TE (Transverse Electric Mode)

TE (transverse electric mode) in the context of oil and gas well logging refers to the transverse electric propagation mode in electromagnetic (EM) induction and propagation resistivity tools — an electromagnetic wave mode in which the electric field vector lies perpendicular to the direction of wave propagation while the magnetic field has a component in the propagation direction, as opposed to the transverse magnetic (TM) mode — with the distinction between TE and TM modes being relevant to the interpretation of directional resistivity measurements, formation anisotropy characterization, and the response of multi-component induction tools to tilted beds and resistivity anisotropy.

Key Takeaways

In multi-component induction logging (triaxial induction tools), the TE and TM modes correspond to different transmitter-receiver coil orientations: TE mode responses are dominated by the horizontal resistivity (Rh) of the formation, while TM mode responses are sensitive to both horizontal and vertical resistivity (Rv), enabling the decoupling of Rh and Rv in anisotropic formations from the combination of TE and TM measurements.
Resistivity anisotropy (Rv/Rh greater than 1) is common in laminated sand-shale sequences in clastic reservoirs and in naturally fractured carbonates — thin shale laminations have high vertical resistivity (current cannot cross the resistive shale) but lower horizontal resistivity, creating bulk anisotropy that can cause conventional induction logs reading primarily Rh to significantly underestimate true formation resistivity in thinly laminated sections and thereby overestimate water saturation.
Propagation resistivity measurements in logging-while-drilling (LWD) tools use electromagnetic wave attenuation and phase shift at two frequencies and multiple transmitter-receiver spacings to determine resistivity in both TE and TM sensitive orientations, providing real-time anisotropy information for geosteering decisions in horizontal wells drilling along laminated or dipping formations.
The response of resistivity logging tools to dipping beds and formation anisotropy is described by the TE/TM decomposition: when the borehole axis is oblique to the formation boundaries (deviated or horizontal wells intersecting dipping beds), TE and TM mode responses mix in the measured signal, and anisotropy inversion software decouples the two modes to recover the true horizontal and vertical resistivities.
Formation evaluation error from ignoring resistivity anisotropy can be significant: in a thinly laminated sand-shale sequence where the net-to-gross is 50%, the induction log reading Rh may give water saturation calculations suggesting the interval is wet, while the true sand resistivity (corrected for anisotropy using Rv from TM mode) reveals the sand laminations are hydrocarbon-bearing and the apparent wet reading is caused by the conductive shale component dominating the Rh measurement.

Fast Facts

Resistivity anisotropy ratios (λ = sqrt(Rv/Rh)) of 2 to 5 are common in laminated sand-shale sequences with net-to-gross ratios of 30 to 70 percent, and ratios above 10 can occur in highly laminated sections with thin (centimetre-scale) sand beds separated by shale laminae. The anisotropy coefficient λ directly affects the apparent resistivity measured by conventional induction tools through the relationship Ra ≈ Rh × (1 + λ² tan²θ) / (1 + tan²θ) for a borehole at angle θ to the formation, showing that horizontal well logging at 90 degrees to the formation provides a resistivity reading that approaches Rh × λ² — potentially orders of magnitude higher than a vertical well reading Rh in the same formation.

What Is TE Mode?

Electromagnetic waves propagating through a medium can be decomposed into two fundamental polarization modes based on the orientation of the electric and magnetic field vectors relative to the direction of propagation. In the transverse electric (TE) mode, the electric field vector lies entirely in the plane perpendicular to the propagation direction — it has no component along the propagation axis. In the transverse magnetic (TM) mode, the magnetic field vector lies in the transverse plane while the electric field has a component along the propagation direction.

This distinction matters in well logging because different coil orientations in resistivity tools couple primarily to different EM modes, and different modes respond with different sensitivity to horizontal versus vertical resistivity. Understanding which mode a given measurement is sensitive to is essential for correctly interpreting the measured resistivity as a formation property rather than a tool geometry artifact.

The practical importance of TE/TM mode analysis increased dramatically with the introduction of horizontal drilling and LWD resistivity tools: when a wellbore is nearly horizontal in a formation with horizontal bedding, the conventional induction tool geometry is rotated 90 degrees relative to its design orientation, fundamentally changing which EM modes it measures and how it responds to formation anisotropy.

TE Mode in Resistivity Tool Response and Anisotropy

Conventional induction logging tools use coaxial transmitter and receiver coils (coils wound around the tool axis). This geometry primarily measures the horizontal component of formation conductivity (σh = 1/Rh) because the induced eddy currents flow in horizontal planes concentric with the tool axis. In the EM mode language, this coaxial measurement is predominantly in the TM mode. For a vertical well in a horizontally layered formation, this measurement provides Rh, which is the relevant property for most vertical well flow calculations.

In a horizontal well drilled perpendicular to vertical formation boundaries (or in a vertical well through steeply dipping beds), the tool axis is perpendicular to the layering, and the coaxial measurement now sees currents flowing up and down across the layering — measuring a response controlled by both Rh and Rv. Multi-component induction tools add transverse coils (perpendicular to the tool axis) that provide complementary measurements in primarily TE mode orientation, sensitive to Rv. The combination of coaxial (TM) and transverse (TE) measurements enables simultaneous determination of Rh and Rv and the anisotropy ratio λ.

