Publications & Papers

Avalon works closely with academia in developing understanding of borehole seismic principles and applications.

By keeping close ties with specialist academic staff, ASL ensures our products remain innovative and responsive to industry requirements.


  • Fracture-generated frequency-dependent seismic Q measured from a VSP in granite
    • EGU General Assembly 2021. Online EGU21-12766.
    • April, 2021

    Authors: Victoria R. Bourne1, C. Dario Cantu Bendeck1,2, Mark W. Hildyard1, Roger A. Clark1, and William Wills3

    • 1School of Earth & Environment, University of Leeds, Leeds LS2 9JT, UK
    • 2now at: CGG, Crompton Way, Crawley RH10 9QN, UK
    • 3Avalon Sciences Ltd, Somerton Business Park, Somerton, Somerset TA11 6SB, UK

    Details

    We integrate two topics – seismic characterisation of fractures, and seismic attenuation quantified as the frequency-dependent Seismic Quality Factor Q, Q(f). The former is vital for predicting and monitoring fluid movement and containment in energy-related settings (hydrocarbons; geothermal; CO2, hydrogen or compressed air storage; radwaste). Fractures control the fluid flow and structural behaviour of a rock mass, yet their expression in Q is poorly studied and not well understood despite it typically being more sensitive than wavespeeds as a rock physics parameter. The latter is long-recognised, little-studied, and a paradigm shift from frequency-independent Q (‘constant-Q’, a routine signal-processing and image enhancement tool in hydrocarbon exploration), despite theory, laboratory, and field data showing that Q must be frequency dependent due to varying scale-lengths of the physical-mechanical phenomena causing attenuation.

    We therefore measure Q(f) from the downgoing direct P-wave arrival in a near-offset vertical seismic profile in granite at a former geothermal test site in Cornwall, SW England, where vertical and horizontal fracturing is seen at surface: horizontal fractures are confirmed at depth by well-log data. Sensors were 3-component 15Hz geophones at 15m depth spacing: the source was a single vibrator, linear 8-100Hz up-sweep, 30m offset from the wellhead in the azimuth of well deviation: record length was 1000ms at 1ms sample interval. We analyse only the deeper cased interval, from 700m to 1735m. Pre-processing was geometric spreading correction, hodogram-based component rotation toward the source, and wavefield separation using a 7-point median filter to suppress interference from upgoing energy. Measured attenuation Qeff is the harmonic sum of intrinsic Q, Qint, and apparent attenuation, Qapp. Qint in massive granite is typically 500-1000, yet we find Qeff(f) is 50-70 at >60Hz and only ≈30 at <30-35Hz, features masked in the constant-Q result of 55±11 over our working bandwidth of 25-90Hz.

    One contribution to Qapp is ‘stratigraphic attenuation’, forward-scattering interference of short-path internal multiple reflections superimposed on direct arrivals, and quantifiable from sonic and density well-logs using O’Doherty-Anstey-Shapiro methodology. We find it is indeed frequency-dependent (peaking at ≈50-60Hz, 10-40% lower at our bandwidth limits) but its absolute magnitude is insignificant (Q≈20,000-30,000) and unable to explain the measured Qeff(f). We therefore investigate the effect of fracturing directly using finite difference models in which fractures are defined explicitly as displacement discontinuities with opposing surfaces connected by a normal and shear stiffness. An individual fracture acts somewhat like a low pass filter: more complex frequency behaviour emerges from multiple fractures, particularly when fracture stiffness, spacing and size can vary. We concentrate first on large horizontal fractures perpendicular to the borehole receiver array, and find that these can indeed influence effective attenuation within the 25-90Hz bandwidth. We then discuss the range of fracture spacings and stiffnesses capable of explaining the data and whether they are sufficiently physically credible as an explanation of the observed Q(f).

  • Analysis of thermal performance of geophonic down-hole measuring tools; anumerical and experimental investigation
    • Applied Thermal Engineering. Elsevier 134 p504-512.
    • April, 2018

    Authors: Rohitha Weerasinghe⁎, Thomas Hughes

    (Department of Engineering Design and Mathematics, Faculty of Environment and Technology, University of the West of England).

    Details

    Downhole tools encounter harsh environmental conditions due to pressure and elevated temperatures. Use of Peltier cooling in down-hole seismic tooling has been restricted by the performance of such devices at elevated temperatures. Present paper analyses the performance of Peltier cooling in temperatures suited for down-hole measuring equipment using measurements, predicted manufacturer data and computational fluid dynamic analysis. A critical analysis of Peltier performance prediction techniques is presented with measurements. Validity of the extrapolation of thermoelectric cooling performance at elevated temperatures has been tested using computational models for thermoelectric cooling device. This method has been used to model cooling characteristics of a prototype downhole tool and the computational technique used in has proven valid. Further, an CFD analysis of the performance of two heat sink metals has been presented. The experimental and modelling exercise was targeted at achieving cooling performance that would enable the tool withstand temperatures near 200 °C.
  • Borehole Seismic Technology, What’s Needed, What’s New, What’s Next

    Authors: Gary Tubridy, William Wills (ASL)

    Details

    Presented by Charles Naville (IFPen)  on behalf of Avalon Sciences at the April 2008 SAID technical meeting, this technology overview discusses the industry requirements, offerings and future innovation direction from the point of view of a UK based borehole seismic tool manufacturer.
  • Orientation of a 3-C VSP Dataset Acquired by Integrated Geophone Sensor and MEMS Inclinometer Devices
    • EAGE Passive Seismic Workshop Proceedings, EarthDoc
    • 19 November 2017

