Weight Drop Source Research Articles

Developing a distributed acoustic sensing seismic land streamer: Concept and validation

Motivated by existing cabled seismic land-streamer designs, we develop a distributed acoustic sensing (DAS) land-streamer system for high-resolution near-surface seismic data acquisition. The system consists of a DAS interrogator unit (IU), a fiber-optic cable attached beneath a fire-hose assembly for environmental isolation and improved fiber-ground coupling, and a vehicle-mounted accelerated weight-drop source. The DAS land streamer is easily deployed and towed along the ground surface, allowing for spatially dense data acquisition. We report two field tests with the developed hardware to evaluate the DAS land-streamer performance. The first test investigates the effects of hose weight on the surface-deployed fiber and indicates that this approach improves fiber-ground coupling and leads to an improved signal-to-noise ratio (S/N). The second test demonstrates that the DAS land streamer records waveforms with similar phase and moveouts as those recorded by horizontal geophones but offers a higher native spatial density advantage than standard geophone arrays, leading to spatially dense waveforms, improved S/N after postprocessing, and superior surface-wave acquisition array mobility. Our findings suggest that a DAS land streamer is a promising alternative to traditional geophone-based surveys and may offer several advantages, such as faster survey acquisition speed and lower field costs due to reduced acquisition hardware requirements. However, methodological limitations include recording a single horizontal ground-motion component, a dependence on favorable fiber-ground coupling conditions, and the upfront cost of IU procurement. A DAS land streamer may be useful in numerous subsurface applications, such as (pseudo) 1D multichannel analysis of surface waves or 2D surface-wave inversion for S-wave velocity model estimation for geophysical, geologic, geotechnical, and environmental investigations.

A novel weight-drop seismic energy source for subsurface characterization

Multichannel Analysis of Surface Waves (MASW) studies for subsurface characterization in Central Ganga Basin of India, where thick cover of alluvium absorbs most of the vibrations generated by a sledgehammer source, give low quality seismic data at large offsets and generally provide imaging depth lesser than 30 m due to low quality of dispersion curves at lower frequencies. Therefore, a novel seismic energy source is designed and developed to produce large amplitude surface waves even at longer offsets and for deeper depths of imaging using dispersion analysis of surface waves. The developed source for seismic data acquisition is low-cost, repeatable, portable, and easy to operate with no environmental impact. Unlike the sledgehammer source where the quality of surface waves generated also depends on the person hitting the metal plate, the seismic data quality from the novel weight-drop source is independent of the operator.The developed source is an impact type where a free-falling mass accelerated due to gravitational pull hits onto a base plate coupled with the earth generating vibrations in the ground. The performance of the source was evaluated in a field trial. A receiver array of 24-channel cabled geophones with 3 m spacing was planted on the ground. Seismic data were acquired using the source at 5 m negative offset inline from the first geophone. Seismic data were also recorded with the sledgehammer as a source for comparison. Five shot gathers for each source were acquired with the same acquisition geometry and source position.Shot gathers were analyzed and compared using trace attributes, 1D and 2D Fourier spectra, signal-to-noise ratio, relative energy, and dispersion images. The study shows that the developed source produces low frequency high amplitude signal and better dispersion image, notably in the lower frequency range, in comparison with the sledgehammer source. Therefore, the developed source is adapted to investigate deeper subsurface velocity structure with dispersion curve from high amplitude surface wave analysis. Moreover, the repeatability study suggests that the repeat shot gathers acquired from the developed source have remarkably high degree of similarity. Therefore, seismic data acquisition with the developed source does not require repeat shots for vertical stacking as often done in case of a hammer source. Sound pressure level measurement indicates that the developed source has no adverse environmental impact on the operator. From the field trial of the developed source, it is convinced that the source is cost-effective, portable, easy to operate and handle and is environmentally friendly.

