EM Glossary

The mathematical process of inversion in which the modelled resistivity is only allowed to vary with depth.

Simulating the controlled-source electromagnetic or magnetotelluric response of a geological resistivity model that varies only with depth. Also called plane-layer modelling, as resistivity is assumed to be laterally constant.

The mathematical process of inversion in which the modelled resistivity is allowed to vary vertically and horizontally in the same plane as a receiver line, but is made constant in the horizontal plane at 90° to the inline direction. The model is therefore 2D, but the source is simulated in 3D, which is why it is called 2.5D inversion.

2.5D inversion provides a relatively rapid method of inversion compared with full 3D inversion.

The mathematical process of inversion used in magnetotelluric surveys to obtain a 2D resistivity model of the subsurface. The modelled resistivity is allowed to vary vertically and horizontally in the same plane as a receiver line, but is made constant in the horizontal plane at 90° to the inline direction.

The mathematical process of inversion in which the modelled resistivity is allowed to vary in all directions.

Simulating the controlled-source electromagnetic or magnetotelluric response of a geological resistivity model that varies in three dimensions.

See also forward modelling.

The electromagnetic signal that has travelled from the source to the receiver along the air–sea interface (see illustration).

The airwave is more pronounced in shallow waters, but advances in processing have effectively removed its effect as a water-depth restriction. Surveys can now be performed in any water depth as long as the vessel can operate safely.

The degree to which the measurement direction affects a subsurface property. See also isotropy.

All modelling and inversion schemes should, by default, take anisotropy into account to fully explain electromagnetic data. For example, for the same geological formation, the resistivity in the vertical direction is often higher than the resistivity in the horizontal direction.

A property derived from electromagnetic data by a simple transfer function such as magnitude versus offset or phase difference versus offset.

The angle between the vector from the source to a particular receiver and the source dipole direction, which is close to the towing direction. See also wide azimuth.

When this angle is 0°, the receiver is described as inline; when it is close to 90°, the receiver is broadside. Using receivers away from the source towing line greatly increases the survey coverage (see illustration).

A regional resistivity model showing the general trends against which target responses can be modelled and resistivity anomalies can be examined.

Used to describe those receivers to either side of the towing line.

Using receivers away from the source towing line greatly increases the survey coverage (see illustration).

Using measured (control) values to adjust a model or compare measurement devices.

In the case of marine controlled-source electromagnetic surveying:

a) resistivity models may be calibrated using resistivity values from a well log through or close to the survey area.

b) all receiver sensors are calibrated using reference electrical and magnetic fields to ensure that all amplitude and phase measurements are accurate. The receiver clock and other measurement devices are also calibrated.

In marine controlled-source electromagnetic surveying, the independent measurements of the electromagnetic field recorded by a receiver.

Measuring the complete electromagnetic field requires six channels: electric channels in three spatial directions (commonly denoted Ex, Ey and Ez) and magnetic channels in three spatial directions (commonly denoted Hx, Hy and Hz).

The point halfway between the source and the receiver.

CMP offset plots show attributes derived from the electromagnetic measurements plotted at CMPs. They are often one of the first data analyses performed and serve mainly as a quality control check. CMP offset plots are relatively quick analysis tools and thus a useful early step in electromagnetic processing. CMP inversion is a quick and robust inversion method.

Data from well logs and seismic horizons (constraints) are used to control (constrain) an inversion.

It is a way of incorporating information from the interpretation of other data types into the controlled-source electromagnetic interpretation. See also unconstrained inversion.

In the marine environment, a survey or method in which an electromagnetic field is generated in the substrate using a controlled source and measured at different offset locations using surface/seabed receivers

In principle, CSEM techniques cover all geophysical, electromagnetic surveys with an active source. The CSEM method when applied offshore may be known as marine CSEM, mCSEM or seabed logging (SBL).

See resistance.

See resistivity.

In the case of controlled-source electromagnetic surveying, the EM field is generated by a controlled source, whereas the magnetotelluric method uses naturally occurring EM fields.

As the name suggests, EM fields have electrical and magnetic components. EM receivers should have the ability to record both the electrical and magnetic fields as independent datasets.

Measurements made to determine the electromagnetic field.

A group of methods for measuring natural (see magnetotelluric) or generated (see CSEM) electromagnetic fields at the surface/seabed or in boreholes to map variations in subsurface electrical properties, most notably resistivity.

See electromagnetic field or electromagnetic surveying.

See sensitivity study.

The use of physical equations to determine the receiver responses for a given geological situation or resistivity model. See also 3D modelling and modelling.

In controlled-source electromagnetic (CSEM) surveying, the data after a Fourier transform has been applied to the time series of electromagnetic measurements recorded by a receiver.

In frequency domain CSEM methods, the Fourier transform is applied early in the processing sequence and all subsequent data analysis and interpretation are performed in the frequency domain.

