Walter Söllner

Stochastic joint inversion of 2D seismic and seismoelectric signals in linear poroelastic materials: A numerical investigation

The interpretation of seismoelectrical signals is a difficult task because coseismic and seismoelectric converted signals are recorded simultaneously and the seismoelectric conversions are typically several orders of magnitude smaller than the coseismic electrical signals. The seismic and seismoelectric signals are modeled using a finite-element code with perfectly matched layer boundary conditions assuming a linear poroelastic body. We present a stochastic joint inversion of the seismic and seismoelectrical data based on the adaptive Metropolis algorithm, to obtain the posterior probability density functions of the material properties of each geologic unit. This includes the permeability, porosity, electrical conductivity, bulk modulus of the dry porous frame, bulk modulus of the fluid, bulk modulus of the solid phase, and shear modulus of the formations. A test of this approach is performed with a synthetic model comprising two horizontal layers and a reservoir partially saturated with oil, which is embedded in the second layer. The result of the joint inversion shows that we can invert the permeability of the reservoir and its mechanical properties.

Increased Resolution of Seismic Data From a Dual Sensor Streamer Cable

Traditionally, marine cables measure the seismic wavefield pressure using hydrophones. A new solid core dual sensor cable has been introduced (Tenghamn et.al., SEG submitted abstract, 2007). This cable measures the pressure wave-field using hydrophones and simultaneously measures the vertical component of the particle velocity using motion sensors. The advantages of measuring the complete wave field in this manner are numerous. These advantages include reduced acquisition noise and improved ability to acquire data during rough weather. The advantages arise from the fact that the cable can be towed deeper and that both wave-field measures are at exactly the same location. The simultaneous recording at the same location avoids cable positioning difficulties.

Elimination of harmonic distortion in Vibroseis data

We present a new approach to eliminate the harmonic distortion in vibroseis data. This technique is based on the definition of the sweep for the case where the instantaneous frequency varies linearly with time. The method requires no more than the computation of a purely deterministic phase‐shift filter that has the phase spectrum of a hypothetical “kth harmonic distortion.” With this particular phase‐shift filter, all harmonic distortions in the base‐plate signal and those caused by the first strong arrival in the vibroseis trace (which leads to the “correlation ghost sweeps”) are removed. The process consists of (1) applying the deterministic pure phase‐shift filter to the vibroseis trace and the base‐plate signal; (2) setting time values equal to zero for negative times in both the filtered base‐plate signal and the filtered vibroseis trace; and (3) applying the inverse pure phase‐shift filter to both base‐plate signal and vibroseis trace prior to performing the usual crosscorrelation. The correlated t...

NATURE OF SEISMIC REFLECTIONS AND VELOCITIES FROM VSP-EXPERIMENTS AND BOREHOLE MEASUREMENTS AT THE KTB DEEP DRILLING SITE IN SOUTHEAST GERMANY

Abstract The 1989 VSP program at the KTB pilot hole was complemented in 1992 by a standard VSP in the KTB super-deep borehole from 3000 to 6013 m. P-wave velocities oscillate around 5.8 km/s in the upper 3150 m in accordance with sonic log velocities and correlate with paragneisses which prevail in this depth range. At about 3150 m depth, velocities increase to 6.4 km/s correlating with metabasites which dominate in the depth range 3150 to 7500 m. Laboratory measurements and petrophysical modelling provide evidence that the intrinsic velocities were reduced by 5–10% by fracture density and porosity at all depth ranges. Subvertical dip (50–70°) in structures and textures prevail, causing about 10% S-wave anisotropy, indicated by direct observations of S-wave splitting. Pronounced P-wave reflections, accompanied by P- to S-wave conversions and a lack of S-wave reflections, occur in the lower depth range only (3000–6000 m) and correlate with fluid-filled fracture systems. Lithological contrasts (gneiss-amphibolite) play a minor role in generating reflections. The most prominent reflecting elements known from surface profiling and 3D-surveys (i.e. the ‘Franconian Lineament’ reflector at about 7 km, and P-wave reflections at 8.3 km with notably absent associated S-wave reflections) coincide with pronounced anomalies observed in the logging data indicating the presence of major fracture zones.