Noise suppression using a near-source wavelet

Denoising becomes a nontrivial task when the noise and signal overlap in multiple domains, such as time, frequency, and velocity. Fortunately, signal and noise waveforms, in general, tend to remain morphologically different and such differences can be used to separate body-wave signals from other waveforms such as ground roll and cultural noise. The key in denoising using a near-source wavelet is to find a wavelet that is a close approximation of the true source signature and remains uncontaminated by the Green’s function in any significant manner. An inverse filter designed using such a wavelet selectively compresses the body waves that can be extracted using median and low-pass filters. The overall phenomenon is explained with a synthetic example. The idea is also tested on a land data set generated using a large weight-drop source in which the wavelet recorded approximately 3 m from the source location fulfills the criteria set in our method. The results suggest that the incremental effort of recording an extra trace close to the source location during acquisition may provide previously unavailable denoising opportunities during processing, although the trace itself may be redundant for imaging.

Three-dimensional supervirtual seismic refraction interferometry: A case study in western Saudi Arabia

The semiautomatic seismic refraction supervirtual interferometry (SVI) algorithm has been developed to improve the conventional SVI method. The conventional SVI method uses convolution techniques and involves the raw trace, which reintroduces noise back into the enhanced trace. However, the semiautomatic method uses a first-arrival reference picked from a raw trace to compute the arrival times of all enhanced virtual traces. The semiautomatic SVI method has been extended recently from 2D to 3D geometry and applied on a synthetic 3D seismic data set using the raw traces of only one inline. We have developed a case study of the semiautomatic 3D SVI method by applying the algorithm on an active seismic refraction data set that consists of 82,944 raw traces from 288 shot gathers that use an accelerated weight drop source. Due to possible differences in the source wavelet among shots, the semiautomatic 3D SVI method is applied on the 288 raw traces from each shot gather separately. The SVI technique generates 41,328 distinct correlograms from one shot, which results in the production of a trace with a much better signal-to-noise ratio.

Reflexně seismický výzkum pozdně kenozoické zlomové tektoniky na vybraných lokalitách hornomoravského úvalu

We measured shallow reflection seismic profiles across the assumed faults in the Late Cenozoic (Pliocene – Holocene) Upper Morava Basin (UMB). The faults in the UMB are indicated by horst-and-graben morphology, differential thickness of Pliocene and Quaternary siliciclastic sediments, considerable gravity gradients a present-day seismicity. Four seismic lines, 380 to 860 m long (fixed geophone spread) were designed to cross the assumed faults at three sites, Mezice, Drahlov and Výšovice. The data were acquired by 24-channel ABEM Terraloc Mk-8 seismic system with PEG-40 accelerated weight drop source and processed by Sandmaier ReflexW and Halliburton Landmark ProMax® seismic processing software. The processing included application of filters (DC shift, scaled windowgain, bandpass frequency and muting), stacking using normal moveout constant velocity stack, additional application of subtrack-mean (dewow) filter, topographic correction and low velocity layer static correction. Distinct reflectors were detected up to 400 ms TWT, which corresponds to maximum depth of 280 and 350 m at 1400 and 1750 km.s-1 velocities, respectively. The observed reflection patterns were classified into three seismic facies, which were interpreted as crystalline rocks (Brunovistulicum) and/or well consolidated Paleozoic sedimentary rocks (SF1), unconsolidated Quaternary siliciclastic sediments (SF2) and semi-consolidated Neogene clays (SF3) based on the cores drilled in their close vicinity. Distinct faults were observed at the Drahlov and Výšovice 2 profile, which coincided with the observed topographic steps between the horsts and grabens. Presence of the fault at the Drahlov profile separating the Hněvotín Horst from the Lutín Graben was demonstrated by independent electrical resistivity tomography profile. On the other hand, another topographic step at the Mezice profile, between the Hněvotín Horst and Olomouc Graben, does not correspond to any seismic indication of a fault. The reflection seismic proved to be useful and relatively low-cost method to visualize the shallow subsurface geology in the Upper Morava Basin.

