Accelerated Weight Drop Research Articles

Seismic Characterization of Subsurface Structures at Rock Valley, Nevada

ABSTRACT The Source Physics Experiment (SPE) aims at improving explosion monitoring techniques by investigating source characteristics of chemical explosions in geologic formations. One of the critical tasks in Rock Valley Direct Comparison (RV/DC), SPE phase III, is to prepare for the main experiment by characterizing the subsurface structures at the test site. Based on the seismic data acquired during an accelerated-weight-drop (AWD) seismic survey at Rock Valley, we first pick the P-wave first-arrival travel times and derive a P-wave velocity model using the adjoint-state first-arrival travel-time tomography. We then apply reverse-time migration to the processed seismic data and obtain high-resolution images of the subsurface structures along the two main survey lines. Our migration results show several reflectors corresponding to major geologic formation boundaries. We employ a multitask machine learning model to enhance the reverse-time migration images and identify faults from these images. We find that our automatically picked faults correlate well with the locations of known faults in the region in addition to many geologically undetected faults. Our subsurface characterization results refine our understanding of the geology in this region and provide valuable velocity and structural information for RV/DC geologic model building and fault identification.

An Open-Access Data Set of Active-Source and Passive-Wavefield DAS and Nodal Seismometer Measurements at the Newberry Florida Site

Abstract This article documents a comprehensive subsurface imaging experiment using seismic waves in a well-studied outdoor laboratory at Newberry, Florida, which is known for significant spatial variability, karstic voids, and underground anomalies. The experiment used approximately two kilometers of distributed acoustic sensing (DAS) fiber-optic cable, forming a dense 2D array of 1920 horizontal-component channels, and a 2D array of 144 SmartSolo three-component nodal seismometers, to sense active-source and passive-wavefield seismic waves. The active-source data were generated using a powerful, triaxial vibroseis shaker truck (T-Rex) and impact sources (accelerated weight drop and an eight-pound sledgehammer) that were simultaneously recorded by both the DAS and nodal seismometers. The vibroseis truck was used to excite the ground in three directions (two horizontal and one vertical) at 260 locations inside and outside the instrumented array, whereas the impact sources were used at 268 locations within the instrumented array. The passive-wavefield data recorded using the nodal seismometers comprised 48 hr of ambient noise collected over a period of four days in four 12-hour time blocks, whereas the passive wavefield data collected using DAS consisted of four hours of ambient noise recordings. This article aims to provide a comprehensive overview of the testing site, experiment layout, the DAS and nodal seismometer acquisition parameters, and implemented raw data processing steps. Although potential use cases, such as surface-wave testing, full-waveform inversion, and ambient noise tomography, are discussed relative to example data, the focus of this article is on documenting this unique data set and presenting its initial data quality rather than on generating subsurface imaging results. The raw and processed data, along with detailed documentation of the experiment and Python tools to aid in visualizing the DAS data set, have been made publicly available.

Hydrologic Impacts of a Strike-Slip Fault Zone: Insights from Joint 3D Body-Wave Tomography of Rock Valley

ABSTRACT The Rock Valley fault zone (RVFZ), an intraplate strike-slip fault zone in the southern Nevada National Security Site (NNSS), hosted a series of very shallow (<3 km) earthquakes in 1993. The RVFZ may also have hydrological significance within the NNSS, potentially playing a role in regional groundwater flow, but there is a lack of local hydrological data. In the Spring of 2021, we collected active-source accelerated weight drop seismic data over part of the RVFZ to better characterize the shallow subsurface. We manually picked ∼17,000 P-wave travel times and over 14,000 S-wave travel times, which were inverted for P-wave velocity (VP), S-wave velocity (VS), and VP/VS ratio in a 3D joint tomographic inversion scheme. Seismic velocities are imaged as deep as ∼700 m in areas and generally align with geologic and structural expectations. VP and VS are relatively reduced near mapped and inferred faults, with the most prominent lower VP and VS zone around the densest collection of faults. We image VP/VS ratios ranging from ∼1.5 to ∼2.4, the extremes of which occur at a depth of ∼100 m and are juxtaposed across a fault. One possible interpretation of the imaged seismic velocities is enhanced fault damage near the densest collection of faults with relatively higher porosity and/or crack density at ∼100 m depth, with patches of semiperched groundwater present in the sedimentary rock in higher VP/VS areas and drier rock in lower VP/VS areas. A relatively higher VP/VS area beneath the densest faults persists at depth, which suggests percolation of groundwater via the fault damage zone to the regionally connected lower carbonate aquifer. Potentially, the presence and movement of groundwater may have played a role in the 1993 earthquake aftershocks.

