Borehole Vertical Seismic Profiling (VSP)

Basic Concept

Vertical seismic profiling (VSP) is a seismic-reflection method that records seismic arrivals from surface-activated sources with one or more receivers at depth in a borehole. Initially, VSP was primarily used to produce depth-dependent reflectivity images, which can aid in the determination of the local subsurface structures. Overtime, VSP has evolved and, because it has the potential to provide valuable ancillary information and depth constraints, is often used in conjunction with other techniques.

Due to the survey geometry, VSP can determine in situ formation properties such as seismic-wave velocity, acoustic impedance, seismic anisotropy, and seismic attenuation. VSP also provides information that assists in the understanding, processing, and interpretation of surface seismic data (e.g., depth data, identification of multiples, and 3D-modelling constraints). Vertical seismic profiling has proven very useful in hydrogeophysical studies as well as subsurface monitoring and drilling efforts (Stewart, 2001).

Theory

Seismic-body (i.e., compressional (P) and shear (S)) waves are waves of acoustic energy that propagate through, interact with, and are influenced by earth materials. A seismic-body wave travels through a material at a specific seismic velocity, the value of which is related to intrinsic material properties. Seismic energy is reflected at interfaces with a sufficient contrast in acoustic impedance, which is a function of the seismic velocity and density of a material.

Seismic-wave propagation occurs semispherically, and waves can be described by their individual rays, which are perpendicular to the wavefront. In VSP, seismic rays travel from the source to receiver(s) as either a direct ray, primary reflection, or secondary multiple. Direct rays arrive first at a receiver and can provide seismic-velocity structure information. The primary reflected rays are the second arrivals and allow the position of acoustic discontinuities (i.e., reflecting interfaces or “reflectors”) to be determined.

The receivers continuously record seismic-energy amplitude data, and these seismic traces are used to identify the type and travel time of each detected seismic ray (i.e., arrival). The first-arrival times are used to determine the time-depth relation and velocity model for the site, which aid calculations of interval-, average-, and root-mean-square velocities. Because the receivers are at depth, VSP can detect the seismic wavefield between the source and reflectors and directly correlate seismic data to depth (Mokhtari and Pourhossein, 2003).

Because it examines the seismic wavefield in situ and can detect both the down- and up-going seismic rays, VSP has advantages over surface surveys. Surface surveys are limited by their inferences of the subsurface that are derived only from measuring up-going reflected rays. By considering the down-going rays, VSP can be used to identify and eliminate multiples in surface data. Additionally, three-dimensional seismic models can be produced by integrating VSP with surface data (Rector and Mangriotis, 2011).

Applications

The seismic-energy sources used for VSP can be impulsive (e.g. explosives), controlled (e.g., mechanical vibrator), or an active drill bit, and the source strength is site- and project-specific. In the case of logging-while-drilling (LWD), the noise produced during drilling is the signal that is received by a surface-geophone array. At a single site, the VSP-survey source is ideally the same as the surface-survey source, but, if impossible, inconsistencies can be mitigated through data processing (Mokhtari and Pourhossein, 2003).

Typically, VSP employs a multi-level, wireline array of receivers with fixed spacing. VSP receivers require contact with the borehole environment, and, as such, there is a variety of receiver types that are designed for different well construction. Geophones contact the borehole wall via a sidewall tool in open boreholes or magnets in steel casing. If contact is impossible, hydrophones can be used in fluid-filled wells. Additionally, VSP data can be collected in various survey setups.

Checkshot- and zero-offset surveys have shot points (i.e., source locations) near the wellhead and measure the travel times of the near-vertical direct rays from source to receiver(s). Checkshot- and zero-offset VSP relate seismic travel time to depth, which allows for determination of a vertical velocity profile or interval velocity of a formation. Checkshot- surveys may only use one geophone, whereas zero-offset VSP involves evenly spaced geophones that are positioned or moved up the length of the borehole.

