Bill Dragoset

Introduction to air guns and air-gun arrays

The effect of seismic operations on marine mammals has been debated vigorously for years. Some feel that these operations could harm the animals. Others, based on anecdotal evidence of marine mammals swimming (or even playing) near active air-gun arrays, feel that harmful effects are unlikely. They claim that such evidence indicates, at the very least, that air guns do not physically damage the animals. Still others have relied on acoustic measurements to argue that any potential effect on the marine mammal population within a seismic survey area is negligible. Because of the importance of the discussion, the seismic industry has conducted numerous tests and monitoring studies to try to address this issue. To date, those studies have not identified any harmful long-term effects due to the proximity of marine mammals to air guns. Why then, does the debate continue? The answer may lie in the shear complexity of the issues. Air-gun design, underwater acoustics, animal behavior, and marine mammal physiology are complex subjects and interactions between them are even more complicated. Although many debate participants are experts in one or more of these fields, none is an expert in all. Thus, individuals can interpret the same data in different ways and report their interpretations using different terms. Suppose two individuals with varying backgrounds observed a dolphin jumping near an active air-gun array. One might see a dolphin “leaping from the water to avoid the noise” while the other may conclude that same dolphin is “playing in the air bubbles.” Because of this communication problem and a lack of definitive scientific studies, no clear consensus has been reached about how air guns affect marine mammals. Thus, many organizations have recommended mitigation practices until a clear answer is found. One common mitigation procedure is air-gun ramp-up, in which guns in an array …

Introduction to this special section Low-frequency seismic

Geophysicists in the seismic exploration industry have long recognized the potential benefits of extending the bandwidth of seismic measurements to well below 10 Hz. Low frequencies generated by a seismic shot travel much farther through the subsurface than do higher frequencies, just as the low frequencies in thunder travel farther in the atmosphere. Thus, the amount of power available in the low end of the seismic spectrum is especially important for deep exploration targets, such as subsalt and sub-basalt plays, and for deep crustal studies. In many areas of the world higher frequencies are often too attenuated, scattered, and dispersed to provide useful signal for probing such objectives. Also, low frequencies play a critical role in the inversion process by which seismic data are converted into acoustic impedance sections and ultimately into reservoir properties. Surface seismic data usually lack low frequencies necessary for accurate inversion, so they are commonly augmented by data from well-log me...

A practical approach to surface multiple attenuation

Multiply reflected events in marine seismic data can be divided into two classes: Surface multiples are events that have at least one downward reflection from the water surface; and internal multiples, on the other hand, have all of their downward reflections at the water bottom or below. Surface multiples are usually stronger, more broadband, and more of a problem than internal multiples because the reflection coefficient at the water surface is much larger than the reflection coefficients found in the subsurface.

Predicting and removing complex 3D surface multiples with WEM modeling--an alternative to 3D SRME for wide azimuth surveys?

Summary Wave equation modeling of multiples predicts multiples by performing one way propagator modeling using only a previously produced velocity model and migrated seismic image. Test results show that wave equation modeling of multiples using a v(x,y,z) velocity and the complete seismic image is effective for predicting complex 3D multiples for subsequent removal. The results appear competitive with 3D SRME methods in certain situations where multiples have 3D complexity or the acquisition geometry provides a challenge for 3D SRME. One situation where this approach is promising is wide-azimuth surveys.

Ocean-bottom cable dual-sensor scaling

Seismic data recorded at the ocean bottom using either pressure or velocity sensors suffer from the presence of the receiver water-surface ghost. The two sensor types are complementary, however; data recorded by both velocity and pressure phones can be scaled and summed so as to not only cancel the ghost effect, but also all water column reverberations at the receiver end of reflection event raypaths. Traditionally, the scale factors that match a geophone data set to its hydrophone complement are determined by recording and analyzing special calibration shots. If seismic data are not too noisy, calibration scale factors can also be derived directly -- and more economically -- from the data, based on the criterion that the proper scalars are those that best whiten the summed data. Both methods were applied to a line recorded in the Gulf of Mexico. The two stacked sections are visually identical.

Air-gun source instabilities

The signature of an air‐gun array can change over a period of time or even from one shot to the next. If the signature variations are large, then deterministic deconvolution, with an operator designed from a single signature or from an average signature, could produce errors significant enough to affect data interpretation. Possible sources of air‐gun instability include changes in gun positions, firing times, and pressures, gun failures, and scattering from the fluctuating rough ocean surface. If an air‐gun array were perfectly stable, after application of signature deconvolution the residual signatures for a sequence of shots would be identically shaped, broadband, zero‐phase wavelets. In practice, air‐gun instabilities lead to two major defects in band‐ limited residual signatures: the central portion of the wavelet can become asymmetrical, and unsuppressed energy can occur in the residual bubble region. Processing experiments done with synthesized air‐gun array signatures show that of all types of air...

Robust internal multiple prediction algorithm

Multiple attenuation is an important data processing step for both marine and land data. Techniques for surface related multiple elimination have improved rapidly in the past years. Internal multiple attenuation can still be very challenging due to poor discrimination between and multiples. A data-driven internal multiple method with minimum requirement of a priori information is presented. The method is an extension of surface multiple prediction and is suitable for all acquisition geometries.

Practical, 3D surface‐related multiple prediction (SMP)

Prediction of surface multiples via 2D algorithms (eg, SRME) is now routine in data processing, and is often effective in removing those multiples when combined with an adaptive subtraction. There are, however, many situations in which the 2D assumptions made on the geology and the acquisition geometry are invalid to the extent that the resultant errors in the predicted multiples are too large to allow those multiples to be effectively subtracted. A 3D prediction is theoretically possible and would eliminate these errors, but it requires data that are well sampled in all dimensions, and can be very expensive. This paper covers the following aspects of 3D surface multiple prediction. 1. Determination of the timing errors associated with a 2D prediction, in order to determine the spatial locations where a 3D prediction is appropriate. This error analysis is model-based, and does not require the 2D prediction to be run.

An introduction to this special section: The new world of multiple attenuation

The application of seismic imaging techniques is a highlight of the last decade. These imaging methods, however, assume that the input data are free of multiples. We know, of course, that this is not true and that the presence of multiples leaves us with spurious images and amplitudes. It is also apparent that, as those imaging techniques increase in sophistication, multiple attenuation becomes crucial. Subsalt imaging is a dramatic example. As we have enjoyed success in the mechanics of depth migration, we see that the large salt reflection coefficients afford nature the opportunity to create outstanding multiples to perplex and perturb seismic interpreters.

An introduction to this special section—Multiple attenuation

The discipline of multiple attenuation continues to make significant steps forward in the ultimate goal of eliminating multiples from various types of seismic data. As a retrospective on this, we refer back to the last TLE special section on multiple attenuation (January 1999) and compare it to the papers in this issue and the recent literature. The 1999 issue featured significant progress in the attenuation of multiples using wave-equation-based methods, which are popularly referred to as surface-related multiple elimination or SRME. A significant portion of that section was populated with papers from a multiple attenuation workshop held at SEGs 1997 Annual Meeting in Dallas. At that workshop, there was a growing acceptance that SRME was not only a viable method, but that it could attenuate multiples that previous methods could not. This was, of course, the promise of theory borne out in practice; however, one important fly in the ointment was that all applications were limited to 2D. While hopeful, the...