Fluid reservoir (?) beneath the KTB drillbit indicated by seismic shear-wave observations

Indications for the possible presence of a fluid accumulation below the drillbit of the KTB (German Continental Deep Drilling Project)-Hauptbohrung are provided by comparisons of seismic P-wave and S-wave reflection sections. A bright reflection with negative polarity at approx. 8 km depth is seen only on P-wave sections. It is absent on S-wave sections in contrast to deeper reflections. This observation can be explained by the following geological models or combinations of them: (1) brine accumulation within a strongly fractured rock reservoir, (2) liquid/gas interface within strongly fractured rocks, (3) rock composition with increased quartz content and corresponding low Poissons ratio, and (4) varying symmetry system of seismic anisotropy due to rock foliation. Predictions concerning the nature and location of this reflector are expected to be verified by drilling and direct probing in spring-summer, 1993.

Imaging the sea surface using a dual-sensor towed streamer

Sea-surface profile and reflection coefficient estimates are vital input parameters to various seismic data processing applications. The common assumption of a flat sea surface when processing seismic data can lead to misinterpretations and mislocations of events. A new method of imaging the sea surface from decomposed wavefields has been developed. Wavefield separation is applied to the data acquired by a towed dual-sensor streamer containing collocated pressure and vertical particle velocity sensors to obtain upgoing and downgoing wavefields of the related sensors. Time-gated upgoing and downgoing wavefields corresponding to a given sensor are then extrapolated to the sea surface where an imaging condition is applied so that the time-invariant shape of the sea surface can be recovered. By sliding the data time-window, the temporal changes of the sea surface can be correspondingly estimated. Ray tracing and finite-difference methods were used to generate different controlled data sets used in this feasibility study to demonstrate the imaging principle and to test the image accuracy. The method was also tested on a first field data example of a marginal weather line from the North Sea.

Imaging of Primaries And Multiples Using a Dual-sensor Towed Streamer

Summary Multiple reflections are commonly treated as noise in oneway imaging methods. High effort is put into research and data processing worldwide in an effort to suppress this source of noise. In a new perspective multiples are treated as valuable imaging information. Based on dual-sensor towed streamer measurement, we decompose the wavefield and apply up/down imaging of primary and multiple reflections. This approach is tested using shallow water synthetic data and finally applied on dual-sensor field data.

Surface Related Multiple Suppression in Dual- sensor Towed Streamer Data

Conventional surface related multiple prediction for towed streamer configuration is flawed by the sea surface level variation and sea surface reflection coefficient fluctuation. Knowledge of the sea surface and reflection coefficient allows approximate correction of the prediction errors. Based on a novel dual-sensor towed streamer acquisition we propose a multiple prediction approach from the down-going vertical velocity field and up-going pressure field. This approach handles the obliquity factor and sea surface variations implicitly and may reduce bad-weather-related acquisition downtime.

Diffraction response simulation: A 3D velocity inversion tool

The concept of velocity determination by Diffraction Response Simulation (DRS) consists of the following steps: (1) Determination of the normal moveout velocities; (2) Determination of the zero offset reflection slopes in the stacked cube; (3) Transformation of selected zero offset traces into diffraction point responses – at pre-selected CDP positions; (4) Application of poststack depth migrations in a velocity analysis loop. The focusing of the simulated diffraction response is used as a measure for velocity adjustment. The shape of the interfaces is automatically obtained together with the optimal focus of diffraction energy.

On the Free Surface Assumption for Marine Data Driven Demultiple Methods

In the past, integral formulations for marine data driven demultiple methods have been derived from reciprocity theorems. Two fundamental assumptions in these derivations were that the sea surface is flat and has a known reflection coefficient, often take