Practical Application of UAV Source in Complex Area Exploration

Unmanned Aerial Vehicle(UAV) source is a new source for shallow seismic exploration. UAV source is based upon the principle of an accelerated weight drop from a high altitude. A 5-kilogram object is carried 500 meters above the surface, controlled by special remote control equipment to free fall at the exact moment we need it, and by hitting the ground at high speed, it generates a powerful amount of seismic waves. Different from traditional seismic sources, such as explosive sources and vibroseis, UAVs are not limited by terrain and it can generate seismic waves in complex environments such as mountains and rivers, with strong flexibility. Meanwhile, it’s impact object is recyclable, so the cost per excitation is minimal. In this paper, Firstly, we analyze the frequency information of the reflected signal in the seismic record obtained by the UAV source in the actual mountainous area, and then discuss the influence of different excitation positions on the record. Finally, we obtain the reflected seismic profile of the region through seismic data processing. It has been proven that UAV source can achieve low-cost and high-quality acquisition in complex environments

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.

WSGF — Time for change part 2: Forces and energies acting on the seismic source and earth systems

In land geophysical exploration an impulsive or vibratory source, operating at the earth’s surface, is used to generate the seismic signal during the field acquisition. Comparisons of the manufacturer’s output specifications will often be a factor in selecting a seismic source for ‘good S/N’ (signal to noise ratio) in a particular survey area. For impulsive sources, such as dynamite, weight drop, and accelerated weight drop (AWD), the source specifications are given in energy units (Joules, foot-pounds) whereas vibroseis sources are rated in force units (Newtons, pounds-force). For either type of source, the manufacturer’s specifications are the output ‘power’ of the source mechanism and do not take into account any inefficiencies/losses in energy in the source mechanism or the interacting earth volume when generating the seismic signal. In this paper, I examine some of the factors that determine the transfer of energy from the source mechanism to the seismic signal and show, for AWD sources, the energy transfer during the impulse can be greatly improved by optimization of the source parameters. For vibrators, the energy transfer to the ground may be a better indicator of the propagating wave than the current WSGF estimates.

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.

Dense Seismic Array Study of a Legacy Underground Nuclear Test at the Nevada National Security Site

ABSTRACTThe complex postdetonation geologic structures that form after an underground nuclear explosion are hard to constrain because increased heterogeneity around the damage zone affects seismic waves that propagate through the explosion site. Generally, a vertical rubble-filled structure known as a chimney is formed after an underground nuclear explosion that is composed of debris that falls into the subsurface cavity generated by the explosion. Compared with chimneys that collapse fully, leaving a surface crater, partially collapsed chimneys can have remnant subsurface cavities left in place above collapsed rubble. The 1964 nuclear test HADDOCK, conducted at the Nevada test site (now the Nevada National Security Site), formed a partially collapsed chimney with no surface crater. Understanding the subsurface structure of these features has significant national security applications, such as aiding the study of suspected underground nuclear explosions under a treaty verification. In this study, we investigated the subsurface architecture of the HADDOCK legacy nuclear test using hybrid 2D–3D active source seismic reflection and refraction data. The seismic data were acquired using 275 survey shots from the Seismic Hammer (a 13,000 kg weight drop) and 65 survey shots from a smaller accelerated weight drop, both recorded by ∼1000 three-component 5 Hz geophones. First-arrival, P-wave tomographic modeling shows a low-velocity anomaly at ∼200 m depth, likely an air-filled cavity caused by partial collapse of the rock column into the temporary postdetonation cavity. A high-velocity anomaly between 20 and 60 m depth represents spall-related compaction of the shallow alluvium. Hints of low velocities are also present near the burial depth (∼364 m). The reflection seismic data show a prominent subhorizontal reflector at ∼300 m depth, a short-curved reflector at ∼200 m, and a high-amplitude reflector at ∼50 m depth. Comparisons of the reflection sections to synthetic data and borehole stratigraphy suggest that these features correspond to the alluvium–tuff contact, the partial collapse cavity, and the spalled layer, respectively.

