Brad Artman

Imaging passive seismic data

Imaging passive seismic data is the process of synthesizing the wealth of subsurface information available from reflection seismic experiments by recording ambient sound using an array of geophones distributed at the surface. Crosscorrelating the traces of such a passive experiment can synthesize data that are identical to actively collected reflection seismic data. With a correlation-based imaging condition, wave-equation shot-profile depth migration can use raw transmission wavefields as input for producing a subsurface image. Migration is even more important for passively acquired data than for active data because with passive data, the source wavefields are likely to be weak compared with background and instrument noise — a condition that leads to a low signal-to-noise ratio. Fourier analysis of correlating long field records shows that aliasing of the wavefields from distinct shots is unavoidable. Although this reduces the order of computations for correlation by the length of the original trace, the...

Signal-to-noise estimates of time-reverse images

Locating subsurface sources from passive seismic recordings is difficult when attempted with data that have no observable arrivals and/or a low signal-to-noise ratio (S/N). Energy can be focused at its source using time-reversal techniques. However, when a focus cannot be matched to a particular event, it can be difficult to distinguish true focusing from artifacts. Artificial focusing can arise from numerous causes, including noise contamination, acquisition geometry, and velocity model effects. We present a method that reduces the ambiguity of the results by creating an estimate of the (S/N) in the image domain and defining a statistical confidence threshold for features in the images. To do so, time-reverse imaging techniques are implemented on both recorded data and a noise model. In the data domain, the noise model approximates the energy of local noise sources. After imaging, the result also captures the effects of acquisition geometry and the velocity model. The signal image is then divided by the noise image to produce an estimate of the (S/N). The distribution of image (S/N) values due to purely stochastic noise provides a means by which to calculate a confidence threshold. This threshold is used to set the minimum displayed value of images to a statistically significant limit. Two-dimensional synthetic examples show the effectiveness of this technique under varying amounts of noise and despite challenging velocity models. Using this method, we collocate anomalous low-frequency energy content, measured over oil reservoirs in Africa and Europe, with the subsurface location of the productive intervals through 2D and 3D implementations.

Teleseismic shot-profile migration

The shot-profile migration approach of wave-equation migration generates subsurface images using the interferometric principle of crosscorrelating two passive wavefields. These wavefields are typically a source wavefield containing energy from an excited source and a receiver wavefield comprised of scattered-source wavefield energy by the discontinuous earth structure. Shot-profile migration can be recast as a novel way of imaging the earth’s lithosphere using teleseismic wavefield data, where the source wavefield is the directly arriving wavefront and the receiver wavefield is the following wavefield coda. We demonstrate that the shot-profile technique can be tailored to suit teleseismic acquisition geometry and wavefields. Assuming an acoustic framework and 2.5D experimental geometry, we develop procedures that enable kinematic and structural imaging (migration) using both transmission and free-surface reflected passive wavefields. Experiments with synthetic data demonstrate the method’s applicability a...

Image-space surface-related multiple prediction

A very important aspect of removing multiples from seismic data is accurate prediction of their kinematics. We cast the multiple prediction problem as an operation in the image space parallel to the conventional surface-related multiple-prediction methodology. Though developed in the image domain, the technique shares the data-driven strengths of data-domain surface-related multiple elimination (SRME) by being independent of the earth (velocity) model. Also, the data are used to predict the multiples exactly so that a Radon transform need not be designed to separate the two types of events. The cost of the prediction is approximately the same as that of data-space methods, though it can be computed during the course of migration. The additional cost is not significant compared to that incurred by shot-profile migration, though split-spread gathers must be used. Image-space multiple predictions are generated by autoconvolving the traces in each shot-gather at every depth level during the course of a shot-profile migration. The prediction in the image domain is equivalent to that produced by migrating the data-space convolutional prediction. Adaptive subtraction of the prediction from the image is required. Subtraction in the image domain, however, provides the advantages of focused energy in a smaller domain since extrapolation removes some of the imperfections of the input data.

Migration methods for passive seismic data

Passive seismic imaging is based on the fact that by cross-correlating the transmission responses of a medium, one can reconstruct its reection response. Here, we show a method to directly migrate the transmission responses measured at the surface, based on the shot-prole migration. We also show that the results from direct migration of passive data and from migration of the simulated reection response are identical. At the end, we also show comparisons between the behavior of the results from the migration process and the simulated reection responses.

