front normal faults terminate at, or sole into, moderately dipping normal‐sense detachments at depth that have undergone substantial footwall uplift and rotation during extension. This geometry implies that the extension of the Tibetan Plateau crust has been accommodated at depth by lateral flow within the middle crust. It also suggests that E‐W extension of the plateau may have been substantially greater than has generally been presumed.

Contribution of volcanism and tectonism to axial and flank morphology of the southern East Pacific Rise, 17°10′–17°40′S, from a study of layer 2A geometry

Multichannel seismic images from the East Pacific Rise 17°10′–17°40′S are used to study the geometry of seismic layer 2A in the near‐axis region. Wave‐equation datuming is applied to stacked common midpoint data to remove the distorting effects of seafloor topography on the layer 2A event. Processed stacks are compared with seafloor morphology and tectonic fabric imaged in side‐scan sonar data. Assuming layer 2A corresponds with the extrusive crust, these data are used to study the relationship between volcanism and tectonism in the accumulation of the extrusive layer and to assess the contribution of extrusives to ridge‐crest and ridge‐flank morphology. We find variations in axial thickening of layer 2A which imply twofold variation in the width of the extrusive layer accumulation zone, as well as systematic changes in the pattern of accumulation of this layer. On the ridge flanks the layer 2A horizon mimics abyssal hill relief, consistent with a horst and graben origin for this topography. Short‐wavelength variations in the thickness of layer 2A are superimposed on this relief, which we attribute to volcanic modification of tectonic topography at the edge of the neovolcanic zone. The integrated off‐axis thickness of layer 2A is correlated with ridge cross‐sectional area and indicates persistent (100,000 years) spatial gradients in extrusive layer thickness. Systematic changes in the cross‐axis shape of the ridge, observed over distances of 5–10 km, cannot be attributed to the extrusive layer, and it appears that axial structure beneath the volcanics governs the cross‐axis morphology of the ridge.

Seismic data filtering using a Gabor representation

Gabor representations are signal expansions using sets of functions that are localized and concentrated in time and frequency. This characteristic makes them suitable candidates for filtering data where the desired signal or noise is nonstationary or time-dependent. It is shown that Gabor representations formulated with frame theory can be used for time-dependent noise removal. Furthermore, their ability to filter noise in the presence of a nonstationary signal enables them to outperform singular value decomposition eigenimage filtering techniques in the removal of incoherent noise in common midpoint data with a moderate fold. >

Incomplete AVO near Salt Structures and Impact for AVO Analysis

Amplitude versus offset (AVO) measurements for deep hydrocarbon-bearing sands can be compromised when made in close proximity to a shallow salt piercement structure. Anomalous responses are observed, particularly on low acoustic impedance bright spots. Common-midpoint (CMP) data from key seismic profiles traversing the bright spots do not show the expected Class 3 offset responses as defined by Rutherford and Williams (1989). On these CMPs, significant decrease of far trace energy is observed. CMP data from other seismic profiles off-structure do exhibit the Class 3 offset responses, implying that structural complications may be interfering with the offset response. A synthetic AVO gather was generated using well log data, which supports the off-structure Class 3 responses, further reinforcing the concept of structurally-biased AVO responses. Acoustic, pseudo-spectral modelling of the structure substantiates the misleading AVO response. Pseudo-spectral modelling results suggest signal degradation observed on the far offsets is caused by wavefield refraction — a shadow zone, where the known hydrocarbon-bearing sands are not completely illuminated. Such shadow zones obscure the correct AVO response, which may have bearing on exploration and development.

Application of model-based transformation for seismic interpretational analysis

A new method for seismic interpretational analysis is proposed. The method is based on the model-based transformation (MBT) described by Nass (1989). This transformation was originally proposed as a useful tooI for noise analysis and for improving signal-to-noise ratio on common midpoint (CMP) data. The latter was achieved by optimized muting applied to traces of transformed CMP gathers. In this paper a different application of MBT for detailed seismic interpretational analysis will be discussed and demonstrated using field data examples.