A field test and performance evaluation of accelerated weight drop (awd250 v1.0) for shallow seismic survey

The acquisition of seismic reflection involves generating and recording seismic wave field on/near the surface of the earth. Success of producing high quality seismic data depend on choice of seismic sources. There are commonly seismic sources for land seismic surveys such as explosive sources, vibrators and Accelerated Weight Drop (AWD). In some cases, the use of explosive sources often faces many problems, especially related to the costs, permission and environmental issues. The use of vibrators is also similar, facing problems such as the availability of access roads, permits and the environment impact. The use of AWD seems to be a solution as an alternative seismic sources when explosive or vibroseis cannot be used for some reasons. In this research, we have designed and built a mechanical seismic source named AWD250V1.0. This hand-made AWD prototype was developed for shallow seismic surveys. To evaluate the performance of this tool and to improve the design of the prototype, a field tests have been carried out. The test was conducted in Ciparay village, Bandung Regency, West Java. The survey area is mostly paddy field with the near surface covered by clay and highly seismic attenuation. The purposes is simply whether AWD can work properly or not while maintaining data quality and safety standards. In addition, the test is intended to find out how deep penetration of seismic waves using this source. Seismic recorders used were 96 channels with interval between receivers was 10 meters. Findings showed that the deepest reflection can be seen around 450 millisecond or approximately 400 meters. To sum up, the device is work well and is able to produce good quality of seismic sources. The prototype can also meet design criteria that are repeatability, portability, economical and environmetal friendly.

3-D seismic travel-time tomography validation of a detailed subsurface model: a case study of the Záncara river basin (Cuenca, Spain)

Abstract. A high-resolution seismic tomography survey was acquired to obtain a full 3-D P-wave seismic velocity image in the Záncara river basin (eastern Spain). The study area consists of lutites and gypsum from a Neogene sedimentary sequence. A regular and dense grid of 676 shots and 1200 receivers was used to image a 500 m×500 m area of the shallow subsurface. A 240-channel system and a seismic source, consisting of an accelerated weight drop, were used in the acquisition. Half a million travel-time picks were inverted to provide the 3-D seismic velocity distribution up to 120 m depth. The project also targeted the geometry of the underground structure with emphasis on defining the lithological contacts but also the presence of cavities and fault or fractures. An extensive drilling campaign provided uniquely tight constraints on the lithology; these included core samples and wireline geophysical measurements. The analysis of the well log data enabled the accurate definition of the lithological boundaries and provided an estimate of the seismic velocity ranges associated with each lithology. The final joint interpreted image reveals a wedge-shaped structure consisting of four different lithological units. This study features the necessary key elements to test the travel time tomographic inversion approach for the high-resolution characterization of the shallow subsurface. In this methodological validation test, travel-time tomography demonstrated to be a powerful tool with a relatively high capacity for imaging in detail the lithological contrasts of evaporitic sequences located at very shallow depths, when integrated with additional geological and geophysical data.