Offset and multi-offset surveys have sources that are single- or multiple distances away from the wellhead. Both have the potential for a large lateral coverage and depth of investigation. Offset VSP is often used for imaging a target feature and either has a source or receiver with a fixed position. Multi-offset VSP provides two-dimensional sections away from the well and employs multiple source locations offset from one or both sides of the well. Walk-away VSP surveys use multiple borehole receivers to measure signals transmitted at multiple, regularly spaced shot points. Walk-above VSP is similar but involves a horizontal/deviated well, which allows a vertical line to exist between source and receiver. Walk-away VSP can aid amplitude-variation-with-offset (AVO) analysis, walk-above VSP can deduce vertical travel times, and both can produce two-dimensional images of formations adjacent to and/or below the well. Additionally, the general applications of vertical seismic profiling include the following:

Integration with and expansion of seismic-reflection surface geophysical surveys
Improvement of velocity analysis) for constraining seismic-refraction profiles (i.e., differentiation of S- and P wave reflections/velocities)
Imaging of subsurface geologic structures (e.g., faults, salt domes)
Reduction of risk in well placement by directing drilling operations
Monitoring of hydraulic fracturing
Determination of amplitude-variation-with-offset (AVO) response
Estimation of seismic anisotropy, attenuation profiles, and true-reflection coefficients
Improvement of reservoir characterization and fluid-drainage monitoring
Study of lithological and hydrocarbon effects on propagating wavelets

Examples/Case studies

Beydoun, W.B., Cheng, C.H., and Toksöz, M.N., 1985, Detection of open fractures with vertical seismic profiling: Journal of Geophysical Research: Solid Earth, v. 90, no. B6, p. 4557-4566, doi:10.1029/JB090iB06p04557.

Abstract: In vertical seismic profiling surveys, tube waves are generated by compressional waves impinging on subsurface fractures or permeable zones. The problem of generation of these waves by a nonnormal incident P wave for an inclined borehole intersecting a tilted parallel‐wall fracture is formulated theoretically. The amplitude of tube waves depends on the permeability, the length of the fracture, and the frequency. The relative effects of these parameters are studied individually. The problem is also formulated for a thin oblate ellipsoidal (penny‐shaped) fracture. The results for the two fracture models are compared and contrasted. Field data from Tyngsboro, Massachusetts, are shown for open fractures in granite. From tube wave amplitudes normalized to P wave amplitudes, calculated permeabilities are of the order of 100 mdarcy.

Chavarria, J.A., Malin, P., Catchings, R.D., and Shalev, E., 2003, A Look Inside the San Andreas fault at Parkfield Through Vertical Seismic Profiling: Science, v. 302, no. 5651, p. 1746-1748, doi:10.1126/science.1090711.

Abstract: The San Andreas Fault Observatory at Depth pilot hole is located on the southwestern side of the Parkfield San Andreas fault. This observatory includes a vertical seismic profiling (VSP) array. VSP seismograms from nearby microearthquakes contain signals between the P and S waves. These signals may be P and S waves scattered by the local geologic structure. The collected scattering points form planar surfaces that we interpret as the San Andreas fault and four other secondary faults. The scattering process includes conversions between P and S waves, the strengths of which suggest large contrasts in material properties, possibly indicating the presence of cracks or fluids.

Hayward, N., Westbrook, G.K., and Peacock, S., 2003, Seismic velocity, anisotropy, and fluid pressure in the Barbados accretionary wedge from an offset vertical seismic profile with seabed sources: Journal of Geophysical Research: Solid Earth, v. 108, no. B11, p. 2515-2531, doi:10.1029/2001JB001638.

Abstract: The state of compaction and fluid pressure in the Barbados accretionary wedge near its toe, at Ocean Drilling Program Site 949, were investigated by modeling travel times of seismic waves from ocean bottom shots to a borehole geophone array. The model, constrained by a three‐dimensional seismic survey and well logs, shows (1) a velocity gradient of about 1–1.25 s−1 in the uppermost 180–230 m of the wedge; (2) a zone of variable, but no net change in, velocity between 230 and 350 m depth; (3) a low‐velocity zone 40–50 m thick just above the décollement at 391 m; and (4) a displacement of the low‐velocity zone by thrust faults. Pore fluid pressure sections derived from P wave velocity show that the upper half of the wedge is normally pressured while the lower half is overpressured. The ∼160 m thick, underconsolidated basal zone shows anisotropy, which increases downward. The lowest 40–50 m has velocity varying (1) azimuthally (3%), being fastest in the direction of plate convergence, and (2) in the vertical plane (2–5%), horizontal faster than vertical. After correction for the effect of anisotropy in the derivation of effective stress from seismic velocity the calculated pore fluid pressure ratio λ does not exceed 0.9 and in the lowest 40–50 m of the basal zone, is between 0.71 and 0.82, with λ* [(fluid pressure − hydrostatic)/(lithostatic pressure − hydrostatic)] between 0.5 and 0.65, in accordance with in situ measurements of fluid pressure in the décollement zone beneath. These indicate that the accretionary wedge is stronger and less overpressured than was previously supposed.

