Ultrashallow Seismic Reflection Research Articles

Integrated reflection and refraction processing of an ultra-shallow seismic survey

SUMMARY Velseis Pty Ltd acquired and processed an ultra-shallow seismic reflection survey designed to image targets with a depth of less than 50m, including the structure of the weathering layer. Several experimental sources were implemented, each with unique frequency and amplitude characteristics. Reflection processing was not routine since the target of interest was the weathering zone itself. Due to this, a combination of reflection and refraction processing was used in order to develop an integrated image and interpretation of the near-surface. The results from the different processing techniques, including refraction (reciprocal method and tomography), reflection, and a depth converted stack, provide an internally consistent interpretation of the base of weathering and layering within the weathering.

超磁歪振動子を利用した極浅層における地中映像化システムに関する研究

Recently, Water and gas pipes are developed frequently because of the increased urban population. When the underground piping is constructed, drawings and signs are needed to drill the ground. However, underground pipings and other buried objects are often buried in different locationscompared with drawings. Underground pipings, the rock and the gravel interfere with construction. It is necessary to avoid this situation. To determine the construction of underground pipes, the non-destructive imaging method previously detects the pipe inside the ground. This paper describes a system of ultrashallow underground imaging method using seismic reflection and seismic giant magnetostrictive transducer. We apply a surface wave analysis using a giant magnetostrictive transducer. We improve the accuracy of P wave velocity. For the high-efficiency setup and the underground imaging succeed. This system combines ultrashallow seismic reflection and the ultra-magnetostrictive transducer. It provides the practical system.

Rapid seismic reflection imaging at the Clovis period Gault site in central Texas

AbstractUsing a modified seismic reflection imaging system with rapid translation of receivers, stratigraphic profiles were collected at the Gault site in central Texas. For rapid data collection, spikeless geophone receivers were placed in sand‐filled bags at tight spacing, and these receivers were rapidly pulled along the ground surface between shots. Shots were produced by a small hammer strike to a vertical pipe at 20‐cm intervals. High quality ultrashallow seismic reflection profiles were collected at a rate of 25 m h−1, significantly faster than what is possible with conventional seismic reflection imaging using individually planted geophones. Ground‐penetrating radar was attempted, but abandoned owing to the poor penetration of the radar signals in the clay soils present at the Gault site. Electromagnetic induction grids were collected surrounding each seismic reflection profile, and provided information on near‐surface ground water. Seismic reflection images of Gault site stratigraphy provided greater depth penetration than accessible from backhoe trenching and coring, and helped to better outline the site geological context. Seismic images reveal coherent reflections at shallow depths (0–2.5 m), and extensive scattering at deeper levels (2.5–8 m), underlain by reflection‐free zones. These data are interpreted as clay and gravel layers overlaying palaeostream channels carved into the limestone bedrock. Where comparative data were available, the geophysical findings were corroborated by observations of site stratigraphy in archaeological excavation units, backhoe trenches and cores. Seismic reflection studies at the Gault site revealed a palaeochannel filled with pre‐Clovis age sediments. Pre‐Clovis age sediments are not known to occur at other locations within the Gault site. They provide a unique opportunity to test for cultural remains of great antiquity. Copyright © 2007 John Wiley & Sons, Ltd.

Combined seismic tomographic and ultrashallow seismic reflection study of an Early Dynastic mastaba, Saqqara, Egypt

AbstractMastabas were large rectangular structures built for the funerals and burials of the earliest Pharaohs. One such mastaba was the basic building block that led to the first known stone pyramid, the >4600‐year old Step Pyramid within the Saqqara necropolis of Egypt. We have tested a number of shallow geophysical techniques for investigating in a non‐invasive manner the subsurface beneath a large Early Dynastic mastaba located close to the Step Pyramid. After discovering that near‐surface sedimentary rocks with unusually high electrical conductivities precluded the use of the ground‐penetrating radar method, a very high‐resolution seismic data set was collected along a profile that extended the 42.5 m length of the mastaba. A sledgehammer source was used every 0.2 m and the data were recorded using a 48‐channel array of single geophones spaced at 0.2 m intervals. Inversions of the direct‐ and refracted‐wave travel times provided P‐wave velocity tomograms of the shallow subsurface, whereas relatively standard processing techniques yielded a high‐fold (50–80) ultrashallow seismic reflection section. The tomographic and reflection images were jointly interpreted in terms of loose sand and friable limestone layers with low P‐wave velocities of 150–650 m s−1 overlying consolidated limestone and shale with velocities > 1500 m s−1. The sharp contact between the low‐ and high‐velocity regimes was approximately horizontal at a depth of ca. 2 m. This contact was the source of a strong seismic reflection. Above this contact, the velocity tomogram revealed moderately high velocities at the surface location of a friable limestone outcrop and two low‐velocity blocks that probably outlined sand‐filled shafts. Below the contact, three regularly spaced low velocity blocks probably represented tunnels and/or subsurface chambers. Copyright © 2005 John Wiley & Sons, Ltd.

