Dave Monk

Seriously southerly seismic: 3D acquisition in Tierra del Fuego

In late 2006, Apache became operator of several license blocks totaling 2950 km2 in the province of Tierra del Fuego, Argentina. As the first part of an extensive exploration and development program, Apaches Exploration and Technology group immediately started work on covering the newly acquired blocks with fit-for-purpose 3D seismic, intending to acquire all the areas in a single campaign lasting up to 12 months. The imaging objectives of the surveys ranged from development of existing fields to pure exploration in the sparsely mapped blocks that formed the majority of the area, and led to several geometries and acquisition solutions being developed, some redesigned and implemented on the fly during acquisition.

Pitfalls in seismic acquisition

Welcome to a new series of articles that focuses on interpretation pitfalls. These articles are part of a larger project, sponsored by SEG’s Interpretation Committee and AAPG’s Geophysical Integration Committee (the same dynamic duo which brings you Geologic Column each month in TLE and Geophysical Corner each month in the Explorer). The intent of these articles is to update the classic Pitfalls in Seismic Interpretations by Tucker and Yorstan. The original Pitfalls was published in 1973, more than 25 years ago! It was invaluable to a generation or more of seismic interpreters but as all of us know, the world of seismic interpretation has changed dramatically in recent years.

3-D Seismic Acquisition In Tierra Del Fuego, Argentina: A Case History.

The initial scout of the area found the landscape to be low rolling grassland hills separated by broadly meandering rivers and “chorrillos”, or streams, in wide valleys. The primary land use in the region is sheep farming, the hillside grasslands being irrigated by man-made channels. The project area appeared to be ideal for vibroseis, however a need for up to 40% explosive sources was anticipated in the soft wet valleys where vibrator access was thought unlikely, and on the coastal flats of the northern programs.

Seismic Sans Frontières - Cross-border 3D Acquisition In Tierra Del Fuego

In 2008, Apache acquired the Lenga / Russfin 3-D survey in the Magallanes region of Chile, on La Isla Grande de Tierra del Fuego. The survey was adjacent to Chile’s border with Argentina and also to one of Apache’s recently acquired 3D surveys in Argentina. The survey was recorded as an extension of the Argentina 3D through the innovative combination of conventional and wireless seismic recording systems.

Technical Program highlights

The 75th Annual Meeting in Houston attracted large submissions of abstracts for consideration as technical presentations. This created an interesting dilemma for the organizing committee—whether to accept more papers than has been customary and expand the program, or limit the number of accepted papers and keep the number of parallel sessions roughly the same as in previous recent exhibitions. In the end it was decided that more parallel sessions would be detrimental to the overall success of the Technical Program, causing more conflicts for the attendees interested in several areas of expertise, and generally “lowering the bar” for acceptance of abstracts. The result was an unusually high rejection rate for abstracts (in some areas of submission this was as high as 50%), but a level of paper which was suited for such an auspicious event. In the end a program of nearly 700 papers was put together which included both oral and poster sessions.

Lena A seismic model

In the May 2000 issue of TLE, Chris Liner raised the issue of available bandwidth in seismic data and suggested that it is the lack of low frequencies in recording which limits our ability to properly image the subsurface. In this entertaining article, Liner uses a standard image, Lena, to demonstrate what happens to a familiar image when we lose the high and low frequencies and suggests that perhaps the poor response of geophones to low frequencies is a problem. Those who attended SEGs 2001 Annual Meeting in San Antonio had the opportunity to experiment with “LenaSeis” at the convention and look at the impact of different high and low frequency limits.

Azimuthally dependent Amplitude vs. Offset in 3D seismic: An automated statistical measurement and case study

This paper describes a practical method, illustrated with examples, of evaluating true AVAZ (AVO as a function of azimuth) effects in 3D. Seismic data may be recorded with different offsets and azimuths at each mid point, and there are geologic factors which cause the AVO response to vary with respect to azimuth. Conventionally extracted AVO attributes are incorrect if the azimuth dependencies are not considered. There are a number of reasons why AVAZ may be present in seismic data, some of these have been discussed in previous literature (see for example Hall 1996), and there is considerable current interest in a potentially improved understanding of the subsurface that AVAZ can yield.

Survey geometries that achieve more uniform offset and azimuth sampling

Most preand post-stack 3D imaging methods require that the seismic wavefield and the subsurface image be sampled in 3 dimensions, preferably uniformly. The more dense and uniform the offset and azimuth sampling is, the smaller the resulting acquisition and processing artifacts are, regardless of the method being applied. This paper examines how more uniform sampling of offsets and azimuths can be achieved in an environment where the number of active recording channels continues to increase while source points remain comparatively expensive. Methods to compactly quantify the “goodness” of the offset and azimuth distribution are proposed.