Multi-Frequency CSEM Data Processing for Shallow Resistivity Masking

Resolve Bottlenecks,
Find Innovative SolutionsGenerate Solutions

Solution Overview

Problem

Conventional methods for interpreting marine controlled-source electromagnetic (CSEM) data struggle to distinguish between shallow and deep resistivity anomalies, leading to false negatives and false positives, as shallow resistivity structures can mask or misinterpret deeper resistivity responses, particularly in offshore environments.

Innovation Solution

A data-processing method that selects survey data sets at different frequencies to penetrate shallow and deeper resistivity structures, calculates shallow resistivity structures using electromagnetic field equations, and distinguishes between shallow and deep responses, either through iterative forward modeling or inversion, to produce a two-dimensional anomaly map with reduced shallow resistivity effects.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If a single frequency (typically 1/4 Hz) is used for CSEM data interpretation, then the electromagnetic response can be optimized for reservoir depth detection, but shallow resistivity anomalies mask deeper anomalies causing false negatives and false positives

Engineering Contradiction:

Improvedetection of deeper resistivity anomaliesVSAvoidaccuracy of anomaly interpretation

Core Design Contradiction:

Measurement precisionVSReliability

Solution Approach 1:

The patent segments the electromagnetic survey into multiple frequency components, using high frequencies to probe shallow structures and low frequencies to penetrate to deeper reservoirs. This frequency-based segmentation allows separate analysis of shallow and deep anomalies, eliminating the masking effect where shallow features obscure deeper targets in single-frequency surveys

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent adds the frequency dimension to the traditional single-frequency CSEM survey. By collecting and analyzing data across multiple frequencies rather than a single frequency, the method creates an additional analytical dimension that enables separation of shallow and deep resistivity anomalies through their differential frequency responses

Inventive Principle:

Principle #17Another dimension (Dimensionality change)

2Measurement precision

If high-frequency electromagnetic data is used, then shallow resistivity structures are detected with high resolution, but the data is rapidly attenuated and cannot penetrate to deeper anomalies

Engineering Contradiction:

Improveresolution of shallow resistivity structuresVSAvoidpenetration depth to deeper anomalies

Core Design Contradiction:

Measurement precisionVSLength of stationary object

Solution Approach 1:

The patent employs dynamic frequency selection where different frequencies are applied at different survey locations based on the expected depth and nature of subsurface features. High frequencies are used where shallow features are of interest, while low frequencies are used for deeper targets, allowing the survey to dynamically adapt to varying geological conditions and depth requirements

Inventive Principle:

Principle #15Dynamics

3Length of stationary object

If low-frequency electromagnetic data is used, then deeper resistivity structures can be detected, but shallow anomalies also appear causing misinterpretation

Engineering Contradiction:

Improvepenetration depth to deeper structuresVSAvoiddiscrimination of shallow vs deep anomalies

Core Design Contradiction:

Length of stationary objectVSMeasurement precision

Solution Approach 1:

The patent uses frequency as an intermediary parameter to mediate between shallow and deep anomaly detection. By analyzing the frequency-dependent response characteristics, the method can identify which anomalies are shallow (affecting multiple frequencies) versus deep (affecting primarily low frequencies), thereby resolving the misinterpretation problem

Inventive Principle:

Principle #24Intermediary (Mediator)

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 effectively reduces the masking effects of shallow resistivity on deeper anomalies, enhancing the discrimination and resolution of resistivity anomalies at different depths, allowing for more accurate mapping of deeper resistivity structures and reducing non-uniqueness in the solution.

Implementation Method 1

controlled source electromagnetic surveys in offshore environments (where a controlled electromagnetic transmitter is towed above electromagnetic receivers fixed on the sea floor)

Methodology Applied

Scientific EffectElectromagnetic induction: Electromagnetic Induction

Implementation Method 2

The amplitude of the data is attenuated to 1/e (e is the base of natural logarithms) at a distance δ=503(R/f)1/2 where R is the resistivity in Ohm-m, f is the frequency in Hertz and δ is the skin depth in meters

Methodology Applied

Scientific EffectSkin effect: Skin Effect

Data Source

PatentUS7333893B2Method for spatially interpreting electromagnetic data using multiple frequencies

Publication Date: 2008.02.19 EXXONMOBIL UPSTREAM RESEARCH COMPANY(US)

AI summary

Method for removing effects of shallow resistivity structures in electromagnetic survey data to produce a low frequency resistivity anomaly map, or alternatively imaging resistivity structures at their correct depth levels. The method involves solving Maxwell's electromagnetic field equations by either forward modeling or inversion, and requires at least two survey data sets, one taken at the source frequency selected to penetrate to a target depth, the other a higher frequency able to penetrate only shallow depths.