Steven D. Sloan

Acquisition and processing pitfall associated with clipping near-surface seismic reflection traces

The processing of clipped seismic traces may produce high-frequency wavelets that can be misinterpreted as reflections in filtered shot gathers and common-midpoint (CMP) stacked sections. To illustrate this effect, a near-surface CMP seismic reflection survey was conducted using two sources to compare the effects of various band-pass frequency filters on clipped traces. An event observed in the clipped data set replicated the frequency of the filter operators applied, similar to the effect of convolving a boxcar function with the filter operator. The anomaly exhibited hyperbolic moveout and imitated a reflection during the processing stages. The hyperbolic event was flattened by NMO corrections chosen for the target reflection, and it stacked in as a coherent event in the final section. Clipped data should be removed or corrected before processing to prevent misinterpreting high-frequency reflection artifacts in trace gathers and stacked sections.

Shallow seismic AVO variations related to partial water saturation during a pumping test

This is the published version. Copyright 2007 by the American Geophysical Union. All Rights Reserved.

Near-surface Void Identification Using MASW And Refraction Tomography Techniques

Subsurface voids may manifest themselves as natural or anthropogenic dissolution features, illegal cross-border tunnels, or abandoned mines. Detection of these voids using geophysical methods has often proven difficult due to multiple factors including depth-to-diameter ratio, lack of resolution, non-uniqueness, etc. Experiments were conducted at a test site with a known subsurface void to determine the capability of multiple near-surface seismic methods as applied to void detection. In this study, refraction tomography and multichannel analysis of surface wave methods successfully identified a man-made void approximately three meters deep. Results of these experiments correlated well with the expected results, exhibiting decreased velocity, the absence of seismic ray coverage, and a high shear-wave velocity halo above the void.

Fixed-source and fixed-receiver walkaway seismic noise tests: A field comparison

Seismologists and geophysical literature often use the term “walkaway” to describe any survey used to analyze wavetrains based on source-to-receiver offset. A distinction should be made between receiver-group moveout (fixed-source walkaway) and source moveout (fixed-receiver walkaway) when multiple channels simultaneously record signal from multiple independent geophones. Three data sets are presented that illustrate this distinction: one collected in an area where a fixed-receiver walkaway survey recorded similar data and was more time efficient than a fixed-source walkaway survey, and two others collected in an area where dipping reflectors and laterally varying velocities caused the fixed-receiver walkaway data to be significantly different than the fixed-source walkaway data. The results show that, while still useful, clarity in recorded data is lost when fixed-receiver walkaway surveys are substituted for fixed-source walkaway surveys in areas with uneven surface topography, dipping interfaces, or la...

Combining near‐surface seismic reflection and ground‐penetrating radar data in the depth domain

Shallow seismic reflection (SSR) and ground-penetrating radar (GPR) data of approximately the same wavelength were collected over a coincident volume of geologic material in an effort to image the water table at a test site near Clay Center, KS. Although the top of the saturated zone was successfully imaged using both techniques, no significant reflectors were imaged above the water table using SSR or below the water table using GPR. The SSR and GPR data sets were depth converted and vertically stacked to provide a more complete image of the subsurface. Instead of having two different sections representing depths of ~0-5 m (GPR) and ~4-30 m (SSR), the combined section includes the near-surface stratigraphy, water table, and bedrock, among other geologic features. It is important to note, however, that this method of data combination is intended for qualitative purposes only because the two methods measure different physical properties of the subsurface and quantitative analysis of the vertically stacked section may lead to erroneous results.

Joint Shear‐Wave Analysis Using MASW and Refraction Traveltime Tomography

We use the multichannel analysis of surface waves (MASW) and SH-wave refraction traveltime tomography to constrain each method’s starting layer-model at a site located at the Yuma Proving Ground, AZ. Our treatment resulted in a more stratified, higher-resolution MASW S-wave velocity (Vs) model that deviated from the tomographic SH-wave solution by approximately 15%. The top 12 m were our target at the test site, which is comprised of roughly 30 m of unconsolidated sediments overlying bedrock. A bungee-assisted drop-weight source was used for MASW and a hammer and block were used for the refraction survey. A preliminary MASW profile served as the starting model for refraction inversion. The resultant Vs tomogram contained slightly higher velocity ranges and imaged a highvelocity layer not apparent with initial MASW results. Using this a priori information, along with density values from well logs, more layers were added to an updated Vs model for MASW inversion. The subsequent higher-resolution MASW profile moved toward the tomographic solution. The use of two inversion schemes allowed a more constrained Vs model of the site, while limiting resolution uncertainties for the MASW Vs section. Moreover, the possible over-parameterization problem seen with increased layers is defended through refraction comparison. We recommend the combined approach as a means to constrain MASW inversion and increase confidence in future Vs appraisals.