Joint Body‐ and Surface‐Wave Tomography of Yucca Flat, Nevada, Using a Novel Seismic Source

AbstractIn preparation for the next phase of the Source Physics Experiments, we acquired an active‐source seismic dataset along two transects totaling more than 30 km in length at Yucca Flat, Nevada, on the Nevada National Security Site. Yucca Flat is a sedimentary basin which has hosted more than 650 underground nuclear tests (UGTs). The survey source was a novel 13,000 kg modified industrial pile driver. This weight drop source proved to be broadband and repeatable, richer in low frequencies (1–3 Hz) than traditional vibrator sources and capable of producing peak particle velocities similar to those produced by a 50 kg explosive charge. In this study, we performed a joint inversion of P‐wave refraction travel times and Rayleigh‐wave phase‐velocity dispersion curves for the P‐ and S‐wave velocity structure of Yucca Flat. Phase‐velocity surface‐wave dispersion measurements were obtained via the refraction microtremor method on 1 km arrays, with 80% overlap. Our P‐wave velocity models verify and expand the current understanding of Yucca Flat’s subsurface geometry and bulk properties such as depth to Paleozoic basement and shallow alluvium velocity. Areas of disagreement between this study and the current geologic model of Yucca Flat (derived from borehole studies) generally correlate with areas of widely spaced borehole control points. This provides an opportunity to update the existing model, which is used for modeling groundwater flow and radionuclide transport. Scattering caused by UGT‐related high‐contrast velocity anomalies substantially reduced the number and frequency bandwidth of usable dispersion picks. The S‐wave velocity models presented in this study agree with existing basin‐wide studies of Yucca Flat, but are compromised by diminished surface‐wave coherence as a product of this scattering. As nuclear nonproliferation monitoring moves from teleseismic to regional or even local distances, such high‐frequency (>5 Hz) scattering could prove challenging when attempting to discriminate events in areas of previous testing.

Deep onshore reflection seismic imaging of the chalk group strata using a 45 kg accelerated weight-drop and combined recording systems with dense receiver spacing

The Chalk Group forms important hydrocarbon reservoirs offshore and water aquifers onshore Denmark. Within a day of fieldwork, a 450 m long reflection seismic profile was acquired onshore in an area in southeast Denmark, where the Chalk Group extends almost to the surface and is approximately 900 m thick. The main objective of the study was to image the complete Chalk Group in high resolution and to study the origin of reflectivity within the different chalk units. A 45 kg accelerated weight-drop source, in combination with dense receiver spacing using microelectromechanical sensors mounted on a streamer and 48 planted geophones, was used for data acquisition. The profile runs subparallel to the cliffs of Stevns, and the recorded signal reaches the base of the Chalk Group at approximately 600 ms. The fully cored 443 m-deep Stevns-1 borehole, which is located at the recorded seismic line, provides excellent control on lithologic and facies changes. Comparison with the borehole data demonstrates that our seismic data set provides a high-resolution image of the internal layering of the Chalk Group. We find that the internal reflection coefficients of the Chalk Group are, in general, small based on wireline-log data. However, the reflected amplitudes are just big enough to be recorded with the receiver setup used, even from the pure chalk beds of the Chalk Group. The reflectivity seen on the high-resolution seismic profile is influenced by occurrences of clay-enriched chalk layers. Flint bands consisting of numerous flint nodules are a characteristic of the uppermost part of the Chalk Group at Stevns. The flint nodules appear to produce significant scattering of the seismic signals, and flint-rich layers appear with diffuse internal reflectivity characteristics. Outcrop-scale mound structures in Danian and Upper Cretaceous outcrops are for the first time seismically resolved.