Spatial Sampling Effects of a Multi-Source Multi-Cable Recording Configuration

Acquiring large volumes of marine 3-D seismic data using conventional single-source single-streamer recording vessels is no longer considered cost effective. By comparison a dual-source dual-cable recording configuration quadruples the number of subsurface kilometres recorded for each sail line. Taken to the extreme, a 'quadquad' system can record twelve common-midpoint (CMP) lines per sail line. In this manner large data volumes can be acquired economically over a shorter time interval. This efficient method of data capture does have the unfortunate characteristic of low-fold coverage and non-uniform offset distribution for each subsurface line. For a dual-source dual-receiver configuration the offset distribution, within the CMP gather of any single subsurface line, repeats every four CMPs. For a 'quad-quad' system the CMP offset distribution repeats every eighth CMP Data acquired in the above manner deviates from the stack-array criterion proposed by Anstey, and as a consequence a residual of the coherent noise present in the raw data will pass through to the stack. It is demonstrated that the 'herring-bone' pattern observed on some stacked data is directly related to the acquisition configuration. The North West Shelf of Australia is characterized by a hard limestone water bottom. This reflector generates a series of strong periodic multiple events which dominate the amplitude spectrum of the data. The paper will illustrate that the wavelength of the residual multiple is related to both the recording configuration and the water depth. Synthetic models are used to simulate data collected using a multi-source multi-receiver recording configuration. f-k domain displays of synthetic stacked data are used to illustrate why the residual multiple energy is concentrated in particular areas of f-k space. Having established the origin of the residual noise trains on the stacked data, we suggest methods to remove them from the data. Traditional multiple-attenuation techniques, post-stack f-k notch filters and shot-record interpolation are suggested as possible processing techniques which will attenuate the 'herring-bone' signature from the stacked section.

Seismic reflection results from an ice island over the Arctic continental Shelf

In 1982 an Arctic ice island, 3 km by 7 km and 41 m thick, broke away from the Ward-Hunt Ice Shelf on Ellesmere Island and began drifting southwest with the ocean currents. This island provided a stable platform in the polar pack ice, for the seismic reflection exploration of the Arctic Continental Shelf north of Axel Heiberg Island. A seismic reflection array, consisting of a 107 station, 3.53 km, main arm and a twelve station, 2.4 km, cross arm, was constructed on the island. A set of 6 s common midpoint (CMP) data was recorded from the thirteen shotpoints for each kilometre the island drifted. Each charge, 1 kg of 60% Geogel, was detonated 110 m beneath the ice surface. Over July and August, 1985, a total of 87 sets of shots were recorded. The raw record sections were dominated by very strong low-velocity waves and water layer reverberations. Up to fourteen distinct water bottom multiples are seen in both the raw shot gathers and the CMP-stacked sections. Median, ( f– k) and optimum array filters were used along with CMP stacking, deconvolution and common shot gather stacking to reduce the water-bound noise. These processes were tested on both simulation data and the real data. A P-wave travelling through the ice layer dominated most of the first arrivals. First break refraction analysis, where possible, revealed 600–800 m of locally varying shallow sedimentary rock (4–5 km s −1) overlying a higher velocity layer (5.5–6 km s −1). The CMP sections contain numerous short discontinuous reflections indicating a lack of simple structure in the high-velocity rock. The larger (107) fold of the common shot gather stack increased the signal to noise ratio and revealed an extensive, roughly horizontal reflection at a two-way time of 1.6 s. This corresponds to a depth of about 4.2 km and is interpreted as a reflection from the top of the Precambrian basement.

Interval Velocity Analysis by Tau-P Methods

Good velocity control is extremely important in seismic data processing. Stacking velocities are needed to perform normal moveout correction before compositing common midpoint data (Taner and Koehler, 1969). Interval velocities serve as a lithology indicator and are also needed for depth migration (Schultz and Sherwood, 1980).