    Authors: William Wills (ASL) Charles Naville (IFPEN), Max Nott-Bower (ASL), Gary Tubridy (ASL)

    Poster Download

    Details

    This paper seeks to gauge the performance of a new commercial downhole VSP tool electronics module, which has been recently implemented with a 3C solid state inclinometer system called a High Side Indicator . The HSI measures the direction of the pull of gravity and calculates the angles of roll and vertical inclination in each shuttle of the borehole seismic toolstring. By so doing, the VSP tool manufacturer expects to replace the previous gimbal settings systems, which proved voluminous, fragile, inaccurate at low inclination, and mechanically difficult to maintain through the lifetime of a VSP tool, and encourage full 3C processing of all downhole datasets, even those acquired within rig source VSP survey geometries. The first objective of this test was to check that the VSP hardware and acquisition software worked as expected, and secondly to define a minimal field QC procedure which includes at least the roll angle rotation and associated 3C VSP displays. From this the field engineer and the client supervisor can be reassured as soon as possible about the overall VSP data quality and usability for advanced full 3C VSP processing by the geophysicists and interpreters.

  • Triggered or Induced Seismicity; A Long Term Approach
    • GeoExpro Vol. 13 No.3 2017
    • May 1, 2017

    Authors: William Wills (ASL)

    Details

    William Wills (Avalon Sciences) briefly summarises some of the long term passive monitoring requirements for the appropriate identification and regulation of human-induced seismicity.
  • Constant and frequency-dependent attenuation from vertical seismic profiles in fractured granite and thinly layered sediment
    • EAGE Paris Conference
    • June 14, 2017

    Authors:  CD. Cantú Bendeck* (School of Environment, University of Leeds), RA. Clark (School of Earth and Environment, University of Leeds), AD. Booth (School of Earth and Environment, University of Leeds), W. Wills (Avalon Sciences Ltd, UK)

    Details

    A collaboration between the University of Leeds and ASL, which will be presented on the 14th of June at 16:20 in Room A2.

    Abstract: The seismic quality factor, Q, is generally treated as frequency-independent, yet theory and field evidence suggest it may not be.  We measure constant-Qeff and frequency-dependent Qeff(f), from downgoing direct-P arrivals in VSP data, over a 1000 m-thick granite interval in Cornwall, SW England, and a 700 m-thick interbedded shale-carbonate sequence in the Barents Sea, and find two significant results.

    First, we obtain consistent constant-Qeff and Qeff(f) values, although Qeff(f) values vary smoothly with frequency-specific ‘attenuation peaks’.  In the granite, over 25-90 Hz, constant-Qeff =75±47, and Qeff(f) ≈35-100.  For the sediments, over 15-110 Hz, constant-Qeff=152±40, and Qeff(f) ≈70-200.  We conclude that our Q(f) workflow (using logarithmic decrement of individual frequencies’ amplitudes after geometric spreading corrections, so not imposing an analytic Q(f) form) is workable.

    Second, attenuation in this granite is comparably high to that in these sediments.  The granite has only naturally-occurring fractures, but they appear sufficient to increase attenuation above an intuitively-expected level for crystalline rocks. Well-log-based predictions of 1-D scattering contributions to attenuation, for both datasets, are contradictory between Qscat and Qscat(f). We believe the use of elastic synthetic seismograms could clarify the origins of our measured Qeff and

    Overall, we urge that more Q(f) measurements are made, to support of studies of subsurface petrophysical properties and application of signal-processing tools that account for attenuation in seismic imaging.

    .

  • Challenges of the Deep
    • Oilfield Technology 2015
    • November 1, 2015

    Authors: William Wills (ASL)Joe Killen (ASL)

    Details

    This article discusses the requirements and delivery of borehole seismic monitoring within extreme high pressure and temperature borehole environments .
  • Reducing velocity model uncertainty and improving microseismic event location accuracy: crosswell seismic tomography using a repeatable downhole sparker source
    • SEG Beijing 2014 – Borehole Seismic Workshop
    • April 20, 2014

    Authors: William Wills (ASL), Dr. James Verdon (Outer Limits Geophysics)

    Details

    This paper demonstrates how a commercial down-hole sparker source can be used to produce crosswell tomography surveys that result in much improved velocity profiles. The paper aims to address the impact of differences between acoustic log and crosswell-derived velocity models acquired from a North Belt Test Well, Texas USA on microseismic event location.
  • Into the Abyss. Borehole Seismic – the technological challenge
    • Geo Expro Volume 9 2012
    • September 1, 2012

    Authors: William Wills (ASL)

    Details

    Discussion of some of the principle requirements driving borehole seismic technology innovation.
  • Borehole Seismic – Using multiple stacked geophones
    • International Oil & Gas Engineer Article
    • February 4, 2012
    Authors: William Wills (ASL)

    Details

    Discussion of the advantages of employing multiple stacked geophones within a borehole seismic monitoring system in order to accurately map fracture induced microseismic events.
  • Reservoir characterization: microseismics now a key tool
    • National Hydrographic and Seismic Search – August 2009
    • August 1, 2009
    Authors: William Wills (ASL)

    Details

    Broad upstream industry article commenting on the increasing use of borehole passive seismic and microseismic monitoring within the oil and gas industry.

 

Publications