Longitudinal Plot
A longitudinal plot (also called a longitudinal dip component plot or L-plot) is one of two complementary dipmeter log presentations that displays the component of formation dip in the direction along the borehole's reference azimuth (typically the direction of maximum borehole deviation in a deviated well, or north in a vertical well), computed from the four-pad or six-pad resistivity sensors of a dipmeter tool (or from the image pixels of a formation microimager) to indicate whether beds are dipping toward or away from the reference direction; the longitudinal plot is used in conjunction with the transverse plot (T-plot, showing the dip component perpendicular to the reference azimuth) to fully characterize the spatial pattern of formation dip and dip azimuth as a function of depth, enabling the identification of depositional environments, structural geometry, and unconformity boundaries from the systematic patterns of dip variation observed in the borehole; longitudinal plots are particularly valuable in deviated wells where the borehole trajectory intersects formations at an oblique angle, causing apparent dip (the dip angle measured in the borehole cross-section) to differ significantly from the true dip of the formation, with the longitudinal and transverse dip components enabling calculation of the true formation dip after correction for the borehole deviation.
Key Takeaways
- The dipmeter tool computes formation dip by correlating the resistivity variations measured by pads pressed against opposite sides of the borehole, detecting the depth offset at which the same resistivity pattern appears on different pads to determine the strike and dip of the bedding plane that caused the pattern: in a four-pad dipmeter, four pads spaced 90 degrees apart around the borehole circumference each record a resistivity microlog as the tool is pulled uphole, and correlation of the waveforms from pads on opposite sides of the borehole (the two 180-degree pairs) provides two independent dip computations that are combined to give the dip magnitude and azimuth; the longitudinal component of the dip is calculated as the dip magnitude times the cosine of the angle between the dip azimuth and the reference azimuth, providing the component of the true dip vector in the reference direction; in formation image logs (FMI, OBMI, STAR), the same dip computation is performed on the digitized resistivity image pixels rather than on manually correlated pad curves, providing higher-resolution and more continuous dip data than the older dipmeter correlation methods.
- Structural and stratigraphic dip patterns identified from longitudinal and transverse plots include: constant dip (uniform dip magnitude and azimuth over a long interval, indicating a simple homoclinal structure or uniformly dipping formation); upward-decreasing dip (dip magnitude decreasing from bottom to top, classic pattern of a progressive unconformity or growth fault where the older beds have greater structural tilt than younger beds deposited after the deformation was underway); opposite dip (adjacent intervals dipping in opposite directions, indicating an anticlinal or synclinal fold axis between them); and cross-bedding patterns (rapidly alternating dip azimuths within a sand unit, indicating the presence of cross-bedded sandstone where individual foresets dip in different directions than the bounding surfaces); the combination of systematic dip patterns from the longitudinal and transverse plots with the borehole image texture provides a much richer interpretation of the depositional and structural environment than the dip magnitude alone from the historical stick-plot presentation.
- Sedimentological interpretation of dipmeter longitudinal plots uses the systematic dip patterns associated with different depositional environments to identify fluvial channels, deltaic foresets, turbidite sequences, and other sedimentary architectures in wells without core: fluvial channel fills show a characteristic fining-upward grain size trend (visible on the gamma ray log) combined with dipmeter patterns of high-angle foreset dips (20 to 35 degrees) within the channel sand, changing to low-angle dips in the point bar accretion surface geometry (epsilon cross-stratification) that records lateral migration of the channel; offshore bars (subaqueous dune fields) show landward-dipping or seaward-dipping foresets depending on the dominant current direction; turbidite sequences show chaotic dip patterns in the sandy turbidite units (reflecting the complex internal geometry of high-energy turbidity current deposits) contrasting with more consistent, low-angle dips in the hemipelagic mudstone intervals between turbidites; these sedimentological interpretations from dipmeter longitudinal plots guide the placement of horizontal wells to intersect the highest-quality reservoir facies and avoid the low-permeability baffles in complex depositional systems.
- True dip calculation from deviated wells requires decomposing the measured apparent dip (the dip as seen in the borehole cross-section, which is a combination of the true formation dip and the borehole deviation) into true formation dip using the vector addition of the apparent dip and the borehole inclination and azimuth vectors: the longitudinal and transverse dip components provide the two horizontal components of the apparent dip vector, which is then corrected for the borehole inclination (using the borehole survey data from the directional drilling instruments) to yield the two horizontal components of the true dip vector; the true dip magnitude is the vector sum of the two horizontal dip components, and the true dip azimuth is the angle of the horizontal dip vector from north; in highly deviated wells (above 60 degrees inclination), small errors in the apparent dip measurement produce large errors in the computed true dip, making accurate borehole survey data and high-quality dipmeter data quality control essential for reliable true dip calculation; wellbore image software packages (Schlumberger Techlog, Halliburton Geolog) perform this dip tensor computation automatically from the image log dip picks and the borehole survey data, producing true dip rose diagrams and stereonet plots for structural and sedimentological analysis.
