An example of extreme near-surface variability in shallow seismic reflection data

2016 ◽  
Vol 4 (3) ◽  
pp. SH1-SH9
Author(s):  
Steven D. Sloan ◽  
J. Tyler Schwenk ◽  
Robert H. Stevens
Keyword(s):  
Seismic Reflection ◽  
Field Data ◽  
Low Velocity ◽  
Seismic Reflection Data ◽  
Near Surface ◽  
Shallow Subsurface ◽  
Reflection Data ◽  
Shallow Seismic ◽  
Low Velocity Layer ◽  
Velocity Layer

Variability of material properties in the shallow subsurface presents challenges for near-surface geophysical methods and exploration-scale applications. As the depth of investigation decreases, denser sampling is required, especially of the near offsets, to accurately characterize the shallow subsurface. We have developed a field data example using high-resolution shallow seismic reflection data to demonstrate how quickly near-surface properties can change over short distances and the effects on field data and processed sections. The addition of a relatively thin, 20 cm thick, low-velocity layer can lead to masked reflections and an inability to map shallow reflectors. Short receiver intervals, on the order of 10 cm, were necessary to identify the cause of the diminished data quality and would have gone unknown using larger, more conventional station spacing. Combined analysis of first arrivals, surface waves, and reflections aided in determining the effects and extent of a low-velocity layer that inhibited the identification and constructive stacking of the reflection from a shallow water table using normal-moveout-based processing methods. Our results also highlight the benefits of using unprocessed gathers to pragmatically guide processing and interpretation of seismic data.

Low velocity layer characterization in the Niger Delta: Implications for seismic reflection data quality

2017 ◽  
Vol 90 (2) ◽  
pp. 187-195
Author(s):  
A. I. Opara ◽  
C. C. Agoha ◽  
C. N. Okereke ◽  
U. P. Adiela ◽  
C. N. Onwubuariri ◽  
...  
Keyword(s):  
Data Quality ◽  
Niger Delta ◽  
Seismic Reflection ◽  
Low Velocity ◽  
Seismic Reflection Data ◽  
Reflection Data ◽  
Low Velocity Layer ◽  
Velocity Layer

Application of waveform tomography to marine seismic reflection data from the Queen Charlotte Basin of western Canada

Geophysics ◽  
2011 ◽  
Vol 76 (2) ◽  
pp. B55-B70 ◽  
Author(s):  
E. M. Takam Takougang ◽  
A. J. Calvert
Keyword(s):  
Seismic Reflection ◽  
Field Data ◽  
Velocity Model ◽  
P Wave ◽  
Low Velocity ◽  
Seismic Line ◽  
Seismic Reflection Data ◽  
Reflection Data ◽  
Sonic Log ◽  
Waveform Tomography

To obtain a higher resolution quantitative P-wave velocity model, 2D waveform tomography was applied to seismic reflection data from the Queen Charlotte sedimentary basin off the west coast of Canada. The forward modeling and inversion were implemented in the frequency domain using the visco-acoustic wave equation. Field data preconditioning consisted of f-k filtering, 2D amplitude scaling, shot-to-shot amplitude balancing, and time windowing. The field data were inverted between 7 and 13.66 Hz, with attenuation introduced for frequencies ≥ 10.5 Hz to improve the final velocity model; two different approaches to sampling the frequencies were evaluated. The limited maximum offset of the marine data (3770 m) and the relatively high starting frequency (7 Hz) were the main challenges encountered during the inversion. An inversion strategy that successively recovered shallow-to-deep structures was designed to mitigate these issues. The inclusion of later arrivals in the waveform tomography resulted in a velocity model that extends to a depth of approximately 1200 m, twice the maximum depth of ray coverage in the ray-based tomography. Overall, there is a good agreement between the velocity model and a sonic log from a well on the seismic line, as well as between modeled shot gathers and field data. Anomalous zones of low velocity in the model correspond to previously identified faults or their upward continuation into the shallow Pliocene section where they are not readily identifiable in the conventional migration.

Large near‐surface velocity gradients on shallow seismic reflection data

Geophysics ◽  
1998 ◽  
Vol 63 (4) ◽  
pp. 1348-1356 ◽  
Author(s):  
Richard D. Miller ◽  
Jianghai Xia
Keyword(s):  
Seismic Reflection ◽  
Surface Velocity ◽  
Coherent Noise ◽  
Lower Velocity ◽  
Seismic Reflection Data ◽  
Near Surface ◽  
Reflection Data ◽  
Shallow Seismic ◽  
Velocity Gradients ◽  
Large Velocity

Extreme velocity gradients occasionally present within near‐surface materials can inhibit optimal common midpoint (CMP) stacking of near‐surface reflection arrivals. For example, abrupt increases in velocity are observed routinely at the bedrock surface and at the boundary between the vadose and the saturated zone. When a rapid increase in near‐surface velocity is found, NMO correction artifacts manifested on CMP gathers as sample reversion, sample compression, or duplication of reflection wavelets can reduce S/N ratio on stacked data or can stack coherently. Elimination of these nonstretch‐related artifacts using conventional NMO-stretch muting requires near‐vertically incident reflection arrivals and allowable stretch ratios as small as 5% in some shallow environments. Radical allowable stretch mutes are not a feasible means to subdue these artifacts if high‐amplitude coherent noise on near‐offset traces inhibits identification and digital enhancement of shallow reflections. On most shallow seismic reflection data, long‐offset reflection arrivals (but less than wide angle) are critical to the generation of an interpretable stacked section. The difference in offset between the optimum window for shallow reflections within unsaturated sediments and reflections from the underlying saturated or consolidated‐material portion of the section inherently limits the effectiveness of conventional NMO corrections. Near‐surface average velocity increases of 200% in less than two wavelengths and at two‐way traveltimes less than 60 ms are not uncommon on shallow reflection data. Near‐surface reflections separated by large velocity gradients can rarely be accurately or optimally CMP processed using conventional approaches to NMO corrections. Large velocity‐gradient shallow reflection data require segregation of shallow lower velocity reflections from higher velocity reflections during processing to maximize the accuracy and resolution potential of the stacked section, as shown by examples herein.

