scholarly journals Avoiding pitfalls in shallow seismic reflection surveys

Geophysics ◽  
1998 ◽  
Vol 63 (4) ◽  
pp. 1213-1224 ◽  
Author(s):  
Don W. Steeples ◽  
Richard D. Miller
Keyword(s):  
Seismic Reflection ◽  
Seismic Waves ◽  
Oil And Gas ◽  
Digital Processing ◽  
Petroleum Exploration ◽  
Near Surface ◽  
Surface Reflection ◽  
Oil And Gas Exploration ◽  
Reflection Data ◽  
Shallow Seismic

Acquiring shallow reflection data requires the use of high frequencies, preferably accompanied by broad bandwidths. Problems that sometimes arise with this type of seismic information include spatial aliasing of ground roll, erroneous interpretation of processed airwaves and air‐coupled waves as reflected seismic waves, misinterpretation of refractions as reflections on stacked common‐midpoint (CMP) sections, and emergence of processing artifacts. Processing and interpreting near‐surface reflection data correctly often requires more than a simple scaling‐down of the methods used in oil and gas exploration or crustal studies. For example, even under favorable conditions, separating shallow reflections from shallow refractions during processing may prove difficult, if not impossible. Artifacts emanating from inadequate velocity analysis and inaccurate static corrections during processing are at least as troublesome when they emerge on shallow reflection sections as they are on sections typical of petroleum exploration. Consequently, when using shallow seismic reflection, an interpreter must be exceptionally careful not to misinterpret as reflections those many coherent waves that may appear to be reflections but are not. Evaluating the validity of a processed, shallow seismic reflection section therefore requires that the interpreter have access to at least one field record and, ideally, to copies of one or more of the intermediate processing steps to corroborate the interpretation and to monitor for artifacts introduced by digital processing.

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.

Near‐surface seismic reflection profiling of the Matanuska Glacier, Alaska

Geophysics ◽  
2003 ◽  
Vol 68 (1) ◽  
pp. 147-156 ◽  
Author(s):  
Gregory S. Baker ◽  
Jeffrey C. Strasser ◽  
Edward B. Evenson ◽  
Daniel E. Lawson ◽  
Kendra Pyke ◽  
...  
Keyword(s):  
High Resolution ◽  
Seismic Reflection ◽  
Longitudinal Axis ◽  
Horizontal Distribution ◽  
P Wave ◽  
Near Surface ◽  
Surface Reflection ◽  
Reflection Data ◽  
Basal Ice ◽  
Matanuska Glacier

Several common‐midpoint seismic reflection profiles collected on the Matanuska Glacier, Alaska, clearly demonstrate the feasibility of collecting high‐quality, high‐resolution near‐surface reflection data on a temperate glacier. The results indicate that high‐resolution seismic reflection can be used to accurately determine the thickness and horizontal distribution of debris‐rich ice at the base of the glacier. The basal ice thickens about 30% over a 300‐m distance as the glacier flows out of an overdeepening. The reflection events ranged from 80‐ to 140‐m depth along the longitudinal axis of the glacier. The dominant reflection is from the contact between clean, englacial ice and the underlying debris‐rich basal ice, but a strong characteristic reflection is also observed from the base of the debris‐rich ice (bottom of the glacier). The P‐wave propagation velocity at the surface and throughout the englacial ice is 3600 m/s, and the frequency content of the reflections is in excess of 800 Hz. Supporting drilling data indicate that depth estimates are correct to within ± 1 m.

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

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.

scholarly journals Formation environments and mechanisms of multistage paleokarst of Ordovician carbonates in Southern North China Basin

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Haitao Zhang ◽  
Guangquan Xu ◽  
Mancai Liu ◽  
Minhua Wang
Keyword(s):  
Carbonate Rocks ◽  
North China ◽  
Oil And Gas ◽  
Carbonate Reservoir ◽  
Carbonate Reservoirs ◽  
Geochemical Data ◽  
Near Surface ◽  
Oil And Gas Exploration ◽  
Pore Types ◽  
North China Basin

AbstractWith the reduction of oil and gas reserves and the increase of mining difficulty in Northern China, the carbonate rocks in Southern North China Basin are becoming a significant exploration target for carbonate reservoirs. However, the development characteristics, formation stages, formation environments and mechanisms of the carbonate reservoirs in Southern North China Basin are still unclear, which caused the failures of many oil and gas exploration wells. This study focused on addressing this unsolved issue from the Ordovician carbonate paleokarst in the Huai-Fu Basin, which is located in the southeast of Southern North China Basin and one of the key areas for oil and gas exploration. Based on petrology, mineralogy and geochemical data, pore types, distribution characteristics, and formation stages of the Ordovician paleokarst were analyzed. Then, in attempt to define the origins of porosity development, the formation environments and mechanisms were illustrated. The results of this study showed that pore types of the Ordovician carbonates in the Huai-Fu Basin are mainly composed of intragranular pores, intercrystalline (intergranular) pores, dissolution pores (vugs), fractures, channels, and caves, which are usually in fault and fold zones and paleoweathering crust. Furthermore, five stages and five formation environments of the Ordovician paleokarst were identified. Syngenetic karst, eogenetic karst, and paleoweathering crust karst were all developed in a relatively open near-surface environment, and their formations are mainly related to meteoric water dissolution. Mesogenetic karst was developed in a closed buried environment, and its formation is mainly related to the diagenesis of organic matters and thermochemical sulfate reduction in the Permian-Carboniferous strata. Hydrothermal (water) karst was developed in a deep-buried and high-temperature environment, where hydrothermal fluids (waters) migrated upward through structures such as faults and fractures to dissolve carbonate rocks and simultaneously deposited hydrothermal minerals and calcites. Lastly, a paleokarst evolution model, combined with the related porosity evolution processes, nicely revealed the Ordovician carbonate reservoir development. This study provides insights and guidance for further oil and gas exploration in the Southern North China Basin, and also advances our understanding of the genesis of carbonate paleokarst around the world.

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