Robert Laws

University of New Hampshire

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Co-Authors

E. Kragh

Schlumberger (Norway)

Jon‐Fredrik Hopperstad

Schlumberger (British Virgin Islands)

David Halliday

Schlumberger (British Virgin Islands)

Ed Kragh

Schlumberger (Norway)

AÖ

Ali Özbek

ETH Zurich

Johan O. A. Robertsson

ETH Zurich

Lasse Amundsen

Equinor (United Kingdom)

Emily Shuckburgh

University of Cambridge

Timothy J. Grant

University of Tennessee at Knoxville

Les Hatton

Kingston University

Cooperative Institutions

Schlumberger (British Virgin Islands)

Schlumberger (United Kingdom)

Schlumberger (Norway)

Shearwater Health

Netherlands Organisation for Applied Scientific Research

National Physical Laboratory

University of New Hampshire

Equinor (Norway)

Norwegian University of Science and Technology

University of Edinburgh

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Estimating the signature of a marine vibrator by joint inversion of hydrophone and accelerometer measurements

R. Telling Arash JafarGandomi Robert Laws S. Grion

Summary This paper examines a method of signature estimation for marine vibrators that jointly uses measurements of the acceleration of the vibrating surfaces and the pressure in the near-field to invert for notional sources. The method enables an optimal estimate of the wavefield emitted in the water by a vibrator unit and has the flexibility to take into account transducer-related distortions, complexity in the radiation pattern and interaction with nearby interfaces. Using simple approximations for the relationship between the measurements and the expected form of the wavefield, we apply the method to data acquired in a lake test. Predicted pressures are calculated and compared with observed hydrophone measurements made at locations in the water surrounding the vibrator, demonstrating the robustness and accuracy of the method.

Hydrophone

Vibrator (electronic)

Robustness

10.3997/2214-4609.202310584

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Citations (3)

An Experimental Comparison of Three Direct Methods of Marine Source Signature Estimation

Proceedings (1995)

Robert Laws Martin Landrø Lasse Amundsen

Three, published methods for estimating the source signature of a marine seismic array are compared experimentally. The methods each use measurements of the pressure-field near to the array to predict the pressure-field at arbitrary positions further from the array. All three methods offer the possibility of computing the far-field signature, in absolute amplitude, as a function of radiation angle on a shot by shot basis during the acquisition. Such an estimate is of great value in acquisition quality-control and in data-processing.

Signature (topology)

Basis (linear algebra)

10.3997/2214-4609.201409276

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Citations (1)

Rough seas and statistical deconvolution

Geophysical Prospecting (2006)

Ed Kragh Robert Laws

ABSTRACT The rough‐sea reflection‐response varies (1) along the streamer (2) from shot to shot and (3) with time along the seismic trace. The resulting error in seismic data can be important for time‐lapse imaging. One potential way of reducing the rough‐sea receiver error is to use conventional statistical deconvolution, but special care is needed in the choice of the design and application windows. The well‐known deconvolution problem associated with the non‐whiteness of the reflection series is exacerbated by the requirement of an unusually short design window – a requirement that is imposed by the non‐stationary nature of the rough‐sea receiver wavelet. For a synthetic rough‐sea data set, with a white 1D reflection series, the design window needs to be about 1000 ms long, with an application window about 400 ms long, centred within the design window. Although such a short design window allows the deconvolution operator to follow the time‐variation of the rough‐sea wavelet, it is likely to be too short to prevent the non‐whiteness of the geology from corrupting the operator when it is used on real data. If finely spatial‐sampled traces are available from the streamer, the design window can be extended to neighbouring traces, making use of the spatial correlations of the rough‐sea wavelet. For this ‘wave‐following’ approach to be fruitful, the wind (and hence the dominant wave direction) needs to be roughly along the line of the streamer.

Reflection

Operator (biology)

10.1111/j.1365-2478.2006.00549.x

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Citations (14)

Noise Characteristics of Near-Gun Hydrophone Data used in Imaging

81st EAGE Conference and Exhibition 2019 (2019)

P. A. F. Christie Robert Laws E. Kragh Peter Nevill K. Davies

Summary Near-field hydrophones (NFH) record both airgun signatures in a source array and reflections from the sub-surface. Previous work has found such data suitable for imaging to 1,000 ms or more. Using flip-flop NFH data from a Chevron seabed node survey, we characterise noise in data recorded in both passive mode (flop source NFH sensors listen while flip source fires) and in active mode (flip sensors listen while flip source fires). We find passive NFH data are good quality, though low-frequency, direct bubble arrivals from the active source require attenuation to retain low-frequency signal needed for penetration. Ambient noise increases aftwards in passive sub-arrays with the tail sensor strongly affected by noise and signal loss, probably due to water aerated by the previous shot. Only leading NFH sensors record good reflection signal on active data. Bubble arrivals dominate and following sensors show significant signal loss. We suggest that trailing NFH sensors enter water aerated by the preceding cluster, reducing signal sensitivity. Our analysis finds that the compact isotropic source generates a localized, cavitational, secondary source that is relatively low amplitude and high frequency but is likely to be visible in direct-arrival signatures at seabed nodes.

Hydrophone

Ambient noise level

SIGNAL (programming language)

10.3997/2214-4609.201901407

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Citations (1)

Where is the center of a multi‐depth marine source array?

Jon‐Fredrik Hopperstad Robert Laws Ed Kragh

In previous work, we discussed how to design a marine source array, using multiple source depths, that emits the same pulse shape (including the ghost reflection) in all directions. In this abstract, we address the question: Where is the (acoustic) center of a multi‐depth source array? The acoustic center is the point from which the signal appears to have radiated. The center might be different for different frequencies; the one that minimizes the phase error across the whole bandwidth is termed the "phase center". The phase center of a single depth array is at the sea surface. For multi‐depth arrays, the optimum location of the center is complicated, because of the delay that is applied to the deeper elements in order to align the vertically propagating wavefields. In the examples discussed in this document, it is found that the optimum location (i.e., the phase center) varies from the depth of deepest source layer to about the same depth as the shallowest layer, depending on the ratio of the depths.