Using receiver measurements taken at all angles around the source.

Using receivers away from the source towing line greatly increases survey coverage.

See also azimuth.

See survey grid.

Used to describe those receivers in line with the source towing direction (see illustration).

The combination of two or more datasets.

Electromagnetic data is typically integrated with seismic and well data so that the most-informed interpretation can be made.

Drawing conclusions from data.

Electromagnetic data is usually integrated with seismic and other subsurface data to enable the best-informed interpretations to be made.

A mathematical process that uses data to generate a model that is consistent with that data

For controlled-source electromagnetic methods, inversion is used to convert electromagnetic data into resistivity values located in space. The underlying models can be simplified to save time, for example, by making the resistivity constant in one dimension (2.5D inversion). In 3D inversion, the resistivity is allowed to vary in all directions.

A physical property, such as resistivity, that is independent of the direction of measurement has isotropy or can be described as being isotropic. See also anisotropy.

An electromagnetic surveying method used to map subsurface resistivity variations by measuring naturally occurring electric and magnetic fields on the seabed or the Earth’s surface

Low-frequency natural electromagnetic fields are generated by the solar wind as it interacts with the Earth’s magnetic field. Because MT surveys have relatively low resolution but excellent depth penetration, the measurements are used to help interpret regional geology. The MT method is particularly useful in areas where topography, high-impedance volcanic rocks or salt make other geophysical methods difficult. MT data are acquired inherently during electromagnetic surveys when the controlled source is inactive.

The variation in the magnitude of the frequency domain of the controlled-source electromagnetic field for any one receiver with the distance from the source as it is towed towards and then away from the receiver.

The magnitude of the electromagnetic response can also be mapped for a grid of receivers for a fixed offset. MVO measurements are usually normalised using results from a reference receiver.

MVO is an attribute derived from the electromagnetic measurements. It is the first data analysis performed and used for quality control, and may sometimes give an early indication of whether a significant normalised anomalous amplitude response has been measured. Although the approach does not provide reliable depth information, it has a short turnaround time and enables a qualitative assessment to be made that shows the relative changes in resistivity, which can help to verify the accuracy of the initial resistivity model.

Controlled-source electromagnetic (CSEM) surveys performed in the offshore environment.

An electromagnetic processing step in which the resistivity data are moved to their correct depths.

Migration is a set of imaging techniques that provide relative resistivity values. In controlled-source electromagnetic, these techniques have been superseded by inversion methods, which provide physical parameters.

The difference between two datasets.

Misfit analysis is used in controlled-source electromagnetic modelling studies. Different resistive bodies are added to a background model to represent competing hypotheses that may explain the target zone. The simulated electromagnetic response for each hypothesis can then be compared with the measured data. The misfit plot shows how well the datasets agree.

Misfit analysis is also an important as a quality control measure for inversion, as it shows how well the inverted resistivity model explains the measured data.

Marine magnetotelluric surveying.

In controlled-source electromagnetic surveying, simulation of the propagation of electromagnetic fields in a resistivity model

Forward modelling is used to test geo-model hypotheses in the survey-planning and interpretation stages.

In seismic surveying, modelling is often used informally for the process of “model building”, or constructing a rock property model of the earth, such as a resistivity model.

A modelling study includes both model building and forward modelling, followed by misfit analysis to assess the accuracy of the model.

See magnetotelluric.

See magnitude versus offset.

Used to describe receivers at acute angles to the source towing direction. See also azimuth.

The maximum variation in a response that is anomalous compared with the background resistivity trend.

Normalised magnitude versus offset. MVO data that has been normalised using measurements from a reference receiver.

In controlled-source electromagnetic surveying, the horizontal distance between the source and a receiver at a given time.

Phase versus offset data that has been normalised using results from a reference receiver.

PDVO is an attribute derived from electromagnetic measurements. It is the first data analysis performed and used for quality control. Although the approach does not provide reliable depth information, it has a short turnaround time and enables a qualitative assessment to be made that shows the relative changes in resistivity that can help to verify the accuracy of the initial resistivity model.

The variation in the phase of the electromagnetic response for any one receiver with the distance from the source.

PVO measurements are usually normalised using results from a reference receiver.

PVO is an attribute derived from electromagnetic measurements. It is the first data analysis performed and used for quality control. Although the approach does not provide reliable depth information, it has a short turnaround time and enables a qualitative assessment to be made that shows the relative changes in resistivity that can help to verify the accuracy of the initial resistivity model.

See 1D modelling.

Preparing data for inversion.

Processing starts with raw data from the receivers then transforms it into generic electromagnetic field data, taking into account, for example, system responses, calibration of data and navigation parameters.

An exploration area in which the existence of an economic quantity of hydrocarbon is predicted.