Hernandez, G., Casares, M., Perez, R., Barrios, O., Bautista, R., Brewer, R.J., and Torne, J., 2007, 'Look Ahead' Applications Of Vertical Seismic Profiles As A Litho-Structural Tool Combined With Dipole Sonic Logging: Case Histories-Burgos Basin-Northern Mexico, in Proceedings, SPWLA 48th Annual Logging Symposium: Austin, Texas, Society of Petrophysicists and Well-Log Analysts, 13 p.

Abstract: Vertical seismic profile (VSP) technology have allowed data acquisition with good accuracy of reflectors from 500 to 700 below deepest VSP level in wells in the Burgos Basin (Figure 1); wells as deep as 5400 m with temperatures up to 428°F have been logged successfully. The VSP makes it possible "look ahead" into zones that are being considered for drilling and shows a good correlation with surface seismic data. When combined with compressional slowness data from the latest technology dipole sonic tools, it is possible to observe the compaction trend and the pressure increment, when it is shown in a compressed scale. By integrating the VSP and dipole low frequency sonic data, it is possible to detect over-pressured zones and drilling risks in advance. This helps to optimize costs and to make decisions while drilling deep and offshore wells. This paper focuses on three wells as case studies. In all cases, a VSP look ahead survey was acquired in the intermediate phase and the results were compared to surface seismic data. After the well was drilled, the VSP data was compared to a synthetic seismogram. The results indicate that the VSP was accurate in depth and confirms the predictive value of the VSP look ahead application for litho-structural changes. Recent applications show the value of the VSP survey as a litho-structural evaluation tool. The original lithostructural model was used to consider a thin shale layer through a producing field. After the VSP was acquired, it was evident that the shale was actually at least twice as thick as what was originally shown in a model. A check shot velocity survey was recorded later in a deeper wellbore to confirm the VSP look ahead results. The exploration team changed the litho-structural model and subsequent exploration well locations were more accurate. Another well drilled later provided additional confirmation of the new model.

Whitmore, N.D., and Lines, L.R., 1986, Vertical seismic profiling depth migration of a salt dome flank: Geophysics, v. 51, no. 5, p. 1039-1155, doi: 10.1190/1.1442164.

Abstract: Vertical seismic profiles (VSPs) can supply information about both velocity and subsurface interface locations. Properly designed VSPs can be used to map steeply dipping interfaces such as salt dome flanks. Mapping subsurface interfaces with VSP data requires careful survey design, appropriate data processing, interval velocity estimation, and reflector mapping. The first of these four ingredients is satisfied, in most cases, by preacquisition modeling. The second is accomplished by careful data processing. Initial velocity estimates are provided by seismic tomography. Velocity‐model refinement is accomplished by a combination of iterative modeling and iterative least‐squares inversion. Finally, the resultant interval velocities are used in depth migration of the processed VSP. These four ingredients have been combined to map a salt dome flank.

Yadav, U.S., Shukla, K.M., Ojha, M., Kumar, P., and Shankar, U., 2019, Assessment of gas hydrate accumulations using velocities derived from vertical seismic profiles and acoustic log data in Krishna-Godavari Basin, India: Marine and Petroleum Geology, v. 108, p. 551-561, doi:10.1016/j.marpetgeo.2019.02.001.

