Borehole Seismic Source Research Articles

Field Evaluation of Two Impulsive Downhole Seismic Sources in Crosswell and Reverse VSP Geometries and High-Resolution Characterization of Near-Surface Texas Gulf Coast Sediments

Borehole seismic methods have been widely used for characterizing the shallow subsurface. Accurate analysis of their data is aided by a solid understanding of the borehole sources' characteristics. This study presents a field evaluation of two impulsive borehole seismic sources (Trident's Scorpion sparker and RT Clark's Ballard weight drop) in crosswell and reverse vertical seismic profile (RVSP) geometries at a Gulf of Mexico coastal site with two shallow vertical wells. The acquired data is then utilized to characterize the near-surface coastal sediments. The Scorpion source generated P-wave dominant frequencies that were recorded as 650 Hz and 250 Hz in the crosswell and RVSP geometries, respectively. For the Ballard source in the two geometries, the P-wave dominant frequencies were 1100 Hz and 250 Hz. We were also able to pick direct S-wave arrivals with the Ballard source, and their dominant frequencies were 100 Hz and 40 Hz for in situ and surface recordings, respectively. The average signal-to-noise ratio (SNR) recorded with the Scorpion data for the crosswell geometry and RVSP, respectively, is 13 and 6, and for the Ballard source, 62 and 30. We also investigated the source radiation patterns and signature wavelets. Seismic tomography was performed for the area between the two wells. Low P-wave and S-wave velocities correspond to three fresh water-saturated sand zones identified from drilling cuttings and previous well log data. Also, the plot of velocity of P-wave (Vp) versus S-wave (Vs) fits reasonably to the Mudrock Line. Both sources can excite repeatable energetic seismic signals up to 150 m away and could be useful in many geotechnical settings and even single-well imaging.

Depth Effect of Rock-Breaking Seismic Source by Drilling Jumbo in Subsea Tunnel with the Drill and Blast Method and Its Application Prospects

The drilling source of a drilling jumbo contains a great amount of noise in the construction of a subsea tunnel, which leads to a large prediction error for the conventional source selection method. To address this problem, this paper proposes an optimization method for the source selection of a drilling jumbo. The optimal truth element is selected for the advanced prediction of a subsea tunnel to ensure the safety of the subsea tunnel construction. First, a spectrum analysis of borehole seismic sources with different drilling depths is carried out, the noise interference contained in the seismic source signal at different drilling depths is analyzed, and the propagation law of a borehole seismic wave field at different depths is studied. Second, the deconvolution-cross-correlation method is used to denoise and reconstruct the seismic records of borehole sources at different depths, and a clutter and signal-to-noise ratio analysis of the reconstructed seismic records is carried out. The principle of optimal selection of the pilot signal time window is established to suppress random noise and surface wave interference. Finally, based on the actual project, the proposed method is evaluated. The results show that when using the optimization method of drilling source interception proposed in this paper, when the drilling jumbo is drilled to the deepest place, the selection of the source signal stationary section for advanced geological prediction is the best. The prediction results are basically consistent with the actual excavation situation, verifying the feasibility and effectiveness of the method.

A borehole seismic source and its application to measure in-situ seismic wave velocities of geo-materials

A borehole seismic source was developed to measure horizontally-polarized shear (SH-) waves in the near surface and to improve drawbacks of conventional seismic sources. An electro-mechanical-type source, called “TahcBalm”, has exceptional repeatability in generating signature SH-waves, while being sufficiently small and light to be fitted in 76mm diameter cased or uncased boreholes. The source has been extensively used for borehole seismic testing at various locations with diverse soil and rock conditions. The cross-hole and in-hole testing signals are strong enough and allow the clear identification of the first arrival of SH-waves in all tested geologic environments. TahcBalm generates SH-waves with proper wavelength of about 1m and 0.5m for cross-hole and in-hole testing configurations respectively, at soil and rock sites. The source performs well in terms of data quality and ease of use.

The Design and Experiment Analysis of Mechanical Borehole Seismic Source

Considering the signal attenuation in deep well affects the use of the new wellbore anti-collision technology based on vibration wave, a kind of borehole seismic source designed. The distance among the risk wells can be identified by collecting the vibration signal produced by this borehole seismic source. The whole device includes energy storage and impact part, control part and auxiliary part. This borehole seismic source bases on the spring energy storage as basic principle. The impact hammer impacts the anvil as the impact institutions. The stuck points consist of arc fastener and arc grooves. The structure is simple and reliable. The function is easy to realize. The laboratory experiment results showed that this mechanical borehole seismic source can generate certain intensity and independent signals. The volume of the impact strength can fulfill the basis prevention warning of the monitoring the wellbore collision in deep layers.

