Christine E. Krohn

The microgeometry and transport properties of sedimentary rock

Abstract This monograph describes recent progress in modelling the transport properties of sedimentary rock. Statistical descriptions are applied to the pore-space geometry and to the transport processes involving pore fluids. Fractals are used to quantify the pore geometry at length scales shorter than grain size. Percolation theory is applied to fluid flow. The permeability can be expressed in terms of a single effective pore diameter measured from mercury injection capillary pressure. This permeability relation is valid for essentially all porous rock and for a broad class of porous media. Mercury injection provides a powerful caliper of the geometry of a percolation cluster in a pore-space and supplies new information about pore space correlations and dynamics of fluid displacements. The statistical description of fluid transport in porous media has analogues in disordered electronic and magnetic materials. Future work may make substantial use of such analogues to solve more complex problems of direct...

Geophone ground coupling

For frequencies much lower than the coupling resonant frequency, the geophone accurately follows the ground motion, but for higher frequencies the coupling can alter both the amplitude and phase of the seismic signal. The normal planting of vertical geophones in the field results in coupling adequate for conventional recording that uses frequencies less than 100 Hz. However, for very loose soils or for high-frequency seismic recording, I recommend that the geophones be buried to place the geophones in firmer soil. The coupling resonant frequency for vertical geophones is determined by the firmness of the soil, and I have measured resonant frequency ranging from 100 to 500 Hz at different locations. Because the firmness of the soil increases with depth, the coupling resonant frequency can be increased by burial of the geophones or by the use of longer spikes. The rocking of horizontal geophones causes a low-frequency coupling resonance. It is crucial that horizontal geophones be planted with their bases firmly contacting the soil. Geophones so planted have a resonance around 130 Hz, whereas those 1 cm off the ground can have a resonance of 30 Hz or lower. Soil conditions have little effect on the resonant frequency. Horizontal geophones with 1-inch spikes are as well coupled as those with longer spikes, but the best coupling is achieved by burial of the geophones.--Modified journal abstract.

Efficient seismic forward modeling using simultaneous random sources and sparsity

The high cost of simulating densely sampled seismic forward modeling data arises from activating sources one at a time in sequence.Toincreaseefficiency,onecouldleveragerecentinnovations in seismic field-data acquisition and activate several e.g., 2‐6 sources simultaneously during modeling. However, such approaches would suffer from degraded data quality because of the interference between the model’s responses to the simultaneoussources.Twonewefficientsimultaneous-sourcemodeling approachesareproposedthatrelyonthenoveltandemuseofrandomness and sparsity to construct almost noise-free model response to individual sources. In each approach, thefirst step is to measure the model’s cumulative response with all sources activated simultaneously using randomly scaled band-limited impulses or continuous band-limited random-noise waveforms. In the second step, the model response to each individual source is estimated from the cumulative receiver measurement by exploiting knowledge of the random source waveforms and the sparsity of the model response to individual sources in a known transformdomaine.g.,curveletdomain.Theefficiencyachievable by the approaches is primarily governed by the sparsity of the model response. By invoking results from the field of compressive sensing, theoretical bounds are provided that assert that the approaches would need less modeling time for sparser i.e., simpler or more structured model responses.Asimulated modelingexampleisillustratedthatshowsthatdatacollectedwithas manyas8192sourcesactivatedsimultaneouslycanbeseparated into the 8192 individual source gathers with data quality comparable to that obtained when the sources were activated sequentially.Theproposedapproachescouldalsodramaticallyimprove seismic field-data acquisition efficiency if the source signatures actuallyprobingtheearthcanbemeasuredaccurately.

HFVS™ : Enhanced data quality through technology integration

Known problems with vibroseis data include difficult-to-pick and inaccurate first-arrival times, poor well ties, correlation side lobes, harmonic ghosts, and coupling differences. Also, to reduce acquisition costs, various methods are used to record and then separate data from different source locations using vibrators sweeping simultaneously, but these methods suffer from poor data separation and harmonic contamination. A novel combination of heritage Mobil High Fidelity Vibratory Seismic (HFVS) and heritage Exxon vibrator technologies solves these problems with vibroseis data. The method involves vibrator separation combined with vibrator signature deconvolution in such a manner that the output is minimum phase and matches impulsive data. Vibrator signatures are calculated from the vibrator accelerometer measurements. The signatures from multiple vibrators and multiple sweeps are used to design a filter that optimally separates the data from each vibrator and replaces the signatures with a specially des...

