Triaxial Geophones Research Articles

Imaging of subsurface faults by walkaway VSP waveguiding

Hydrocarbon reservoirs sometimes occur within low velocity channels, thus forming potential seismic waveguides. The waveguide might be excited at its discontinuities (faults) by seismic waves travelling downward from a surface source. Given that a waveguide would confine scattered energy to the horizontal plane, it should be possible to map fault scatterers at large range by means of a downhole triaxial geophone placed within the reservoir. A simple laboratory scale seismic model of a Southern North Sea gas reservoir was constructed to study the problem. Multicomponent seismic measurements were taken in both walkaway VSP and offset VSP shooting geometries. The results validate the presence of strong waveguiding in a low velocity layer disrupted by faulting. The diffracted arrivals can be imaged to locate the fault 'source'. The model results, when scaled to the field situation, suggest that faults within a reservok can be detected at horizontal distances greater than several kilometres from the drill hole.

A pseudo‐underway geophysical technique for quantifying seabed sediment properties

A towed geophysical device has been developed that will allow the rapid quantification of seafloor sediment properties for engineering and other purposes. The seabed hardware consists of seismic sources, a focused electrode pad (all sledge mounted), and a string of gimballed triaxial geophones. The seismic sources are impulsive devices that are essentially electromagnetic hammers that can preferentially generate shear or compressional waves dependent on their mode of operation. Travel time relationships obtained for seismic waves propagating through the sediment body to the geophone array allow the velocity structure to be defined to a depth dictated by the maximum receiver offset. Of particular interest to the engineers are the velocity gradient effects that for shear waves are indicative of the seabed sediment rigidity. The device can be used to construct distribution maps of geophysically related physical properties of the sediment body, and their variation with depth. Surveys carried out with the device in well‐documented test‐bed sites have produced significant correlations between the geophysical parameters and the known sediment variability.

Numerical modeling of water waves recorded by a subbottom seismometer in the southwest Pacific

The amplitudes of water waves generated by shallow explosive sources and recorded by a subbottom seismometer provide constraints on velocity and attenuation parameters of oceanic sediments and upper oceanic crust. Analysis of data collected during the 1983 Ngendei Seismic Experiment in the southwest Pacific Ocean illustrates this method. The data were recorded by a triaxial geophone emplaced 54 m into the basaltic basement. Synthetic seismograms are computed using wave number integration for several crustal models to determine the sensitivity of water wave amplitudes to specific parameters of the sediments and oceanic crust. The synthetic seismograms fit the recorded data to better than a factor of 2 for frequencies as high as 15 Hz and ranges out to 30 km. The amplitude of the direct water wave beyond critical incidence (“direct wave root”) indicates that the sediment P wave velocity is near 1.8 km/s. Modeling of the attenuation of shear wave reverberations in the sediments indicates that sediment Qβ is between 30 and 50. Modulations in water wave amplitude as a function of receiver offset are related to the velocity of the upper oceanic crust. The distances at which amplitude maxima occur indicate that the shear wave velocity of the upper crust at the Ngendei site is 2.4±0.1 km/s. This is an important result since reliable estimates of the shear wave velocity of upper oceanic crust are rare. The distances at which amplitude minima occur are used to constrain the compressional wave velocity of the uppermost crust. However, the modulations do not resolve the compressional wave velocity of the crust as well as the shear wave velocity. The relative amplitudes of successive water waves indicate that the reflection coefficient at the sediment‐basement interface is 0.66. This further constrains the compressional wave velocity of the upper crust. Using estimates of the sediment properties and crustal shear wave velocity, the compressional wave velocity of the upper crust at the Ngendei site is estimated from this reflection coefficient to be 4.3–4.4 km/s. A method for estimating the velocity gradient in the upper kilometer of the crust from the interference of water waves with upgoing crustal refractions is presented. Application of the method to these data indicate that the shear wave velocity gradient is about 1.0 s−1, which is indistinguishable from the previous determination by Shearer and Orcutt (1986) using a quite different methodology. The velocity of the upper crust at the Ngendei site is consistent with velocities reported for other regions of similar crustal age (>100 m.y.) and is much greater than velocities typically found for young oceanic crust (<5 m.y.). This lends support to current theories regarding the evolution of the uppermost oceanic crust that attribute higher velocities in older crust to the closing of cracks and pores.

Shaker tests on downhole seismic tools

Shaking a downhole seismic tool in situ is a powerful test procedure able to detect abnormal acoustical behavior. It consists of feeding a swept voltage to a special geophone located inside the tool, close to the triaxial measurement geophones. The special geophone creates a swept force, which excites the tool‐formation system. The resulting motion of the tool is recorded through the triaxial geophones. In this paper, a simple model of the tool‐formation system excited by the shaker is given. Predictions from this model compare nicely with results from a field experiment conducted with a very compact seismic tool. Another field experiment, conducted with a longer tool, shows that tool intrinsic modes are clearly visible in the shaker data. Their frequencies and amplitudes depend upon the geometry of the contact between the tool and the formation. A comparison between shaker data and impulsive shot data shows that moderate modal vibrations do not significantly deteriorate the quality of VSP data, but that large modal vibrations, when present, are visible on the shot data and also correspond to an increased sensitivity to tube waves. Recording one shaker trace at each depth allows routine well‐site quality control of VSP data. Such quality control is especially important for the horizontal axes of a tool and in an open hole, where coupling conditions are not easily controlled.

Seismic sensing of shallow water sound

It has long been known that in shallow water low‐frequency sound below first‐mode cutoff travels principally via the bottom. Moreover, a pressure sensitive hydrophone is a poor sensor under these conditions because of the proximity of the pressure‐release sea surface. For these reasons one would suspect that under these conditions a more effective sensor for sound generated in the water would be a vibration‐sensitive geophone tightly coupled to the sea bottom. That such is the case has been indicated by field work done recently by Northrop off San Diego, by Urick off Ft. Lauderdale, Florida, and on two occasions, by McLeroy off Panama City, Florida. In these experiments a triaxial geophone, having two horizontal and one vertical components, was compared with a nearby hydrophone. In each case, a distant underwater source was received on one or more of the geophone components with a higher signal‐to‐noise ratio than on the hydrophone. A likely suggestion is that initially water borne sound travels to long ranges in a sedimentary column as deep‐going low‐loss upward‐refracted compressional waves. More theoretical and controlled field work is needed.