Next Generation Borehole Seismic Source: Dual-Wavefield Downhole Vibrator System

Recent advances in crosswell seismic technologies have lead to the successful development and release of a new downhole seismic source that imparts greater energy into the formation by sweeping frequencies in the range of 30-800 Hz. The result is, for the first time, a powerful, efficient and cost effective vibrator for use in the borehole allowing for enhanced seismic imaging between wells up to 2km apart. In addition, the patented design and operation of the source allows for both compressional and shear wave energy to be acquired simultaneously along with the flexibility of being able to acquire data in reverse vertical seismic profile configuration.

Crosswell seismic imaging in a contaminated basalt aquifer

Multiple seismic crosswell surveys have been acquired and analyzed in a fractured basalt aquifer at Idaho National Engineering and Environmental Laboratory. Most of these surveys used a high‐frequency (1000–10,000 Hz) piezoelectric seismic source to obtain P‐wave velocity tomograms. The P‐wave velocities range from less than 3200 m/s to more than 5000 m/s. Additionally, a new type of borehole seismic source was deployed as part of the subsurface characterization program at this contaminated groundwater site. This source, known as an orbital vibrator, allows simultaneous acquisition of P‐ and S‐waves at frequencies of 100 to 400 Hz, and acquisition over larger distances. The velocity tomograms show a relationship to contaminant transport in the groundwater; zones of high contaminant concentration are coincident with zones of low velocity and high attenuation and are interpreted to be fracture zones at the boundaries between basalt flows. The orbital vibrator data show high Vp/Vs values, from 1.8 to 2.8. In spite of the lower resolution of orbital vibrator data, these data were sufficient for constraining hydrologic models at this site while achieving imaging over large interwell distances. The combination of piezoelectric data for closer well spacing and orbital vibrator data for larger well spacings has provided optimal imaging capability and has been instrumental in our understanding of the site aquifer's hydrologic properties and its scale of heterogeneity.

米国カリフォルニア州ロストヒルズ油田で実施した，坑井間電磁および弾性波トモグラフィーによる貯留層モニタリングの試み

Reservoir monitoring is the key issue to design the optimum plan for recovery enhancements of hydrocarbon production. The Technology Research Center of Japan National Oil Corporation has conducted extensive field measurements to validate the current cross-hole technologies at the Lost Hills Oil Field, California in the first quarter of 2000. The experiments were composed of cross-hole electromagnetic surveys between fiberglass cased boreholes in both frequency domain and time domain methods together with seismic tomography. The time domain electromagnetic survey was designed to test the newly developed time domain down-hole transducer. However, the tool was broken down during the measurement. For the frequency domain electromagnetic method, we conducted three kinds of experiments, which are time lapse analysis with previously measured data, multi-frequency survey for higher resolution analysis, and measurement with setting up transducer in a steel cased hole. Multi-stage multi-disk borehole seismic source, which has been developed in collaboration between JNOC and OYO corporation was employed for seismic tomography.The subsurface velocity profile from seismic tomog raphy was utilized to calculate the formation porosity. Since the standard calculation procedure of sonic porosity is not adequate to apply for the diatomite reservoir because of its nature of high porosity despite very low permeability, a correction function was developed and employed. Then the resistivity profile was incorporated into estimation of fluid saturation in the reservoir by Archie's equation. The resultant values of porosity and water saturation are quite reasonable in comparison with reported ranges for this field.Assuming that the regional compaction being related to reduction of reservoir porosity has not been significant from 1997 to 2000 based on the ground subsidence in the area, we calculated a fluid saturation profile since the beginning of water flooding. We revealed the manners of fluid saturation change in the reservoir during water flooding through the time lapse analysis.

