Anton Ziolkowski

The signature of an air gun array: Computation from near-field measurements including interactions

We designed a system to enable the signature of an air gun array to be calculated at any point in the water from a number of simultaneous independent measurements of the near-held pressure held [subject of a patent application]. The number of these measurements must not be less than the number of guns in the array. The underlying assumption in our method is that the oscillating bubble produced by an air gun is small compared with the wavelengths of seismic interest. Each bubble thus behaves as a point source, both in the generation of seismic waves and in its response to incident seismic radiation produced by other nearby bubbles. It follows that the intcraction effects between the bubbles may be described in terms of spherical waves. The array of interacting guns is equivalent to a notional array of noninteracting guns whose combined seismic radiation is identical. The seismic signatures of the equivalent independent elements of this notional array can be determined from the near-held measurements. The seismic radiation pattern emitted by the whole array can be computed from these signatures by linear superposition, with a spherical correction applied. The method is tested by comparing far-field signatures computed in this way with field measurements made in deep water. The computed and measured signatures match each other very closely. By comparison, signatures computed neglecting this interaction are a poor match to the

Multitransient electromagnetic demonstration survey in France

Wedescribetheacquisition,processing,andinversionofa multitransient electromagnetic MTEM single-line survey, conducted in December 2004 over an underground gas storage reservoir in southwestern France. The objective was to find a resistor corresponding to known gas about 500 m belowthesurveyline.Indataacquisition,wedeployeda100-m inline bipole current source and twenty 100-m inline potentialreceiversinvariousconfigurationsalongthe5-kmsurvey line; we measured the input current step and received voltages simultaneously. Then we deconvolved the received voltagesforthemeasuredinputcurrenttodeterminetheearth impulse responses.We show how both amplitude and traveltimeinformationcontainedintherecoveredearthimpulseresponses reveal the lateral location and approximate depth of the resistive reservoir. Integrating the impulse responses yields step responses, from which the asymptotic DC values were estimated and used in rapid 2D dipole-dipole DC resistivityinversiontofindthetopofthereservoir.Aseriesofcollated 1D full-waveform inversions performed on individual common midpoint gathers of the step responses position the top and bottom of a resistor corresponding to known gas in thereservoirandalsoobtainthetransverseresistance.Theresults imply that the MTEM method can be used as a tool for hydrocarbonexplorationandproduction.

The Critical Reflection Theorem

In predictive deconvolution of seismic data, it is assumed that the response of the earth is white. Any nonwhite components are presumed to be caused by the source wavelet or by unwanted multiples. We show that this whiteness assumption is invalid at precritical incidence. We consider plane waves incident on a layered acoustic half‐space. At exactly critical incidence at any interface in the half‐space, the lower layer acts similar to a rigid plate. The response of the half‐space is then all‐pass, or white. This result we call the critical reflection theorem. The response is also white if the waves are postcritically incident on the lower half‐space. In normal data processing these postcritical components are removed by muting. Thus the whiteness assumption is normally applied to exactly that part of the data where it is invalid. The demarcation between precritical and postcritical incidence can be exploited for the purposes of deconvolution, provided the data can be decomposed into plane waves. To develo...

Removal of the Air Wave In Shallow Marine Transient EM Data

We describe a method to remove the air wave from shallow marine transient EM data. At a long enough offset the air wave dominates the measurement and the response of the earth is negligible. Thus a long-offset measurement can be used to estimate the shape of the air wave which is the same at short offsets, provided the water is shallow. We remove the air wave by filtering. We demonstrate the method on both synthetic and real data.

New Sedimentary Basin on Rockall Plateau

ROCKALL PLATEAU (Fig. 1) is a well defined shoal area of depth less than 1,500 m, some 450 by 600 km in extent and separated from the continental shelf around Great Britain by the Rockall Trough, 2,700 m deep. In the NE corner of Rockall Plateau, Rockall Island is made of aegirine granite of Lower Eocene age1,2. A recent magnetic survey in the vicinity shows this to be part of a planated Tertiary volcanic centre3. A review of the geological and geophysical data over Rockall Plateau and the neighbouring deep ocean (unpublished results of D. G. R.) suggests that the plateau has many of the characteristics of continental structures and that it may be a continental fragment separated from Europe and Greenland during the break-up of Laurasia as suggested by Bullard, Everett and Smith4.