Relative amplitude preservation in high-resolution shallow reflection seismic: a case study from Fore-Sudetic Monocline, Poland

The acquisition parameters and methodology of seismic data processing for high-resolution seismic imaging viewed through relative amplitude preservation are presented. An example of the obtaining of high-quality, shallow seismic data with a variable end-on spread is shown. The source used for the project is an accelerated weight drop. The study area lies within the mine waste disposal area, near Rudna village (Fore-Sudetic Monocline, WS Poland), and results are given for a 2D experimental profile. The aim of the project was to design optimal acquisition and processing parameters for the detailed recognition of Tertiary deposits. The proposed acquisition parameters are a compromise between time, cost and results. High-resolution seismic imaging enables the determining of layers within the range of thicknesses between 5 and 15 m, while the maximal depth of imaging reaches 400 m.

Near-surface structure of the Carpathian Foredeep marginal zone in the Roztocze Hills area

Shallow seismic survey was made along 1280 m profile in the marginal zone of the Carpathian Foredeep. Measurements performed with standalone wireless stations and especially designed accelerated weight drop system resulted in high fold (up to 60), long offset seismic data. The acquisition has been designed to gather both high-resolution reflection and wide-angle refraction data at long offsets. Seismic processing has been realised separately in two paths with focus on the shallow and deep structures. Data processing for the shallow part combines the travel time tomography and the wide angle reflection imaging. This difficult analysis shows that a careful manual front mute combined with correct statics leads to detailed recognition of structures between 30 and 200 m. For those depths, we recognised several SW dipping tectonic displacements and a main fault zone that probably is the main fault limiting the Roztocze Hills area, and at the same time constitutes the border of the Carpathian Forebulge. The deep interpretation clearly shows a NE dipping evaporate layer at a depth of about 500–700 m. We also show limitations of our survey that leads to unclear recognition of the first 30 m, concluding with the need of joint interpretation with other geophysical methods.

Monitoring CO2 Storage at Cranfield, Mississippi with Time-Lapse Offset VSP – Using Integration and Modeling to Reduce Uncertainty

Abstract A time-lapse Offset Vertical Seismic Profile (OVSP) data set was acquired as part of a subsurface monitoring program for geologic sequestration of CO2. The storage site at Cranfield, near Natchez, Mississippi, is part of a detailed area study (DAS) site for geologic carbon sequestration operated by the U.S. Dept. of Energy's Southeast Regional Carbon Sequestration Partnership (SECARB). The DAS site includes three boreholes, an injection well and two monitoring wells. The project team selected the DAS site to examine CO2 sequestration multiphase fluid flow and pressure at the interwell scale in a brine reservoir. The time-lapse (TL) OVSP was part of an integrated monitoring program that included well logs, crosswell seismic, electrical resistance tomography and 4D surface seismic. The goals of the OVSP were to detect the CO2 induced change in seismic response, give information about the spatial distribution of CO2 near the injection well and to help tie the high-resolution borehole monitoring to the 4D surface data. The VSP data were acquired in well CFU 31-F1, which is the ∼3200 m deep CO2 injection well at the DAS site. A preinjection survey was recorded in late 2009 with injection beginning in December 2009, and a post injection survey was conducted in Nov 2010 following injection of about 250 kT of CO2. The sensor array for both surveys was a 50-level, 3-component, Sercel MaxiWave system with 15 m (49 ft) spacing between levels. The source for both surveys was an accelerated weight drop, with different source trucks used for the two surveys. Consistent time-lapse processing was applied to both data sets. Time-lapse processing generated difference corridor stacks to investigate CO2 induced reflection amplitude changes from each source point. Corridor stacks were used for amplitude analysis to maximize the signal-to-noise ratio (S/N) for each shot point. Spatial variation in reflectivity (used to ‘map’ the plume) was similar in magnitude to the corridor stacks but, due to relatively lower S/N, the results were less consistent and more sensitive to processing and therefore are not presented. We examined the overall time-lapse repeatability of the OVSP data using three methods, the NRMS and Predictability (Pred) measures of Kragh and Christie (2002) and the signal-to-distortion ratio (SDR) method of Cantillo (2011) . Because time-lapse noise was comparable to the observed change, multiple methods were used to analyze data reliability. The reflections from the top and base reservoir were identified on the corridor stacks by correlation with a synthetic response generated from the well logs. A consistent change in the corridor stack amplitudes from pre- to post-CO2 injection was found for both the top and base reservoir reflections on all ten shot locations analyzed. In addition to the well-log synthetic response, a finite-difference elastic wave propagation model was built based on rock/fluid properties obtained from well logs, with CO2 induced changes guided by time-lapse crosswell seismic tomography ( Ajo-Franklin et al., 2013 ) acquired at the DAS site. Time-lapse seismic tomography indicated that two reservoir zones were affected by the flood. The modeling established that interpretation of the VSP trough and peak event amplitudes as reflectivity from the top and bottom of reservoir is appropriate even with possible tuning effects. Importantly, this top/base change gives confidence in an interpretation that these changes arise from within the reservoir, not from bounding lithology. The modeled time-lapse change and the observed field data change from 10 shotpoints are in agreement for both magnitude and polarity of amplitude change for top and base of reservoir. Therefore, we conclude the stored CO2 has been successfully detected and, furthermore, the observed seismic reflection change can be applied to Cranfield's 4D surface seismic for spatially delineating the CO2/brine interface.