Numerical simulation of first-order backscattered P and S waves for time-lapse seismic imaging in heterogeneous reservoirs

Summary First-order elastic wave scattering theory, validated in general features by deep-borehole seismic coda wave observations, is used to numerically simulate time-lapse borehole seismic data aimed at monitoring oil–water substitution in heterogeneous hydrocarbon reservoirs. A first-order perturbation solution of the vector wave equation leads to P–P and S–S backscattered displacement vector motion expressed as the angular summation of the second radial derivative of P- and S-wave velocity and density fluctuations α′/α, β′/β, ρ′/ρ over expanding spherical wave fronts of radii ξ = αt/2 and ξ = βt/2. P- and S-wave velocity and density spatial heterogeneity is modelled as long-range correlated random fluctuations consistent with the power-law-scaling character of crustal rock physical properties measured by borehole logs. The spectra of model scattering displacement seismograms for a power-law-scaling volumetric noise distribution duplicate the frequency enrichment observed in the spectra of broadband earthquake coda waves recorded in a deep well. Modelling of time-lapse scattering in power-law-scaling permeability structures suggests that a stable borehole seismic source can locate oil–water substitution in reservoir volumes tens of metres on a side at distances of up to a few hundred metres from an observation well using currently available borehole seismic technology. Time-lapse tracking of oil–water substitution and the monitoring of reservoir stress conditions can lead to spatially well-constrained reservoir models, despite large-scale, large-amplitude correlated random heterogeneity.

Orbital vibrator seismic source for simultaneous P‐ and S‐wave crosswell acquisition

A recently developed borehole seismic source, the orbital vibrator, was successfully deployed in a crosswell survey in a fractured basalt aquifer. This seismic source uses a rotating eccentric mass to generate seismic energy. Source sweeps with clockwise and counter‐clockwise rotations are recorded at each source location. Because this source generates circularly polarized waves, unique processing algorithms are used to decompose the recordings into two equivalent linearly oscillating, orthogonally oriented seismic sources. The orbital vibrator therefore generates P‐ and S‐waves simultaneously for all azimuths. A coordinate rotation based on P‐wave particle motion is used to align the source components from various depths. In a field experiment, both P‐ and S‐wave arrivals were recorded using fluid‐coupled hydrophone sensors. The processed field data show clear separation of P‐ and S‐wave arrivals for in‐line and crossline source components, respectively. A tensor convolutional description of the decomposition process allows for extension to multicomponent sensors.

Development of an advanced vibratory source for borehole seismology

Single-well seismology, reverse vertical seismic profiles (RVSP’s) and Crosswell seismology are three new seismic techniques for obtaining much higher resolution images of oil and gas reservoirs than what is currently obtainable with surface seismic techniques. Borehole seismology involves inserting both the source and the seismic receivers in oil or gas wells. In the past these methods have been limited to short distances between source and receivers in shallow wells. In 1997 a new borehole seismic source and receiver system has been deployed with a capability of handling source–receiver spacings of more than 2000 m in rock with a Q value of more than 20, and well depths as great as 6500 m. This advanced borehole seismic data acquisition system consists of a powerful, clamped, swept-frequency, vibratory source which is nondestructive, a multilevel receiver string of clamped, three-component geophones, and an acquisition system for reverse VSP’s. The downhole vibrator produces high-quality S waves, with controlled polarization, as well as P-wave direct arrivals and reflections. This paper discusses the design and performance of this system.

Borehole seismic‐source radiation in layered isotropic and anisotropic media: Boundary element modeling

In modeling waves radiated from a borehole seismic source in layered isotropic or anisotropic media, the commonly used numerical methods (e.g., finite difference and finite element) encounter difficulties because of the large scale difference between the borehole diameter and the formation extent. To get around this problem, we apply the indirect boundary element method to establish a general algorithm for modeling source radiation from open and cased boreholes in layered transversely isotropic (TI) media. The essence of the algorithm is to use discrete secondary sources (unknowns) on both sides of the borehole wall (formation/cement interface) to represent the influence of the interface on wave scattering, so that wave propagation inside and outside the borehole can be carried out by Green’s functions. The discrete distribution of the secondary sources is determined by matching boundary conditions on the borehole wall. Comparison with the discrete wavenumber method validates the implementation. Applications to fluid‐filled open and cased boreholes in three‐layer media demonstrate the creation of guided modes in low velocity layers. Presence of anisotropy complicates the guided modes as a result of dispersion and P‐ and SV‐waves coupling in homogeneous TI media. Presence of casing and cement enhances the visibility of the guided modes.