High-resolution seismic imaging of Paleozoic rocks in the Mora area, Siljan Ring structure, central Sweden

The Late Devonian Siljan Ring structure in Sweden is the largest known impact structure in Europe. The present-day structure comprises a central dome that is about 20–30 km in diameter, which is surrounded by a ring-shaped depression. In this study, we focus on the southwestern part of the Siljan Ring with the aim to map the structure of the Paleozoic sedimentary rocks. Four 2D high-resolution seismic lines with a total length of about 3 km were acquired in the Mora area. A three component eighty-unit land streamer, combined with wireless recorders, was used for data acquisition along with a weight drop source. Processing of the data shows that clear reflections are present, but results are less distinct where external noise was present during acquisition or the maximum source-receiver offset was too short. Petrophysical measurements on core samples, core log data and a density model along one line were used to guide the interpretation of the seismic sections. These data demonstrate that fault blocks are present in the study area and that the individual blocks have been affected differently by impact-related tectonics.

Experiences with a Permanently Installed Seismic Monitoring Array at the CO2 Storage Site at Ketzin (Germany). - A Status Overview -

Since July 2008, CO2 is injected into a saline aquifer near the town of Ketzin in Germany. For monitoring the CO2- migration close to the injection well, TNO installed a fixed 2D seismic array of 120 meters length in 2009, with 3- component geophones at the surface, 4-component receivers at 50 meters depth and a central vertical array of 4- component receivers. This specific test acquisition set-up was and is being used both for the recording of high-quality active time-lapse seismic data as well as for continuous passive seismic data recording. The latter gave rise to the identification of a large number of surface noise related events and some very weak events possibly originating from the deeper subsurface.The active seismic data acquisition consisted of a conventional repeat survey after 2 years using an accelerated weight drop source, as well as a test with a prototype semi-permanent source located at the site during a period of 3 weeks in which CO2 injection was stopped. In both cases subtle changes at the reservoir level have been observed, though the limitations of the experimental lay-out make it difficult to come up with firm conclusions in terms of CO2 induced pressure and saturation changes. Further analysis of the data is ongoing work.

Seismic monitoring for the Otway CO2 sequestration: acquisition and analysis of borehole seismic data

The Australian Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC) is currently undertaking the Otway Project, which involves the injection and storage of 100,000 tones of carbon dioxide within the subsurface. CO2 injection will be into Naylor onshore depleted gas reservoir and, therefore, the project will provide important experience for monitoring and verification under these conditions. The overall complexities of the field, its deep, small size and in particular the presence of both free and residual gas zones present a serious challenge for time-lapse seismic monitoring. Borehole seismic has a strong advantage over surface seismic: energy crosses surface layers only once, and hence is much less sensitive to the variations in the weathered layer properties than surface seismic. Consequently a comprehensive borehole seismic observational program was designed at the Otway project. In the initial phase a Zero Offset VSP, an Offset VSP, and walkaway VSP data were acquired with a minivibroseis (6000 lb) seismic source in the Naylor-1 well in May 2006. In 2007, in the newly drilled injection well (CRC-1) a series of wireline logs, a Zero Offset, an Offset VSP and the first 3DVSP was acquired with a weight drop source simultaneously with 3D surface seismic. The results of VSP data analysis will be shown and discussed in light of its improved repeatability and image resolution in comparison to surface seismic data. We will also discuss evolving workflows developed to overcome inherently poor land seismic repeatability which is mainly related to changes in the near surface layer conditions.

A Seismic Inversion Experiment to Identify Scatterers in a Granite Outcrop

A scattering inversion experiment was conducted on exposed Panola granite located southeast of Atlanta, Georgia, in the Panola Mountain Research Watershed. The objective was to test a near-surface seismic technique for locating subsurface discontinuities, such as fractures, that can be strong scatterers of seismic waves. Shallow water-productive fractures in the crystalline and metamorphic rocks underlying expanding suburban and urban areas, like those in central Georgia, are important because they have the potential of significantly augmenting surface freshwater supplies. Sixteen vertical [Formula: see text] geophones were placed in an array with aperture of [Formula: see text]. A weight-drop source was deployed around the array at distances of [Formula: see text]. We successfully recorded seismic waves in the frequency range of [Formula: see text] traveling through the unweathered Panola granite. The lack of overburden suppressed the interference from dispersed surface waves and enhanced coupling of the high-frequency energy source to the granite. We calculated semblance coefficients for waves propagating across the array as a function of time, back-azimuth and apparent velocity and used these to locate the source of the scattered waves within a [Formula: see text] area extending [Formula: see text] below the surface. Zones that were strong scatterers of surface waves correlated with topographic irregularities on the rock outcrop and the location of a small creek. Scattered [Formula: see text]- and [Formula: see text]-waves identified a three-dimensional distribution of scatterers which outlines the downward extension of fractures below the stream and the contact of the granite with amphibolite biotite gneiss.