English

  A seismic reflection survey was carried out in Bishop Wood, Yorkshire to determine the subsurface structure and the depth to the prospective gypsum and coal measures. Four 24-channel Geodes were used with a PEG40 accelerated weight drop and 10 Hz geophones at 3 m intervals to shot a 1099 m spread.  The final depth processed section shows some reflectors which are clearly visible and is geologically consistent with borehole information in the area. A strong reflector was found at 25 to 50 m depth, multiple reflections were seen between 70 and 150 m and a strong reflector was seen at 200 to 250 m on the processed seismic section. These were tentatively interpreted as the top of Bunter sandstone, multiple gypsum horizons and the top of coal measures respectively, these observations agreed with the borehole information obtained in the area.  A southwards dipping fault was inferred in the section at approximately 600 m along the line, though this may be an artefact due to acquisition and or processing of the data.   Key words: Reflection, gypsum, coal, geophones, borehole and bishop wood.

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.

A university-developed seismic source for shallow seismic surveys

Abstract The main objectives of this study were to (1) design and develop a low cost seismic source for shallow seismic surveys and (2) test the performance of the developed source at a test site. The surface seismic source, referred to here as a university-developed seismic source is based upon the principle of an accelerated weight drop. A 30 kg activated mass is lifted by a mechanical rack and pinion gear and is accelerated by a mounted spring. When the mass is released from 0.5 m above the surface, it hits a 30 kg base plate and energy is transferred to the ground, generating a seismic wave. The developed source is portable, environmentally friendly, easy to operate and maintain, and is a highly repeatable impact source. To compare the developed source with a sledgehammer source, a source test was performed at a test site, a study site for mapping a major fault zone in southern Thailand. The sledgehammer and the developed sources were shot along a 300 m long seismic reflection profile with the same parameters. Data were recorded using 12 channels off-end geometry with source and receiver spacing of 5 m, resulting in CDP stacked sections with 2.5 m between traces. Source performances were evaluated based on analyses of signal penetration, frequency content and repeatability, as well as the comparison of stacked sections. The results show that both surface sources are suitable for seismic studies down to a depth of about 200 m at the site. The hammer data are characterized by relatively higher frequency signals than the developed source data, whereas the developed source generates signals with overall higher signal energy transmission and greater signal penetration. In addition, the repeatability of the developed source is considerably higher than the hammer source.