An example of the effects of near-surface variability on shallow seismic reflection data

2015 ◽  
Author(s):  
Steven D. Sloan* ◽  
Matt Ralston ◽  
Robert H. Stevens ◽  
J. Tyler Schwenk
Keyword(s):  
Seismic Reflection ◽  
Seismic Reflection Data ◽  
Near Surface ◽  
Reflection Data ◽  
Shallow Seismic ◽  
Surface Variability

High‐resolution shallow‐seismic experiments in sand, Part I: Water table, fluid flow, and saturation

Geophysics ◽  
1998 ◽  
Vol 63 (4) ◽  
pp. 1225-1233 ◽  
Author(s):  
Ran Bachrach ◽  
Amos Nur
Keyword(s):  
High Resolution ◽  
Water Table ◽  
High Tide ◽  
Saturated Sand ◽  
Low Velocity ◽  
Near Surface ◽  
Shallow Seismic ◽  
Low Velocity Layer ◽  
Dry Sand ◽  
Velocity Layer

A high‐resolution, very shallow seismic reflection and refraction experiment was conducted to investigate the seismic response of groundwater level changes in beach sand in situ. A fixed 10-m-long receiver array was used for repeated seismic profiling. Direct measurements of water level in a monitoring well and moisture content in the sand were taken as well. The water table in the well changed by about 1 m in slightly delayed response to the nearby ocean tides. In contrast, inversion of the seismic data yielded a totally different picture. The reflection from the water table at high tide appeared at a later time than the reflection at low tide. This unexpected discrepancy can be reconciled using Gassmann’s equation: a low‐velocity layer must exist between the near‐surface dry sand and the deeper and much faster fully saturated sand. This low‐velocity layer coincides with the newly saturated zone and is caused by a combination of the sand’s high density (close to that of fully saturated sand), and its high compressibility (close to that of dry sand). This low‐velocity zone causes a velocity pulldown for the high‐frequency reflections, and causes a high‐tide reflection to appear later in time than low‐tide reflection. The calculated velocities in the dry layer show changes with time that correlate with sand dryness, as predicted by the temporal changes of the sand’s density due to changing water/air ratio. The results show that near‐surface velocities in sand are sensitive to partial saturation in the transition zone between dry and saturated sand. We were able to extract the saturation of the first layer and the depth to the water table from the seismic velocities. The high‐resolution reflections monitored the flow process that occurred in the sand during the tides, and provided a real‐time image of the hydrological process.

scholarly journals Mapping a bedrock surface under dry alluvium with shallow seismic reflections

Geophysics ◽  
1989 ◽  
Vol 54 (12) ◽  
pp. 1528-1534 ◽  
Author(s):  
Richard D. Miller ◽  
Don W. Steeples ◽  
Michael Brannan
Keyword(s):  
Seismic Reflection ◽  
Dominant Frequency ◽  
Seismic Reflection Data ◽  
Near Surface ◽  
Analog To Digital ◽  
Evaporation Pond ◽  
Reflection Data ◽  
Shallow Seismic ◽  
Analog To Digital Conversion ◽  
Alluvial Material

Shallow seismic‐reflection techniques were used to image the bedrock‐alluvial interface, near a chemical evaporation pond in the Texas Panhandle, allowing optimum placement of water‐quality monitor wells. The seismic data showed bedrock valleys as shallow as 4 m and accurate to within 1 m horizontally and vertically. The normal‐moveout velocity within the near‐surface alluvium varies from 225 m/s to 400 m/s. All monitor‐well borings near the evaporation pond penetrated unsaturated alluvial material. On most of the data, the wavelet reflected from the bedrock‐alluvium interface has a dominant frequency of around 170 Hz. Low‐cut filtering at 24 dB/octave below 220 Hz prior to analog‐to‐digital conversion enhanced the amplitude of the desired bedrock reflection relative to the amplitude of the unwanted ground roll. The final bedrock contour map derived from drilling and seismic‐reflection data possesses improved resolution and shows a bedrock valley not interpretable from drill data alone.

Migration of shallow seismic reflection data

Geophysics ◽  
1994 ◽  
Vol 59 (3) ◽  
pp. 402-410 ◽  
Author(s):  
Ross A. Black ◽  
Don W. Steeples ◽  
Richard D. Miller
Keyword(s):  
Seismic Reflection ◽  
Large Data ◽  
Flow Chart ◽  
Seismic Section ◽  
Data Set ◽  
Seismic Reflection Data ◽  
Near Surface ◽  
Reflection Data ◽  
Shallow Seismic ◽  
Simple Set

We present an analysis of migration effects on seismic reflection images of very shallow targets such as those that are common objectives of engineering, groundwater, and environmental investigations. We use an example of seismic reflection data from depths of 5 to 15 m that show negligible effect from migration, despite the apparent steep dip on the seismic section. Our analysis of the question of when to migrate shallow reflection data indicates it is critical to take into account the highly variable near‐surface velocities and the vertical exaggeration on the seismic section. A simple set of calculations is developed as well as a flow chart based on the “migrator’s equation” that can predict whether migration of an arbitrary shallow seismic section is advisable. Because shallow reflection data are often processed on personal computers, unnecessary migration of a large data set can be prohibitively time‐consuming and wasteful.

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