Direct Shear Registration™

March 2011

Svitzer’s seismic hydrocarbon indicator technique using S-waves to calibrate conventional P-wave 2-D and 3-D seismic data

or “Buddy, can you spare an Attribute?”

 

The vast majority of seismic data used in exploration today is pressure wave, or P-wave, data generated by an airgun, dynamite or vibrator source. Shear waves on the other hand have some distinctive characteristics:

a)     Lower velocity than p-waves – shear waves velocities vary from P-waves by factors of between 2 and 50, most variable in the first 100m of the subsurface. Also strata that have identical P-wave velocities can have different shear velocities, allowing differentiation of similar stratigraphic units

b)    Directional – shear waves may propagate in one direction but the particle motion is at right angles to the direction of travel. Two orientations of a  shear source allow us to generate SH and SV-waves to measure anisotropy, fracture direction and perform other measurements

c)     Polarized two like signals can be 180º out of phase with each other depending on source initiation

Onshore the Direct Shear Registration™ (DSR) technique uses a shear beam source of cone truck to record a shear seismogram, this seismogram reacts only to the lithology of the formations being surveyed. Based on these field measurements we remove the effect of lithology changes from the existing 2-D or 3-D P-wave survey and show only those changes caused by factors other than lithology, such as fluid content. Then, by means of the locally significant attributes selected by the interpreter, the P-S difference section displays attributes which provide the best parameter discrimination. Drilling prospects are re-risked using data from the difference section.

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Some Details

 

Using field techniques outlined in Doug Crice’s TLE paper we acquire a shear wave records with almost zero source-receiver offset. The major change from the techniques described in ASTM specification 7400-08 is that we use both transmitted data and reflected data; the engineers are looking for transmission effects. We use their transmission velocity data as the jump-off point and extend the velocity field below the geophone level, in a similar manner to a VSP.  Using Svitzer’s customized processing techniques we turn these records into a pure shear record – i.e. one with most of the noise and all the P-wave energy removed. This is the “pure shear record”. Then, in a similar manner to 4-D seismic, using a difference section between this “pure shear” trace and the P-wave survey we derive a direct measurement of the fluid content and the ratio Vp/Vs. There are as many pitfalls in creating the Shear-wave/P-wave difference section as there are in 4-D difference sections. A good starting point for using this technique is to record a shear record at a known point, typically a well with known production, and then extrapolate the results by recording along a 2-D transect from the well to a prospect location and on to either another producing location or dry hole.

 

The cost of acquiring sufficient land shear data to provide a meaningful difference section is between 5 and 10% of the cost of a p-wave survey and the processing cost depends on the amount of data calibrated and the status of the existing survey. Please call Alan Foley for more details about scheduling, cone truck mobilization and permitting issues. Offshore it is difficult to generate a good shear signal but it can be done, see Suction Corer

We are interested in uses of this technique in Texas on targets in shallow, i.e. 1,000 to 4,500’ depths, trends to reduce drilling risk and possibly detect natural fracture orientation and extent.

References:

 

In a recent SEG Leading Edge® article (Near-surface, downhole shear-wave surveys, TLE: February 2011, vol. 30 No.2 p. 167 et seq.) Doug Crice described how shear waves are used to measure near surface propagation velocities.

 

Link to Doug’s abstract: http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=LEEDFF000030000002000164000001

ASTM – Engineers do everything by the book and the book, in this instance, the book is the ASTM spec – ASTM 7400-08 “Standard Test Method for Downhole Seismic Testing” available from ASTM International at www.astm.org

We currently seek development partners to streamline field deployment and processing of our Direct Shear Registration™ technique; we offer a limited license for scientific use in exchange for “show and tell” rights to demonstrate the usability of this technique. Alternatively we offer to record the shear data and offer for use on a licensing basis. We are especially interested in uses of this technique in the shale trends to reduce drilling risk and allow for natural fracture detection.

Please Note: The seismic techniques described herein are patent applied for and any duplication or copying of this system and technique without express, written permission of Geonetwork Corporation is prohibited. Direct Shear Registration™ is a trademark of Geonetwork Corporation dba Svitzer Surveys

LogoIf you would like information on seismic data, geohazard data and geotechnical sampling contact us at (713) 526-6832 or contact Svitzer Surveys, 3311 Richmond Avenue, #227, Houston, TX 77098

This webpage is authored by Steve Stephens, Houston, TX Feedback? WebMaster

Graphics on this page are copyright Alan Foley,2009and 2010, used with permission

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