Near-surface shear-wave velocity measurements in unlithified sediment

S-wave velocity can be directly correlated to material stiffness and lithology making it a valuable physical property that has found uses in construction, engineering, and environmental projects. This study compares different methods for measuring S-wave velocities, investigating and identifying the differences among the methods’ results, and prioritizing the different methods for optimal S-wave use at the U. S. Army’s Yuma Proving Grounds (YPG). Multichannel Analysis of Surface Waves (MASW) and S-wave tomography were used to generate S-wave velocity profiles. Each method has advantages and disadvantages. A strong signal-to-noise ratio at the study site gives the MASW method promising resolution. S-wave first arrivals are picked on impulsive sledgehammer data which were then used for the tomography process. Three-component downhole seismic data were collected in-line with a locking geophone, providing ground truth to compare the data and to draw conclusions about the validity of each data set. Results from these S-wave measurement techniques are compared with borehole seismic data and with lithology data from continuous samples to help ascertain the accuracy, and therefore applicability, of each method. This study helps to select the best methods for obtaining S-wave velocities for media much like those found in unconsolidated sediments at YPG.

Shear‐Wave Velocity as an Indicator of Increased Stress and Failure Potential Associated with Dissolution‐Mining Voids

The use of dissolution wells for mining salt has been common practice for over a century, leaving behind brine-filled “salt jugs” or voids in the subsurface which, over time, can migrate through overlying rock formations, potentially leading to sinkhole formation and public safety hazards. In an effort to determine the relative range of stress on the roof rock above these jugs at various stages of failure, evaluate the extent of void migration, and aid in remediation planning, shear-wave reflection surveys were conducted at a well field near Hutchinson, Kansas, where drill-confirmed dissolution features exist in the 125-m deep Hutchinson Salt Member. Since shear-wave velocity (Vs) is directly related to stress through the shear modulus, Vs profiles were analyzed at known well locations to determine if measurable variations in velocity were consistent with zones likely experiencing elevated relative stress and therefore, increased failure potential. Changes in Vs upwards of 20% were observed along lines where voids were known to be present and had experienced significant migration into the shale cap rock. Localized velocity changes were coincident with the findings of drilling and associated borehole investigations. Alluvium velocities for the same lines remained relatively constant, suggesting that Vs changes are constrained to consolidated rock above mapped subsurface dissolution voids and are not related to depositional changes in the unconsolidated overburden.

Ultra-shallow seismic imaging of the top of the saturated zone

This is the published version. Copyright 2010 by the American Geophysical Union. All Rights Reserved.

Automating the acquisition of 3D near-surface seismic reflection data

Summary This paper describes new instrumentation to acq uire threedimensional (3D) shallow seismic reflection (SSR) data in a more cost-effective manner. The “3D Autojuggie”, in its current configuration, is capable of simultaneously planting 220 geophones in less than one minute. All 220 geophones can be picked up in the same amount of time so that the entire spread can be moved and redeployed. This paper presents a field test comparison between conventional hand-planted and hydraulically planted geophones using the 3D Autojuggie during a walk-away test. Data acquired are directly comparable and nearly indistinguishable. The 3D Autojuggie deploys geophones in a regular grid and allows them to remain connected to the cables and seismographs during re-positioning and acquisition, reducing significantly the time necessary to roll geophones and cables and the number of crew members required for 3D seismic surveying. Initial results of a 3D ultra-shallow seismic reflection survey show that data acquisition using the 3D Autojuggie is more efficient than conventional methods, including an increase of ~18% in the area covered per hour, ~65% reduction in labor, and a 500% increase in