Seismoelectric imaging of the vadose zone of a sand aquifer

We have acquired a [Formula: see text] seismoelectric section over an unconfined aquifer to demonstrate the effectiveness of interfacial signals at imaging interfaces in shallow sedimentary environments. The seismoelectric data were acquired by using a [Formula: see text] accelerated weight-drop source and a 24-channel seismoelectric recording system composed of grounded dipoles, preamplifiers, and seismographs. In the shot records, interfacial signals were remarkably clear; they arrived simultaneously at offsets as far as [Formula: see text] from the seismic source. The most prominent signal was generated at the water table at a depth of approximately [Formula: see text] and had peak amplitudes on the order of [Formula: see text]. A weaker response was generated at a shallower interface that is interpreted to be a water-retentive layer. The validity of these two laterally continuous events, and of other discontinuous events indicative of vadose-zone heterogeneity, is corroborated by the presence of reflections exhibiting similar characteristics in a ground-penetrating radar profile acquired along the same line.

Seismic Reflection Images of Shallow Faulting, Northernmost Mississippi Embayment, North of the New Madrid Seismic Zone

High-resolution seismic reflection surveys document tectonic faults that displace Pleistocene and older strata just beyond the northeast termination of the New Madrid seismic zone, at the northernmost extent of the Mississippi embayment. These faults, which are part of the Fluorspar Area fault complex in southeastern Illinois, are directly in line with the northeast-trending seismic zone. The reflection data were acquired using an elastic weight-drop source recorded to 500 msec by a 48-geophone array (24-fold) with a 10-ft (∼3.0 m) station interval. Recognizable reflections were recorded to about 200 msec (100–150 m). The effects of multiple reflections, numerous diffractions, low apparent velocity (i.e., steeply dipping) noise, and the relatively low-frequency content of the recorded signal provided challenges for data processing and interpreting subtle fault offsets. Data processing steps that were critical to the detection of faults included residual statics, post-stack migration, deconvolution, and noise-reduction filtering. Seismic migration was crucial for detecting and mitigating complex fault-related diffraction patterns, which produced an apparent `folding' of reflectors on unmigrated sections. Detected individual offsets of shallow reflectors range from 5 to 10 m for the top of Paleozoic bedrock and younger strata. The migrated sections generally indicate vertical to steeply dipping normal and reverse faults, which in places outline small horsts and/or grabens. Tilting or folding of stratal reflectors associated with faulting is also locally observed. At one site, the observed faulting is superimposed over a prominent antiformal structure, which may itself be a product of the Quaternary deformation that produced the steep normal and reverse faults. Our results suggest that faulting of the Paleozoic bedrock and younger sediments of the northern Mississippi embayment is more pervasive and less localized than previously thought.

A note on pulse broadening and anelastic attenuation in near-surface rocks

A pulse rise-time method has been used to study pulse broadening on seismograms generated by a weight drop source at distances up to 600 m. Both source and receiver were placed on glacial overburden overlying a gneiss-monzonite rock body. One of two data sets showed a significant increase in pulse rise time, τ, as a function of travel time, T. This increase, if due to anelastic attenuation in the uppermost part of the rock body, implies a Q value of 243 ± 53, assuming a linear relationship between τ and T. The data were not capable of discriminating between the models of pulse broadening of Gladwin and Stacey ( τ ∞ T) and Ricker ( τ ∞ T 1 2 ).