Prospects are usually identified using low-resolution electromagnetic or seismic surveying methods, and are then ranked and de-risked using higher resolution surveys.

Devices used on the seabed to measure and record electromagnetic signals.

Controlled-source electromagnetic and magnetotelluric (MT) surveying depend on recording high-quality data from high-sensitivity, low-noise receivers. The receivers need

- to measure field strengths that vary greatly in magnitude, from weak naturally occurring MT signals to strong signals when the source passes over the receiver

- high signal-to-noise ratios

- precise signal timing

to be operationally autonomous for several weeks to allow large surveys.

The distribution of the seabed receivers.

Receivers are widely spaced (2–4 km apart) during surveys designed to identify new prospects across large areas. Receivers are more tightly spaced (0.5–1 km apart) where electromagnetic surveying methods are being used to create 3D resistivity data for reservoir evaluation.

A representation of the changes in resistivity associated with, for example, variations in bathymetry and overburden thickness.

This model is used as a reference for each receiver, rather than normalising the measurements from each against those from a single reference receiver. The model is typically constructed using 1D inversions that are constrained by well log and seismic information. Synthetic receiver responses are generated by forward modelling and used to normalise each receiver. This reduces the impact of the airwave and regional resistivity trends in normalised attribute analysis compared with using a single reference receiver.

A receiver used as a reference for normalising measurements from other receivers.

This can be a real receiver lying outside the target area or synthetic measurements.

The opposition of an object to the passage of an electric current.

An object with a uniform cross section has a resistance proportional to its resistivity and its length, and inversely proportional to its cross-sectional area. The SI unit of resistance is the ohm (Ω).

Controlled-source electromagnetic surveying is based on the resistivity contrast between relatively conductive sedimentary rocks saturated with saline water and relatively resistive hydrocarbon saturated rocks.

How strongly a material opposes the flow of an electric current.

The SI unit of electrical resistivity is the ohm metre (Ωm).

In controlled-source electromagnetic surveying, a body or medium with higher resistance than its surroundings.

In general physics, it is a body of infinite resistance.

See marine CSEM.

Work undertaken to determine the likely sensitivity of electromagnetic surveying for a given geological situation.

Fast 1D (plane layer) modelling based on known or assumed parameters is used to provide rapid analysis. This helps to show whether the detection of economic reservoirs using electromagnetic methods is likely in a particular geological setting.

If the decision is made to proceed, a realistic and detailed 3D resistivity model is constructed and used to run a series of virtual surveys. This process helps to verify that an electromagnetic survey will add value. It also helps optimise the survey design, for example, by ensuring that most appropriate receiver spacing and source frequency mix are used.

In marine controlled-source electromagnetic (CSEM), water depths of less than about 100 m.

In its early application, marine CSEM methods were limited to deep water by the airwave effect. Processing and inversion now effectively account for this effect, and the minimum water depth is limited only by vessel operating conditions.

A device that emits a controlled electromagnetic signal with a frequency distribution and waveform designed to meet the needs of each survey

The source is towed above the seabed during electromagnetic surveys or at the sea surface for shallow water surveys. An electromagnetic field is generated by the source and propagates as a function of the resistivity of the surroundings. Controlled-source electromagnetic surveying methods measure and map contrasts in subsurface resistivity, for example, between hydrocarbon and non-hydrocarbon bearing sediments.

Setting the survey parameters, such as receiver spacing and source frequency mix and towing geometry.

For example, widely spaced receiver grids are used for large-scale surveys designed to find new leads, whereas more densely spaced receiver grids are used where detailed 3D resistivity datasets are required for reservoir appraisals.

See also sensitivity study.

See receiver grid.

The focus of an electromagnetic survey.

The objective in most EM surveys has been to assess the resistivity of a predefined prospect(s). In this case, the prospect(s) is/are the targets of the survey.

A model that includes a target resistive body, whether a hydrocarbon reservoir or another resistor.

Forward modelling is used to produce simulated receiver responses that can be compared with measured data to assess competing hypotheses.

An array of data representing the readings of the electromagnetic field by a channel on a receiver at regular and densely sampled time intervals.

The trajectory of the source while active.

In most survey plans, the towing lines are straight and run directly above a line of receivers. The towing line usually extends for several kilometres beyond the first and last receivers in a line to ensure that a full offset range is recorded for all the receivers.

An inversion which is driven only by controlled-source electromagnetic (CSEM) data

No seismic, well-log or other data are used to constrain or guide the inversion process as its attempts to produce a resistivity model that optimally explains the measured CSEM data.

By adding constraints, the inversion process can be made to produce a resistivity model that fits other datasets.

See also constrained inversion.

A wide angle between the vector from the source to a particular receiver and the source dipole direction, which is close to the towing direction. See also azimuth.

Wide azimuth receivers are those at obtuse angles to the source towing direction.