Cross‐well continuity logging using guided seismic waves

Cross‐well seismic recording with sources and receivers in different wells can potentially image the internal structure of reservoirs with resolution on the order of a few feet. Transmission and reflection images can be made, but both have inherent limitations. The well separation is generally limited by the source strength to less than 2000 ft, and coplanar wells are desired. Also, adequate imaging requires a large number of source and receiver locations, resulting in expensive acquisition costs. Removal of production tubing from both wells is often necessary, adding even more to the cost. Given these limitations, imaging methods are generally restricted to densely drilled mature fields where budgets for data acquisition are limited.

Comparisons of downhole geophones and hydrophones

Receiver tests were conducted to compare the responses of downhole geophones and hydrophones. Commercial receiver tools use a maximum of eight geophone levels; however, we use hydrophones because we can record 48 levels simultaneously. For frequencies above 300 Hz, signal‐to‐background‐noise ratios for hydrophones and geophones in a prototype tool were comparable. (This prototype tool is a lightweight, large‐clamping‐force device that can record higher frequencies than commercial geophone tools.) For frequencies below 300 Hz, signal‐to‐noise ratios were greater for the geophones than for the hydrophones. A commercial geophone tool had lower low‐frequency signal‐to‐background‐noise ratios than the prototype tool, but greater than those of the hydrophones. Further analysis was performed to determine why the signal‐to‐background‐noise ratios for geophones were greater than those for hydrophones at low frequencies. The measured signal level for a hydrophone was 2.4 times that for a geophone, compared with a t...

High Fidelity Vibratory Seismic (HFVS) I: Enhanced Data Quality

We present here a novel combination of heritage-Mobil and heritage-Exxon vibrator technologies, which improves on both. The following problems with vibratory data are solved or reduced: • first-arrival times difficult to pick • first-arrival times not accurate • well ties poor • pulses ring from correlation sidelobes • noise from harmonic ghosts • vibrator coupling differences In addition, data from multiple vibrators can be separated by 60 dB or more. The advantages of processing separated vibrator data as unique source points are discussed in a second paper (Krohn and Johnson, 2003).

Constrained Polarization Filtering For Surface-wave Mitigation

We present a new method of surface-wave mitigation using polarization filtering. The method evolves from the polarization-analysis technique developed by Diallo et al., (2006) and introduces new constraints to effectively detect and mitigate surface waves without damaging the signal. Straightforward application of polarization filtering without these constraints results in ineffective filtering or damage to the signal, due to the complexity of surface-wave wavetrains. We illustrate the performance of the method with examples from multicomponent seismic data, and demonstrate the superiority of the filtering compared to the unconstrained approach.

Exploiting Surface Consistency For Ground-roll Characterization And Mitigation

A tomography-like-method is used to invert surfaceseismic data to estimate variable surface-wave properties. Surface consistency is exploited to decompose the data in the frequency domain into frequency-dependent propagation (i.e., velocity and attenuation) effects and variable sourceand receiver-coupling effects. The inversion can be performed simultaneously for single modes (linear optimization) or for multiple modes (nonlinear optimization). Including sourceand receivercoupling variations improves the ability to estimate velocity and attenuation from multi-source multi-receiver data. Further improvements in the estimation are made by constraining the parameters to be a smooth function of frequency. The estimated model parameters can be used to predict the multi-mode ground roll and subtract it from the data with little damage to reflections.

High Fidelity Vibratory Seismic (HFVS) II: Superior source separation

HFVS recording can be used to increase production rates and reduce costs for acquisition or to increase spatial sampling and improve data quality. With the HFVS technique, data from multiple vibrators sweeping simultaneously can be separated with 60 dB of separation. Superior separation is achieved, because measured vibrator signatures from multiple sweeps and multiple vibrators are used to design an optimal separation and inversion filter. After separation, the data from each vibrator can be processed as unique source points. Examples are shown of data quality improvements compared with traditional vibrator arrays from reduction of intra-array effects and suppression of ground roll. Improvements in data quality from reduced correlation sidelobes, harmonic noise, and coupling variations and the ability to accurately pick firstarrival times and control phase are discussed in a first paper (Krohn and Johnson, 2003).