Abstract: The second Indian National Gas Hydrate Program Expedition (NGHP-02) was executed in 2015 in four areas termed as Areas A, B, C and E. During this expedition, twenty five research sites were drilled and/or cored in Krishna-Godavari (KG) and Mahanadi Basins, eastern Indian offshore. During NGHP-02, zero offset vertical seismic profile (VSP) data were acquired at three sites: Sites NGHP-02-17, −19 and −22 in Area B of the KG Basin. In this study, we focus on the three sites in Area B of the KG Basin, where, zero-offset VSP and downhole acoustic log data are used to assess and characterize the gas hydrate deposits. Zero-offset VSP data are correlated with wireline log, surface seismic and synthetic seismic data to characterize and delineate gas hydrate accumulations in the KG Basin. Low velocities ranging from 1500 to 1650 m/s are observed in the unconsolidated shallow sedimentary section above gas hydrate-bearing units, whereas, very high velocities are observed in the acquired acoustic log and VSP data. In the gas hydrate-bearing sedimentary sections, the VSP derived interval velocities range from 2000 to 3000 m/s in the depth interval between 267 and 287 m below sea floor (mbsf) at Site NGHP-02-17, between 1650 and 1750 m/s in the depth interval between 306 and 366 mbsf at Site NGHP-02-19 and between 1700 and 1800 m/s in the depth interval between 198 and 290 mbsf at Site NGHP-02-22. Gas hydrates are distributed both as pore-filling and fracture-filling at all three sites, however, high concentrations are only observed as pore-filling morphology. We estimate the amount of gas hydrate considering both isotropic (pore-filling) and anisotropic (fracture-filling) acoustic reservoir models. Our estimations of gas hydrate saturation match well with the available pressure core data and suggest that high gas hydrate concentrations (to nearly 85% of pore space) are distributed in a load bearing morphology.

Yang, D., Malcolm, A., Fehler, M., and Huang, L., 2014, Time-lapse walkaway vertical seismic profile monitoring for CO2 injection at the SACROC enhanced oil recovery field: A case study: Geophysics, v. 79, no. 2, p. 1MA-Z52, doi:10.1190/geo2013-0274.1.

Abstract: Geologic carbon storage involves large-scale injections of carbon dioxide into underground geologic formations. Changes in reservoir properties resulting from CO2 injection and migration can be characterized using monitoring methods with time-lapse seismic data. To achieve economical monitoring, vertical seismic profile (VSP) data are often acquired to survey the local injection area. We investigated the capability of walkaway VSP monitoring for CO2 injection into an enhanced oil recovery field at SACROC, West Texas. VSP data sets were acquired in 2008 and 2009, and CO2 injection took place after the first data acquisition. Because the receivers were located above the injection zone, only reflection data contain the information from the reservoir. Qualitative comparison between reverse-time migration images at different times revealed vertical shifts of the reflectors’ center, indicating the presence of velocity changes. We examined two methods to quantify the changes in velocity: standard full-waveform inversion (FWI) and image-domain wavefield tomography (IDWT). FWI directly inverts seismic waveforms for velocity models. IDWT inverts for the time-lapse velocity changes by matching the baseline and time-lapse migration images. We found that, for the constrained geometry of VSP surveys, the IDWT result was significantly more consistent with a localized change in velocity as expected from a few months of CO2 injection. A synthetic example was used to verify the result from the field data. By contrast, FWI failed to provide quantitative information about the volumetric velocity changes because of the survey geometry and data frequency content.

References

Hardage, B.A., 1985, Vertical Seismic Profiling: Principles: Oxford, UK, Elsevier Science Ltd, 552 p.

McCollum, B. and LaRue, W.W., 1931, Utilization of Existing Wells in Seismograph work: Bulletin of the American Association of Petroleum Geologists, v. 15, no. 12, p. 1409-1417, doi:10.1306/3D932A32-16B1-11D7-8645000102C1865D.

Mokhtari, M. and Pourhossein, H., 2003, Significance of VSP Data for Surface Seismic Data; South Ghashu Gas Field, South Iran: Iranian International Journal of Science, v. 4, no. 2, p. 223-240.

Rector III, J.W., and Mangriotis, M.D., 2011, Vertical Seismic Profiling, in Gupta, H.K., ed., Encyclopedia of Solid Earth Geophysics: Dordrecht, Netherlands, Springer, p. 1507-1509.

Stewart, R.R., 2001, VSP: An In-Depth Seismic Understanding: Canadian Journal of Exploration Geophysics, v. 26, no. 7, 13 p.