3-D prestack/poststack multiple prediction

Details 3-D prestack/poststack multiple prediction 3-D prestack/poststack multiple prediction

2003-02-26

2004-05-11

Barlow, John (Department: 2863)

Data processing: measuring, calibrating, or testing

Measurement system in a specific environment

Earth science

C702S016000, C703S010000

Reexamination Certificate

active

06735527

ABSTRACT:

FIELD OF THE INVENTION
The present invention generally relates to seismic exploration, and more particularly to accurate prediction of 3-D acoustic reverberations or multiples for the purpose of coherent noise suppression and improved interpretation of 3-D seismic data.
DESCRIPTION OF THE RELATED ART
Seismic exploration involves the study of underground formations and structures. In seismic exploration, one or more sources of seismic energy emit waves into a subsurface region of interest, such as a geologic formation. These waves enter the formation and may be scattered, e.g., by reflection or refraction. One or more receivers sample or measure the reflected waves, and the resultant data are recorded. The recorded samples may be referred to as seismic data or a “seismic trace”. The seismic data contain information regarding the geological structure and properties of the region being explored. The seismic data may be analyzed to extract details of the structure and properties of the region of the earth being explored.
In general, the purpose of seismic exploration is to map or image a portion of the subsurface of the earth (a formation) by transmitting energy down into the ground and recording the “reflections” or “echoes” that return from the rock layers below. The energy transmitted into the formation is typically sound energy. The downward-propagating sound energy may originate from various sources, such as explosions or seismic vibrators on land, or air guns in marine environments. Seismic exploration typically uses one or more sources and typically a large number of sensors or detectors. The sensors that may be used to detect the returning seismic energy are usually geophones (land surveys) or hydrophones (marine surveys).
During seismic exploration (also called a seismic survey), the energy source may be positioned at one or more locations near the surface of the earth above a geologic structure or formation of interest, referred to as shotpoints. Each time the source is activated, the source generates a seismic signal that travels downward through the earth and is at least partially reflected. Seismic signals are partially reflected from discontinuities of various types in the subsurface, including reflections from “rock layer” boundaries. In general, a partial reflection of seismic signals may occur each time there is a change in the elastic properties of the subsurface materials. Reflected seismic signals are transmitted back to the surface of the earth, where they are recorded as a function of traveltime. Reflected seismic signals are typically recorded at a number of locations on the surface. The returning signals are digitized and recorded as a function of time (amplitude vs. time).
FIG. 1A
illustrates a seismic source S at the earth's surface, referred to as the free surface, generating reflected seismic signals from a sub-surface which are measured at four receivers, R
1
-R
4
, as shown. One should note that all free surface reflected energy acts as a secondary source of seismic signals from the points of reflection.
FIG. 1B
illustrates various examples of primary reflection raypaths, where the signal refracts at each boundary between layers, i.e., at each sub-surface reflector. Note that primary reflections do not involve downward reflections from the free surface or any of the sub-surface reflectors.
In seismic analysis, the term “multiples” refers to multiply-reflected seismic energy, or any event in seismic data that has incurred more than one reflection in its travel path. Depending on their time delay from the primary events with which they are associated, multiples are commonly characterized as short-path, implying that they interfere with the primary reflection, or long-path, where they appear as separate events. Multiples from the water bottom (the interface of the base of water and the rock or sediment beneath it) and the air-water interface are common in marine seismic data. The presence of multiples may obscure or interfere with primary reflection signals and may thus act as “noise” when analyzing seismic data.
FIGS. 2A-2D
illustrate a variety of multiple events, as are well known in the art.
FIG. 2A
illustrates water layer reverberations, where the signals are “trapped” in the water layer between two strong reflectors, specifically the free surface and the bottom of the water layer. These multiples tend to decay slowly and obscure the primary reflection energy from deeper reflectors or sub-surfaces.
FIG. 2B
illustrates slightly weaker multiple events, referred to as “peg-leg” multiple events, characterized by an additional roundtrip through the water layer just after emission (source-side) or just before detection (receiver-side).
FIG. 2C
illustrates a variety of what are known as “remaining” surface-related multiple events, where the first and last upward reflections are below the first (water) layer, and there is at least one reflection at the free surface in between. These multiple events are typically weaker than the water layer reverberations and the peg-leg multiples, but may be considerable if a highly reflective structure (e.g., salt or basalt) is involved. Finally,
FIG. 2D
illustrates “internal” or “interbed” multiple reflections, which are generally much weaker than the surface-related multiple reflections of
FIGS. 2A-2C
. Internal multiple reflections have a downward reflection at a boundary in the subsurface. If the primary reflection events have reflection amplitudes on the order of the reflection coefficient r, then the first order surface-related multiples have amplitudes on the order of r
2
, and the first order internal multiples of r
3
, where |r|<1. However, in many common geologic settings of commercial interest in hydrocarbon exploration, multiple events will interfere with less energetic primary events from deeper reflectors, and even internal multiples may obscure primary reflections from deeper reflecting boundaries.
FIG. 3
illustrates the time relationship between the arrival of a source sample at a receiver R, referred to as the sample time, and the arrival times of a multiple reflection from some representative subsurface reflector. Note that time increases downward along the vertical axis shown. As
FIG. 3
illustrates, the original sample produces the multiple event shown to the right of FIG.
3
. These arrival times are those of a primary reflection from that subsurface reflector due to a source located at the receiver location, but delayed by the sample's arrival time. Note that the sample may itself be a measurement of a multiply-reflected signal from a source located elsewhere, and that any energy reflected from the surface is, according to the principle of superposition, considered to be another source. In discrete terms,
FIG. 3
illustrates the concept that the response of any sample of recorded upcoming energy is a delayed copy of a record from an impulsive seismic source signal at that recording location, appropriately scaled by the sample's amplitude and the free surface reflection coefficient.
Multiple source activation/recording combinations may be collected to create a near continuous profile of the subsurface. In a two-dimensional (2-D) seismic survey, the recording locations are generally laid out along a single line. In a three dimensional (3-D) survey the recording locations are a really distributed across the surface. In simplest terms, a 2-D seismic line can be thought of as providing a vertically-oriented cross sectional picture of the earth layers as they exist beneath the recording locations. A 3-D survey produces a data “cube” or volume that is, at least conceptually, a 3-D picture of the subsurface that lies beneath the survey area. In reality, both 2-D and 3-D surveys interrogate some volume of earth lying beneath the area covered by the survey.
After the seismic data have been collected, the seismic data may be “imaged”, analyzed, or otherwise processed to produce a seismic profile or pattern indicating various characteristic structures or signatures

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