A look beyond the CMP concept

For the majority of present generation geophysicists the common mid-point (CMP) technique is the natural way to acquire and process seismic reflection data. Somehow it is always taken for granted that the paths to improvements in acquisition and processing lead through and not around the CMP concept. More often than not, the question of using non-CMP techniques is not even raised. Over the last quarter of a century our industry has become rigidly attached to a concept which in most geophysical exploration circles is no longer subjected to critical examination. It seems, however, that in the era of interactive graphics and supercomputers the CMP concept is no longer immune to critical review. The CMP method was developed independently by several geophysicists, but the first patent application was apparently filed in 1950 by H. Mayne. With the introduction of computers, digital processing systems began to gradually replace the analogue systems. The CMP concept was admirably suited for digital processing and soon it became almost unthinkable to implement it in any other way but digitally. In the early stages of digital seismic processing the question of whether the CMP method could be dispensed with was not seriously investigated, primarily because the production of CMP stacked traces was considered the zenith of the state of the art and also because the early seismic computer could not do much else. The CMP technique became even more popular when a velocity analysis based on scanning of the CMP gathers was introduced (Taner and Koehier 1969). However, the annoying property of the stacking velocity to vary with azimuth of the seismic line was recognised much later (Hubral 1976). This and other related findings have caused considerable inconvenience to the proponents of 3-D CMP data acquisition and processing. Today the basic elements needed to write a new chapter in seismic data acquisition and processing are in place; most larger oil companies have supercomputers, interactive graphics capabilities and rather well-trained geophysicists. This article is intended to show that CMP techniques as practised today are tantamount to destruction of a good part of the information content of the input field data, and that given sufficient computing power, alternative techniques may be designed which do not suffer similar drawbacks.

Velocity determination in layered systems with arbitrarily curved interfaces by means of wave field extrapolation of CMP data

The paper starts with a brief review of conventional velocity determination procedures for plane‐layered systems. These methods assume hyperbolic moveout curves in common midpoint (CMP) data. It is shown that in layered systems with arbitrarily curved interfaces these methods fail since the moveout curves are nonhyperbolic. The subject of this paper is a wave‐theoretical approach to velocity determination. By means of wave field extrapolation of CMP data, nonhyperbolic moveout curves are transformed into hyperbolic ones. The proposed process is called velocity replacement (VR) since an inhomogeneous overburden is replaced by a homogeneous velocity medium. The effect of VR is illustrated on synthetic data. From the results it may be concluded that velocity determination after VR yields significantly more accurate results than velocity determination before VR. The technique of VR is also proposed as a preprocessing tool prior to stack in situations of nonhyperbolic moveout curves.

The traveltime equation, tau‐p mapping, and inversion of common midpoint data

Seismic traveltime data can be described in terms of instantaneous slope (dT/dX) and intercept time (τ). With this parameterization, the most general form of the traveltime equation is developed and found to be valid for reflections and refractions, all source‐receiver offsets, and all commonly used experimental geometries. For common source‐receiver experiments, the instantaneous slope of the traveltime data represents the true horizontal ray parameter p, and the intercept time is the sum along the ray of thickness‐vertical slowness products referenced to the fixed surface station. For common midpoint (CMP) experiments, we show that the instantaneous slope is the average slowness of upgoing and downgoing rays at the surface, and the intercept time is again the sum of thickness‐vertical slowness products, but referenced to the common midpoint. This inherent averaging immediately explains the superiority of the CMP experiment in the presence of dip. In addition to providing a general traveltime equation, we find that τ-p parameterization reorganizes traveltime data in a way which greatly facilitates interpretation. For reflections, the τ-p trajectories are sums of ellipses and all turning rays (whether refracted or postcritically reflected) combine to form a well defined and easily identified trajectory. We present a method for deriving a velocity‐depth function from this trajectory which is applicable to both refracted and reflected arrivals.