Anisotropy inversion algorithms use the full tensor of multi-component measurements to solve for the complete resistivity anisotropy of the formation, including the dip of the formation relative to the borehole axis, using forward modeling (TE and TM mode responses for assumed Rh, Rv, and dip) iterated to match the observed measurements. This provides the formation true resistivity for saturation calculation even in the challenging geometry of horizontal wells in laminated reservoirs.

TE Mode Across International Jurisdictions

Canada (AER / WCSB): WCSB horizontal Montney and Duvernay shale wells drilled through laminated siltstone and shale sequences require multi-component or propagation resistivity LWD tools that properly account for TE and TM mode responses in the horizontal well geometry. AER resource assessment for WCSB tight reservoirs uses saturation calculations based on corrected resistivity that accounts for formation anisotropy identified from triaxial or multi-frequency LWD measurements. WCSB carbonate horizontal wells in the Swan Hills and Beaverhill Lake formations also benefit from TE/TM anisotropy analysis to characterize natural fracture orientation contributing to permeability anisotropy.

United States (API / SPE): Gulf of Mexico deepwater turbidite reservoirs with thin laminated sand-shale sequences are a major application area for multi-component induction tools and TE/TM anisotropy analysis. ExxonMobil, Shell, and Chevron have published extensively in SPE on resistivity anisotropy impacts on reserve calculations in GoM laminated reservoirs, documenting cases where conventional Rh measurements underestimated hydrocarbon saturation by large margins before anisotropy correction. Permian Basin horizontal wells through heterogeneous Wolfcamp and Bone Spring laminated intervals use LWD propagation resistivity tools with TE/TM sensitivity for real-time formation evaluation and geosteering.

Norway (Sodir / NORSOK): NCS turbidite and deltaic sand reservoirs in Paleogene and Cretaceous successions often exhibit resistivity anisotropy from thin shale drapes between sand beds. Equinor's formation evaluation programs use multi-component induction logging in vertical and deviated exploration wells to characterize anisotropy before designing horizontal development well programs. NORSOK D-010 references applicable well logging standards for LWD formation evaluation, and NCS operators routinely run triaxial or propagation resistivity tools with TE/TM sensitivity in horizontal development wells.

Middle East (Saudi Aramco): Saudi Aramco uses multi-component induction and propagation resistivity logging in Arab Formation carbonate horizontal wells and in clastic Jurassic reservoirs where resistivity anisotropy from fracture networks or lamination affects saturation calculations. Aramco's petrophysical engineering programs document TE/TM response effects in horizontal wells drilled through dipping carbonate layers, using anisotropy inversion to obtain the correct Rv and Rh values for accurate saturation assessment. The deep investigation of Aramco's formation resistivity tools must account for TE mode coupling to both matrix and fracture resistivity in naturally fractured Arab Formation carbonates.

TE refers to transverse electric mode. Related terms include transverse magnetic mode (TM), resistivity anisotropy, multi-component induction log, horizontal resistivity (Rh), vertical resistivity (Rv), propagation resistivity, induction log, and geosteering. The transverse magnetic (TM) mode is the complementary polarization to TE, with the roles of electric and magnetic fields reversed. In waveguide theory more broadly, TE and TM modes are a fundamental classification of electromagnetic field distributions in bounded structures, with applications well beyond oil and gas logging.

Tip: When interpreting resistivity logs from horizontal wells in laminated formations, always check whether the LWD tool run can measure or detect resistivity anisotropy before accepting the horizontal resistivity reading as the true formation Rt for saturation calculation. If the LWD tool is a simple coaxial induction or propagation tool (sensitive primarily to Rh), and the formation is a laminated sand-shale sequence (where Rv significantly exceeds Rh), the Rh reading may dramatically underestimate the true sand resistivity. Check for anisotropy indicators: if the vertical well nearby showed low resistivity in an equivalent interval that nevertheless tested hydrocarbons, anisotropy is likely the explanation. Request a multi-component or triaxial tool run in the first horizontal appraisal well to quantify the Rv/Rh ratio before committing to saturation interpretation from Rh alone across the full horizontal well program.

FAQ

How does formation dip affect the TE/TM mode coupling in resistivity logs?
When the borehole intersects a formation at an angle (deviated well in dipping beds, or horizontal well in any beds), the transmitter and receiver coils are no longer aligned with the symmetry axis of the formation layering. This misalignment causes the tool to simultaneously couple to both TE and TM mode responses of the formation in proportions determined by the intersection angle. As the angle increases from 0 degrees (vertical well, horizontal beds — pure TM response reading Rh) to 90 degrees (horizontal well, horizontal beds — mixed response approaching Rh × λ² for TM and Rh for TE), the apparent resistivity increases progressively. Anisotropy inversion software models this angle-dependent coupling to extract Rh and Rv independently from the multi-component measurement set.