Low frequency measurements of seismic wave attenuation in Berea sandstone

We have designed and set up a pressure vessel for 250 mm long and 76 mm in diameter cylindrical samples to measure seismic wave attenuation in rocks at frequencies between 0.01 and 100 Hz and to verify the occurrence of fluid-flow induced by stress field changes. A dynamic stress is applied at the top of the rock cylinder by a piezoelectric motor generating either a stress step of several kPa in few milliseconds or a mono-frequency force. A load cell measures force and a strain sensor the bulk axial shortening across the sample. Five pressure sensors are buried at different heights of the cylinder to measure pore pressure changes related to stress field changes. The sample is sealed in a pressure vessel that can reach confining pressures of 25 MPa. We present datasets collected at room pressure and temperature. Three attenuation data curves measured on reference samples demonstrate the accuracy of the apparatus. A test of the influence of the static stress applied on the sample on the attenuation measurements and measurements conducted for frequencies between 0.1 and 50 Hz with strain < 5e-6 on partially saturated Berea sandstone are presented. Timeevolution pore-pressure curves due to stress field changes are also given.

Microseismic monitoring — Introduction

Most papers the readers find in this special section were presented at the First International Workshop on Microseismic Technology held in Asheville, North Carolina, USA, on 17–22 August 2014. The workshop’s organizing and technical program committee consisted of Werner Heigl (Apache Corporation), Vladimir Grechka (Marathon Oil), Leo Eisner (IRSM Academy of Sciences of the Czech Republic), Christophe Maisons (Magnitude), Serge Shapiro (Freie University, Berlin), Julie Shemeta (MEQ Geo), and Stephen Wilson (Seismogenic). The workshop, designed to facilitate sharing of microseismic expertise and the key observations and findings, was organized with special attention to balancing the needs of industry and academia. To encourage participation from the industry, only brief abstracts of papers were requested and no recording was permitted at the workshop; to attract academic contributions, the special section in Geophysics was offered to authors who would be willing to develop their workshop presentations into full-scale papers. The workshop, attended by 80 geophysicists and engineers from 11 countries, was enthusiastically supported by operating and service companies, with sponsorship from ITASCA, Magnitude, Marathon Oil, Pinnacle, Read, Sigma 3 , and …

Laboratory Measurements of Seismic Attenuation from 0.01 to 100 Hz

The study of wave attenuation in partially saturated porous rocks over a broad frequency range provides valuable information about the fluid system of reservoirs, which are inherently multiple phase fluid systems. Until now, not much laboratory data have been collected in the seismically relevant low frequency range and existing literature data on experimental measured partially saturated rock are very limited. The main goal of our work is to experimentally measure the bulk seismic attenuation on fluid-bearing rocks, using natural rock samples in an efficient way at in situ conditions and employing linear variable differential transformers (LVDTs). We are currently fine-tuning our attenuation measurement prototype. Bench-top results are promising and show consistency with reported experimental data for dry, partially, and fully fluid saturated rocks. Measurements with the machine are accurate and precise. We are able to detect a wide range of attenuation values, from nearly elastic materials, like aluminum, up to very well characterized viscoelastic material, such as Plexiglas. This can be considered the end-members for a saturated rock in the low frequency range at different degrees of saturation.

Fourier‐domain imaging condition for shot‐profile migration

Cross-correlating up-coming and down-going wavefields inh erently applies a spatial multiplication. This multiplication could be performed in the wave-number domain as a convolution. However, the full imaging condition, including s ubsurface offset, transforms to a Fourier domain equivalent that is also a lagged multipli cation. This fact allows for the simple analysis of anti-aliasing criteria. Migrations with synthetic data with flat and dipping reflectors in a homogeneous medium are produced to ev aluate the Fourier domain algorithm and shots from the Marmousi data set are shown as ex mples of its efficacy. Periodic replications in the image space are introduced whe n solving the imaging condition in the Fourier domain which make results unsatisfactor y. The cost of computing the imaging condition in the Fourier domain is much higher than i ts space domain equivalent since very few subsurface offsets need to be imaged if the vel ocity model is reasonably accurate. Analysis of the Fourier domain imaging condition leads to the conclusion that anti-aliasing efforts can be implemented post-migration.

Operator aliasing in wavefield continuation migration

With the widespread adoption of wavefield continuation meth ods for prestack migration, the concept of operator aliasing warrants revisiting. Whil e zero-offset migration is unaffected, prestack migrations reintroduce the issue of ope rator aliasing. Some situations where this problem arises include subsampling the shot-axes to save shot-profile migration costs and limited cross-line shot locations due to acquisit ion strategies. These problems are overcome in this treatment with the use of an appropriate source function or bandlimiting the energy contributing to the image. We detail a synthetic experiment that shows the ramifications of subsampling the shot axis and the efficac y of addressing the problems introduced with our two approaches. Further, we explain how these methods can be tailored in some situations to include useful energy residing outside of the Nyquist limits.