Minimal-impact seismic acquisition: successful imaging using an accelerated weight drop system

Under a joint agreement with the Cambodian National Petroleum Authority, PGS has commenced the first onshore seismic survey in the Kingdom of Cambodia. The aim of this seismic exploration program is to improve understanding of the hydrocarbon prospectivity of the Tonle Sap and Kompong Som Basins. The current area of operation is focused on the Tonle Sap Basin - with seismic lines extending from Battambang in the north to approximately 50km SE of Pursat in the south. The reconnaissance 2D seismic exploration program has been designed around structural trends and a depth-to-basement model derived from high-resolution aerogravity and aeromagnetic data available in the Tonle Sap Basin. At the time of writing, some 700 km of 2D seismic data have been acquired. The main geophysical and geological objectives of this seismic acquisition program have been to: ? verify the presence of the gravity lows and the depth-to-basement model ? verify the presence of a potential petroleum system; and ? obtain a better understanding of the structural evolution of the basins.

Comparison of surface seismic sources at the CO2SINK site, Ketzin, Germany

ABSTRACTIn 2004 three seismic surface sources (VIBSIST, accelerated weight drop and MiniVib) were tested in a pilot study at the Ketzin test site, Germany, a study site for geological storage of CO2 (EU project CO2SINK). The main objectives of this pilot study were to 1) evaluate the response of the Ketzin site to reflection seismics, especially at the planned injection depth, 2) test different acquisition parameters and 3) use the results to guide the planning of the 3D survey. As part of these objectives, we emphasize the source performance comparison in this study. The sources were tested along two perpendicular lines of 2.4 km length each. Data were acquired by shooting at all stations (source and receiver spacing of 20 m) on both lines, allowing common‐midpoint stacked sections to be produced. The sources' signal characteristics based on signal‐to‐noise ratio, signal penetration and frequency content of raw shot records were analysed and stacked sections were compared. The results show that all three surface sources are suitable for reflection seismic studies down to a depth of about 1 km and provide enough bandwidth for resolving the geological targets at the site, i.e., the Weser and Stuttgart Formations. Near surface conditions, especially a thick weathering layer present in this particular area, strongly influence the data quality, as indicated by the difference in reflectivity and signal‐to‐noise ratio of the two common‐midpoint lines. The stacked sections of the MiniVib source show the highest frequency signals down to about 500 ms traveltime (approximately 500 m depth) but also the shallowest signal penetration depth. The VIBSIST source generates signals with the highest signal‐to‐noise ratio and greatest signal penetration depth of the tested sources. In particular, reflections below 900 ms (approximately 1 km depth) are best imaged by the VIBSIST source. The weight drop performance lies in between these two sources and might be recommended as an appropriate source for a 3D survey at this site because of the shorter production time compared to the VIBSIST and MiniVib sources.

Comparison of high‐frequency seismic sources at the Grimsel test site, central Alps, Switzerland

In August 1995, various high‐frequency seismic sources were tested at the Grimsel Test Site (GTS), located inside a crystalline rock body in the central Swiss Alps. These source tests were designed to facilitate future tomographic studies of potential radioactive waste disposal sites. The principal objective was to identify borehole and tunnel seismic sources capable of generating powerful high‐frequency signals such that frequencies up to 1000 Hz can be observed over distances of 1000 m in crystalline or consolidated sedimentary rocks. Seismic sources were situated in water‐filled boreholes (sparker, two piezoelectric sources, explosives) and at or near the tunnel wall (accelerated weight drop, minivibrator, bolt gun, buffalo gun, explosives). To evaluate and compare the source characteristics, the direct P-wave generated by the various seismic sources was investigated for the decay of its S/N and dominant frequency with offset and for the maximum distance at which first arrivals could be picked. Of the seismic sources tested, small explosive charges (5–100 g) had the most favorable S/N and frequency characteristics. At GTS, the target distance (∼1000 m) was reached with explosive charges of 50 g or more. None of the sources tested was capable of generating signals that sustained frequencies of 1000 Hz over distances in excess of 100 to 200 m. The unusually strong attenuation implied by this observation is likely due to the fact that the rocks at GTS underwent brittle deformation during the Alpine orogeny and therefore contain numerous fractures and shear zones.