Energy and Soil-Moisture Effects on Nonlinear Deformation Associated with Near-Surface Seismic Reflection Sources

The ability to collect seismic-reflection data in the upper [Formula: see text] of the subsurface (ultrashallow seismic reflection: USR) is important because it offers an alternative to ground-penetrating radar in regions exhibiting high attenuation of electromagnetic signal. Previous work has demonstrated that the critical controllable factors in obtaining USR data are dense spatial sampling and high [Formula: see text] frequency content. The frequency content is strongly controlled by the characteristics of the sources used to generate the seismic energy. Qualitative experiments have indicated that the higher-energy near-surface seismic reflection projectile sources lack some of the high-frequency energy exhibited by smaller-energy sources. In addition, soil moisture content has been shown to be an important variable in the frequency content of data even if the same source is used. In both cases, the reduction in the frequency content of the data has been hypothesized to correlate with greater amounts of near-source, nonlinear deformation. A corollary of this hypothesis states that source energy and soil-moisture affect the amount of nonlinear deformation generated by near-surface seismic energy sources. As an initial test of this corollary, experiments have been conducted in a controlled laboratory setting to quantitatively determine the relationship between source energy, soil-moisture conditions, and the amount of near-source, nonlinear deformation. Projectile sources with different energy (ammunition) were fired into a sand pit having varied moisture content, and the resulting voids were filled with plaster. The volume of the casts (representing the most substantial region of nonlinear deformation) was then measured. Results indicate that source energy and soil moisture have a direct correlation with the amount of near-source, nonlinear deformation.

Seismic reflection imaging of an ultrashallow interface from a P‐wave data set with a poor signal‐to‐noise ratio

ABSTRACTIt is well known that pitfalls are commonly encountered in the acquisition and processing of shallow reflection data. Although they can often lead to misinterpretations, the obsession with these difficulties can generate an excessively pessimistic attitude and ultimately lead to rejecting data that contain genuine reflections. The authors revisited a data set acquired during an ultrashallow P‐wave reflection experiment conducted in December 1996 on a subsurface model characterized by one main, complex interface. The model had been purpose‐built to control acquisition and processing decisions, and to contribute to clarifying at least some of the critical aspects inherent in the ultrashallow seismic reflection method. For the experiment, the acquisition geometry was purposely designed regardless of the known characteristics of the model, considering only a target depth of less than ten metres. The data, which had been acquired with CMP techniques, were generally characterized by a poor signal‐to‐noise ratio and had been considered completely useless. Initially, inexperience in acquiring and, above all, processing ultrashallow reflection data, in conjunction with mise‐valuations, had led to disappointing failures. Velocity analysis on reflection data had proved very difficult and standard processing sequences were inadequate. However, in this case a good result was obtained by modelling the near‐surface velocity using direct‐wave traveltimes analysis and by adopting a simple but segregated processing sequence, which proved necessary due to significant velocity gradients and large relative variations of the reflector depth. Time‐to‐depth conversion afforded a more than acceptable result.

Recording seismic reflections using rigidly interconnected geophones

Ultrashallow seismic reflection surveys require dense spatial sampling during data acquisition, which increases their cost. In previous efforts to find ways to reduce these costs, we connected geophones rigidly to pieces of channel iron attached to a farm implement. This method allowed us to plant the geophones in the ground quickly and automatically. The rigidly interconnected geophones used in these earlier studies detected first‐arrival energy along with minor interfering seismic modes, but they did not detect seismic reflections. To examine further the feasibility of developing rigid geophone emplacement systems to detect seismic reflections, we experimented with four pieces of channel iron, each 2.7 m long and 10 cm wide. Each segment was equipped with 18 geophones rigidly attached to the channel iron at 15‐cm intervals, and the spikes attached to all 18 geophones were pushed into the ground simultaneously. The geophones detected both refracted and reflected energy; however, no significant signal distortion or interference attributable to the rigid coupling of the geophones to the channel iron was observed in the data. The interfering seismic modes mentioned from the previous experiments were not detected, nor was any P‐wave propagation noted within the channel iron. These results show promise for automating and reducing the cost of ultrashallow seismic reflection and refraction surveys.

Fast 3D ultra shallow seismic reflection imaging using portable geophone mount

We present the results of three different seismic experiments designed to develop and evaluate the use of 3D Ultra Shallow Seismic Reflection for high‐resolution near‐surface imaging. A feasibility study and the first implementation of a 2D portable geophone mount for fast and cost‐effective ultra‐shallow 3D seismic data acquisition are discussed. The portable geophone mount is made out of an anelastic base with a frame and an array of 72 geophones spaced with an interval of 0.25m both in the inline and crossline direction. The array enables acquisition of very high‐resolution 3D data cubes with bin size of 12.5 × 12.5cm and wavelets of 350Hz. The time for re‐planting the 72‐channels portable geophone mount is about 5min in the field. Thus, we show that high‐resolution near‐surface 3D images can be obtained fast and in a cost‐effective manner. This has the potential of providing effective solutions for many geotechnical and geoenvironmental applications.

Ultrashallow seismic reflection monitoring of seasonal fluctuations in the water table

Abstract Ultrashallow seismic-reflection data were collected at a test site in Great Bend, Kansas. The purpose of the experiment was to image seasonal submeter-scale fluctuations in the water table over a period of one year to identify the factors important in monitoring the water table when using seismic-reflection techniques. The study indicates that detailed velocity information must be used when interpreting water-table levels. Using detailed velocity information as a control when depth-converting the seismic profiles yielded correct positioning of the water table within + or -12 cm at the test site.