- Quality control of longitudinal plots requires verification that the dip computation is reliable and not contaminated by tool motion artifacts, borehole rugosity effects, or non-geological sources of resistivity variation: stick plots (displaying each dip computation as a short line segment indicating the strike and dip magnitude on the depth track) have been largely replaced by summary rose diagrams (polar plots showing the frequency distribution of dip azimuth over a selected depth interval) and stereonets (lower-hemisphere Schmidt or Wulff projection plots that show the orientation of each bedding plane as a point on the sphere) that enable rapid identification of dominant dip directions and structural geometries; coherence filtering of the dip computation (retaining only dip calculations with high waveform correlation coefficients between opposing pad pairs) removes the least reliable measurements and improves the quality of the interpreted structural and sedimentological patterns; the quantitative reliability of dipmeter dip computations in highly irregular (rugose) boreholes is limited because pad contact with the borehole wall is intermittent, producing unreliable resistivity measurements that cannot be meaningfully correlated.
Fast Facts
The dipmeter log was first introduced by Schlumberger in 1936 as a two-electrode tool and evolved through four-electrode (1952), four-pad (1961), and six-pad (1982) designs before being superseded in the 1990s by formation microimager tools (FMI, OBMI) that provide full borehole coverage at resolutions of 0.2 to 0.5 centimeters. The longitudinal and transverse plot presentation, introduced in the 1960s as an alternative to the single-color stick plot, has remained the standard dip analysis presentation in formation evaluation reports because it enables clearer identification of dip trends and patterns than the original stick plot format.
What Is a Longitudinal Plot?
A longitudinal plot is a dipmeter log presentation showing the component of formation dip in the direction along the borehole's reference azimuth, used in conjunction with the transverse plot (dip component perpendicular to the reference azimuth) to fully characterize the spatial pattern of formation dip and enable structural and sedimentological interpretation from the borehole. Longitudinal and transverse dip components together allow calculation of true formation dip from apparent dip measured in deviated wells, and their patterns over depth intervals identify structural features (folds, unconformities, growth faults) and depositional environments (channels, foresets, turbidites) in wells without core.
Synonyms and Related Terminology
Longitudinal plot is also called an L-plot or longitudinal dip component plot; its complement is the transverse plot or T-plot. Related terms include dipmeter (the wireline logging tool that measures the electrical resistivity of the formation at multiple pads pressed against opposite sides of the borehole, computing the dip and azimuth of bedding planes from the depth offsets of correlated resistivity features between pads, with the raw dip computation presented as stick plots, rose diagrams, or longitudinal and transverse component plots for structural and sedimentological interpretation), formation microimager (FMI, the high-resolution version of the dipmeter that uses a dense array of button electrodes on multiple pads to create a continuous, high-resolution resistivity image of the borehole wall, providing both dip computation (from automated pattern correlation across the image) and visual interpretation of sedimentary structures, fractures, and vugs that the lower-resolution dipmeter cannot resolve), apparent dip (the dip angle and azimuth as measured in any vertical section or borehole cross-section that is not parallel to the true dip direction, which is always less than or equal to the true dip angle and is the measured quantity from dipmeter logs in deviated wells that must be corrected to true dip using the borehole inclination and azimuth data), true dip (the maximum dip angle of a planar geological surface measured in the vertical plane perpendicular to its strike, which is the geologically meaningful dip quantity for structural interpretation and map construction, computed from dipmeter or image log apparent dip measurements by correcting for borehole deviation using vector geometry), and borehole image log (the wireline or LWD tool measurement that produces a continuous, oriented visual representation of the borehole wall resistivity or acoustic reflectivity as a function of depth and azimuth, enabling sedimentological, structural, and geomechanical interpretation of features visible at the borehole scale including fractures, vugs, bedding planes, and fault zones).
Why Dip Analysis from Longitudinal Plots Remains a Valuable Petrophysical Tool
In wells without core, the dipmeter and its modern equivalent (the formation image log) are the only tools that provide direct measurement of bedding orientation and sedimentary structure from the wellbore. A longitudinal plot that shows a systematic upward-decreasing dip pattern identifies a growth fault or progressive unconformity, a critical structural observation that changes the interpretation of the basin's tectonic history and guides the placement of the next exploration well. A pattern of high-angle foresets within a channel sand body identifies the azimuth of the paleocurrent direction, guiding horizontal well azimuth selection to intersect the maximum lateral extent of the channel. These observations, invisible on any other wireline log, justify the cost of acquiring and interpreting formation dip data as a routine component of the wireline logging program in wells where structural and sedimentological interpretation are important for development planning.