Weathering Layer Depth Estimation via Gravity Response Separation
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Solution Overview
Problem
Near-surface variations in the weathering layer of earth formations complicate the accurate determination of seismic sensor array responses and seismic data processing, making it challenging to determine the base of the weathering layer effectively.
Innovation Solution
A method involving gravity survey data analysis, where an equivalent source density profile is generated to describe gravitational sources, an equivalent source gravity response is calculated excluding the weathering layer's contribution, and a separated weathering layer gravity response is derived to estimate the weathering layer's depth profile, using Tikhonov regularization for stabilization.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If seismic survey methods are used to determine the base of the weathering layer, then seismic data can be processed, but near-surface variations cause inaccurate determination of the weathering layer base
Solution Approach 1:
The patent introduces gravity survey data as an intermediary measurement to indirectly determine weathering layer depth. Instead of relying solely on seismic data which is directly affected by near-surface variations, the method uses gravity anomalies (which are less sensitive to shallow variations) as a mediator to estimate weathering layer base depth, thereby improving accuracy despite the presence of harmful near-surface variations
Solution Approach 2:
The patent replaces the seismic survey method (mechanical wave propagation) with gravity survey method (gravitational field measurement) for determining weathering layer depth. By substituting the measurement mechanism from seismic waves to gravitational attraction, the method avoids the problem of near-surface variations degrading the signal, as gravity measurements are less sensitive to shallow geological heterogeneities
2Measurement precision
If gravity survey data is used to estimate weathering layer depth, then accurate depth estimation is achieved, but the process requires separating weathering layer response from overall gravity profile
Solution Approach 1:
The patent segments the gravity survey data into distinct components: the total gravity response is divided into the weathering layer contribution and the underlying formation contribution. By separating these components through inverse modeling and equivalence principle applications, the method isolates the weathering layer signal to improve depth estimation accuracy, though this segmentation increases processing complexity
Solution Approach 2:
The patent applies partial action by focusing the inverse modeling process specifically on extracting weathering layer information from the total gravity field, rather than attempting to model the entire subsurface structure. This selective approach achieves the necessary depth estimation without requiring complete characterization of all subsurface features, balancing accuracy with manageable complexity
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach facilitates accurate estimation of the weathering layer's depth, improving seismic data processing and aiding in oil/gas well development operations by separating the weathering layer's gravity response from the overall gravity profile, thus enhancing exploration, drilling, and production operations.
Implementation Method 1
Gravity survey refers to measurements, by air or on land, of the gravity field at a series of different locations over an area of interest. The earth's gravity field is affected by the density of different types of rocks. Precision instruments known as gravimeters are used to measure the changes in the gravity field.
Implementation Method 2
Seismic uphole survey refers to a seismic survey technique in which seismic sources are energized within a borehole and arrival times are recorded by surface geophones. Seismic uphole survey may be used particularly to determine weathering layer velocity.
Data Source
AI summary
A method to estimate a depth profile of a weathering layer in a subterranean formation of a field is disclosed. The method includes obtaining gravity survey data of the field, generating an equivalent source density profile based on the gravity survey data, wherein the equivalent source density profile describes a set of equivalent gravitational sources to substitute rock layers of the subterranean formation, generating an equivalent source gravity response based on the equivalent source density profile, wherein the equivalent source gravity response excludes a gravity contribution from the weathering layer, calculating a separated weathering layer gravity response based on a difference between the gravity survey data and the equivalent source gravity response, wherein the separated weathering layer gravity response corresponds to the gravity contribution from the weathering layer, and generating a modeled weathering layer depth profile based on the separated weathering layer gravity response.