Detection of subsurface resistivity contrasts with application to location of fluids

Abstract

The invention relates to a method of mapping subsurface resistivity contrasts by making multichannel transient electromagnetic (MTEM) measurements on or near the earth's surface using at least one source, receiving means for measuring the system response and at least one receiver for measuring the resultant earth response. All signals from the or each source-receiver pair are processed to recover the corresponding electromagnetic impulse response of the earth and such impulse responses, or any transformation of such impulse responses, are displayed to create a subsurface representation of resistivity contrasts. The invention enables subsurface fluid deposits to be located and identified and the movement of such fluids to be monitored.

Description

PRIORITY APPLICATION [0001] The present application is a continuation of co-pending U.S. patent application Ser. No. 10 / 482,554 filed Jun. 17, 2004, which is a 371 filing of International Patent Application No. PCT / GB02 / 04121, which was filed on Sep. 9, 2002, which claims priority to U.K. Patent Application No. 0121719.9, which was filed on Sep. 7, 2001. Each of the foregoing applications is hereby incorporated by reference.TECHNICAL FIELD [0002] This invention relates to a method of mapping subsurface resistivity contrasts. The method enables the detection and location of subsurface resistivity contrasts, which, in turn, enables the discrimination between, for example, water (brine or fresh water), which is conductive, and hydrocarbons (gas or oil), which are resistive. [0003] Porous rocks are saturated with fluids. The fluids may be water (brine or fresh water), or hydrocarbons (gas or oil). The resistivity of rocks saturated with hydrocarbons is often orders of magnitude greater ...

AI Technical Summary

Benefits of technology

Enables the detection and location of subsurface resistivity contrasts, allowing for the discrimination between water and hydrocarbons, as well as the monitoring of fluid movement, thereby reducing the risk of dry holes and improving hydrocarbon detection.

Problems solved by technology

TDEM methods all fail to recognise the importance of measuring the system response and instead put much effort into generating a transient signal with as small a turn-off time or ramp turn-off time as possible.

In reality it is not possible to input an exact square wave either.

This is impossible to obtain without a deconvolution of the measured data which is stated to be inherently unstable [5].

 The LOTEM method fails to recognise the importance of measuring the system response for each source transient in the field, and fails to recognise that the decay curves are a function of all the intervening material between the source and corresponding receiver where the induced currents flow.

 Their technique fails to recognise the importance of measuring the system response for each source transient and using this to deconvolve the measured transients to obtain the estimated earth impulse response functions.

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