Low‐ and high‐frequency radiation from seismic sources in cased boreholes

An accurate characterization of borehole seismic sources is necessary to model and interpret waveforms observed in crosshole and reverse vertical seismic profiling (VSP) surveys, since the radiation pattern of a source will directly influence the amplitudes of elastic wave arrivals at receiver locations. Any attempt to study these data or perform inversions of amplitude data without incorporating the borehole effects will have serious limitations. Most previous studies of borehole seismic source radiation patterns have applied low‐frequency approximations to develop expressions for the radiation patterns of volume injection or stress sources (Heelan, 1953; White, 1960; White and Senghush, 1963; Lee and Balch, 1982; Lee, 1986; Kurkjian, 1986; Meredith, 1990; Winbow, 1991; Ben‐Menahem and Kostek, 1991). For example, Lee and Balch (1982) used this approach, along with a steepest descent solution, to derive closed‐form analytic expressions for the asymptotic far‐field radiation from sources located in uncased boreholes. Meredith (1990) applied the same methodology to study the radiation patterns of a variety of types of sources, though he also computed full waveform synthetic seismograms using the discrete wavenumber method. Likewise, Greenfield (1978) used full waveform numerical solutions to compute seismograms for force sources applied to the wall of a cylindrical cavity.

A field test of an electrodeless arc discharge, borehole seismic source

Waveforms generated by an impulsive, 1.2 kJ, seven‐conductor wireline electrodeless arc discharge borehole seismic source or sparker at Texaco’s Humble, TX field test site were recorded by three borehole sensor arrays: two free‐hanging hydrophone streamers in in‐line boreholes at 82 m and 170 m from the source well and a grouted, three‐component geophone string in a borehole 110 m from the source well. A repeatability test of the source, consisting of single firings of the source at a rate of 1 firing per 5 s, showed very clean, very strong, Ricker‐like wavelets. Despite a high‐degree of attenuation (exact value of Q is not known), the useful frequency passband of the wavelets was from 200 Hz to 1200 Hz for the data recorded by the 82-m offset hydrophones and 200 Hz to 500 Hz for the 170-m hydrophones. Using 62 single‐firing wavelets recorded in the 82-m offset well gave mean and median crosscorrelations greater than 0.96 with standard deviations less than 0.02. A stack test, consisting of 1, 2, 4, 8, 16, and 32-stacked waveforms, confirmed the shape, strong S/N ratio, and high correlation of the sparker output. The 32-stack, which took less than 3 minutes to generate, was recorded by the noisy, near-surface geophones at a raypath distance of nearly 300m.

A high‐frequency sparker source for the borehole environment

For tomographic investigations of shallow subsurface features of limited lateral extent, a high‐frequency, low‐cost borehole seismic source would be highly desirable, particularly for investigators with limited budgets. We constructed a simple, arc‐discharge seismic source from off‐the‐shelf items. This source consists of a salt water filled bottle containing exposed conductors of a coaxial cable, across which 100 to 300 joules of electrical power were discharged. This source produced a seismic pulse with a dominant frequency in the neighborhood of 1.5 kHz and a half‐power bandwidth in excess of 1 kHz. Repeatability of seismic signatures in a variety of environmental settings was excellent. Sufficient power was generated to observe seismic signals with at least a 35 dB signal‐to‐noise ratio at horizontal borehole separations of 100 m. For a borehole separation of 33.2 m, signals with at least a 35 dB signal‐to‐noise ratio were observed at angular ranges in the vertical plane to 68 degrees. The hydrostatic head limit for this source was determined to be approximately 430 m.

Cross‐borehole geophysics in the petroleum industry

Few disciplines affecting SEG members have developed more rapidly in the last five years than the field of cross‐borehole geophysics. Much of the progress has been driven by the advent of powerful, nondestructive borehole seismic sources and also by the need for enhanced definition of oil producing reservoirs. On 3 March 1992, the Geophysical Society of Tulsa held the symposium “Cross‐borehole Geophysics in the Petroleum Industry” in order to describe the past, present, and future of this rapidly evolving field. This article and accompanying papers in this issue describe the symposium.

A swept-frequency borehole source for inverse VSP and cross-borehole surveying

A swept-frequency borehole seismic source has been constructed and tested that consists of a portion of the borehole isolated from the remainder and driven to build up pressure oscillations by resonance.The length of the isolated portion is changed to vary the resonant frequency. The source radiates an approximately isotropic P-wave whose total energy is comparable in magnitude to that created by a surface vibrator truck. Strong shear waves are also generated.The source has been tested over a frequency range of 30 to 120 Hz, but the design can be operated from 15 to 500 Hz. Because the driven section of the borehole is isolated, strong tube waves are not generated. No damage to the casing-cement bond has been observed after prolonged operation of the source at a fixed depth.This source has the strength and uniform radiation pattern to suit it for both inverse VSP and cross-borehole surveying.