Wavelets, well ties, and the search for subtle stratigraphic traps

We examine the conventional methodology for tying wells to processed seismic data and show why this methodology fails to allow for reliable interpretation of the seismic data for stratigraphy. We demonstrate an alternative methodology that makes the tie without the use of synthetic seismograms, but at the price of measuring the seismic source signature, the cost of such measurements being about 1% of data acquisition costs. The essence of the well tie is (1) to identify geological and seismic interfaces from the logs and core, (2) to measure the one‐way traveltime to these interfaces using downhole geophones, and (3) to use the polarity information from (1) and the timing information from (2) to identify the horizons on the zero‐phase processed seismic data. Conventional processing of seismic data usually causes the wavelet to vary from trace to trace, and conventional wavelet extraction at a well using the normal‐incidence reflection coefficients relies on a convolutional relationship between these coeff...

Determination of the signature of a dynamite source using source scaling; Part I, Theory

It is normally impossible to measure the source signature in land seismic data acquisition with a dynamite source, because it is normally impossible to separate the incident field from the scattered field. Nevertheless, in any serious attempt to invert the seismic data, it is essential to know the source signature; for the dynamite source this is the volume injection function. The problem can be solved by using two different shots at each shot point and relating the source signatures by the source scaling law, which follows from the invariance of the medium parameters with the size of the charge. The volume injection function of the larger shot is an amplified and stretched version of that of the smaller shot, the amplification factor being equal to the ratio of the charge masses and the time stretch factor being equal to the cube‐root of this ratio. At a given receiver, the response to one shot is a convolution of the source signature with the impulse response of the earth, plus noise. The two shots and ...

Determination of the signature of a dynamite source using source scaling, Part 2: Experiment

In April 1990 we performed an experiment in the Netherlands to test the theory of the determination of the signature of a dynamite source using the scaling law. The theory says that the source signature may be determined from the recorded seismic data using two shots of different charge size at the shotpoint; we used 125 g and 500 g charges. The theory was put at risk with a 250-g test charge at each shotpoint. According to the theory, the test record should be different from the other two and, apart from the noise, should be predictable from them. This experiment was repeated 95 times at approximately 50 m shotpoint intervals, using a 240-channel recording system. The results corroborate the theory within an acceptable error. The second‐derivative of the volume injection function was extracted as the source signature; it varied slightly from shot to shot and was minimum phase. This new method of seismic data acquisition allows the signature of the dynamite source to be obtained from the data, uncontamina...

Hydrocarbon Detection With a Multi-channel Transient Electromagnetic Survey

Summary We describe the acquisition of multi-channel transient electromagnetic data over an underground gas storage reservoir in France. We also outline a new pre-processing methodology for handling such data and show 2 preprocessed common-source gathers which indicate the presence of the reservoir. Finally, a processing method is described that will enable resistivity to be determined directly from the data.

Seismic source decomposition

We exploit the differences that exist between the radiation fields of a point source and an array to design a time‐separated marine seismic source array with desired power spectral and directional characteristics, whose far‐field time signature is known precisely from measurements. The desired power spectral characteristics are created by firing a predetermined series of point source units sequentially, such that their time signatures do not overlap. The effective power spectrum of the whole series of time‐distributed signatures can be made to approximate the sum of the power spectra of the individual signatures and can, therefore, be designed to suit the desired application by the appropriate choice of source units. The desired directional characteristics of the array can be created by arranging the source unit separations such that each source unit reaches the desired spatial position at the prescribed firing instant. The key to the subsequent processing of the recorded data is to measure the pressure w...