Center (category theory)

10.1190/1.3054834

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Citations (11)

Rough seas and time‐lapse seismic

Geophysical Prospecting (2002)

Robert Laws Ed Kragh

ABSTRACT Time‐lapse seismic surveying has become an accepted tool for reservoir monitoring applications, thus placing a high premium on data repeatability. One factor affecting data repeatability is the influence of the rough sea‐surface on the ghost reflection and the resulting seismic wavelets of the sources and receivers. During data analysis, the sea‐surface is normally assumed to be stationary and, indeed, to be flat. The non‐flatness of the sea‐surface introduces amplitude and phase perturbations to the source and receiver responses and these can affect the time‐lapse image. We simulated the influence of rough sea‐surfaces on seismic data acquisition. For a typical seismic line with a 48‐fold stack, a 2‐m significant‐wave‐height sea introduces RMS errors of about 5–10% into the stacked data. This level of error is probably not important for structural imaging but could be significant for time‐lapse surveying when the expected difference anomaly is small. The errors are distributed differently for sources and receivers because of the different ways they are towed. Furthermore, the source wavelet is determined by the sea shape at the moment the shot is fired, whereas the receiver wavelet is time‐varying because the sea moves significantly during the seismic record.

Flatness (cosmology)

Repeatability

Synthetic seismogram

10.1046/j.1365-2478.2002.00311.x

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Citations (62)

Exploiting Phase Control Of A Marine Seismic Vibrator For High-Multiplicity Simultaneous Source Acquisition

Proceedings (2018)

David Halliday Robert Laws J-F Hopperstad Mehul Supawala

A high-fidelity marine seismic vibrator would allow control of the phase of the seismic source wavefield. Phase control can be used to encode high-multiplicity simultaneous-source data. By changing the phase of the source wavefield from shot-to-shot following a prescribed sequence of phase delays, energy from selected sources can be coherently shifted into different parts of the frequency-wavenumber spectrum in the common-receiver domain. Results of this approach are demonstrated using realistic synthetic examples. A comparison with the more conventional time-dithered air-gun approach indicates that the phase sequencing approach gives an improved result, with a lower separation error. The phase sequencing approach cannot be used as effectively with air guns because they only allow changes to the overall time delay of the source.

Seismic vibrator

Vibrator (electronic)

Multiplicity (mathematics)

10.3997/2214-4609.201802105

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Citations (4)

Full-wavefield, Towed-marine Seismic Acquisition and Applications

Proceedings (2012)

Johan O. A. Robertsson David Halliday D.J. van Manen Ivan Vasconcelos Robert Laws

A four-component (4C) streamer recording pressure as well as the three-component particle velocity vector, addresses long-standing geophysical problems such as receiver-side sampling and deghosting. In this paper, we introduce multicomponent marine seismic sources generating monopole and dipole responses in the water. We describe a few different alternatives for generating such a source using existing technology. Three different application areas are described in some detail: source-side deghosting, source-side wavefield reconstruction and, finally, a vector-acoustic reverse-time imaging approach that requires monopole and dipole data on both the source and receiver side of the acquisition.

Component (thermodynamics)

10.3997/2214-4609.20148849

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Citations (19)

Full-wavefield, towed-marine seismic acquisition and applications

David Halliday Johan O. A. Robertsson Ivan Vasconcelos Dirk‐Jan van Manen Robert Laws

A four-component (4C) streamer recording pressure, as well as the three-component (3C) particle velocity vector, addresses long-standing geophysical problems such as receiver-side sampling and deghosting. In this paper, we introduce multicomponent marine seismic sources generating monopole and dipole responses in the water. We describe a few different alternatives for generating such a source using existing technology. Three different application areas are described in some detail: source-side deghosting, source-side wavefield reconstruction and, finally, a vector-acoustic reverse-time imaging approach that requires monopole and dipole data on both the source and receiver side of the acquisition.

10.1190/segam2012-0446.1

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Citations (19)

Sea surface shape derivation above the seismic streamer

Geophysical Prospecting (2006)

Robert Laws Ed Kragh

ABSTRACT The rough sea surface causes perturbations in the seismic data that can be significant for time‐lapse studies. The perturbations arise because the reflection response of the non‐flat sea perturbs the seismic wavelet. In order to remove these perturbations from the received seismic data, special deconvolution methods can be used, but these methods require, as input, the time varying wave elevation above each hydrophone in the streamer. In addition, the vertical displacement of the streamer itself must also be known at the position of each hydrophone and at all times. This information is not available in conventional seismic acquisition. However, it can be obtained from the hydrophone measurements provided that the hydrophones are recorded individually (not grouped), that the recording bandwidth is extended down to 0.05 Hz and that data are recorded without gaps between the shot records. The sea surface elevation, and also the wave‐induced vertical displacement of the streamer, can be determined from the time‐varying pressure that the sea waves cause in the hydrophone measurements. When this was done experimentally, using a single sensor seismic streamer without a conventional low cut filter, the wave induced pressure variations were easily detected. The inversion of these experimental data gives results for the sea surface elevation that are consistent with the weather and sea state at the time of acquisition. A high tension approximation allows a simplified solution of the equations that does not demand a knowledge of the streamer tension. However, best results at the tail end of the streamer are obtained using the general equation.

Hydrophone

10.1111/j.1365-2478.2006.00583.x

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Citations (10)