Attitude of linear and planar geological objects with Brunton

Although most geologic structures are generally either curvilinear or curviplanar, they can be approximated as either linear or planar at specific scales or domains. For example, a primary linear structure such as the crest of ripple marks or flute casts on a bed may be folded around the axis of a fold. At the scale of a large fold, these linear objects are curved, i.e., have a systematically distributed orientation (e.g., small circle or great circle distribution). However, within each limb of the fold (a domain), the orientation of these structures may be homogeneous, that is, the flute casts or ripple crest are subparallel to parallel. On each limb, the fabric data of minor folds may have a homogeneous distribution.

The attitude of both linear and planar objects has two general components: bearing and inclination. Bearing is the horizontal angle between a line and a specified reference (N or S). The "line" either is the horizontal projection of an inclined linear object, or a horizontal line on an inclined plane. Bearing is a scalar feature, i.e., it just is a number (e.g., 045o or N45oE). Inclination, on the other hand, is the vertical angle between a linear or planar object and the horizontal. The convention for direction of inclination is down, i.e., we measure the angle from the horizontal down (not up), especially when we process the data on the lower hemisphere, equal area projection (mineralogists also use the upper hemisphere for crystals). Inclination is a vector, meaning that it has two components: an amount (angle below the horizontal), and an orientation specifying the direction to which the planar feature is inclined down (e.g., 30oNW).

Attitude is too general, and its two components: bearing and inclination, take different meanings when dealing with linear and planar element. For planar features such as bedding (the boundaries of a bed), fault, and foliation, the bearing and inclination become strike and dip. Note that strike is a scalar, and dip is a vector. Strike is the bearing of a horizontal line on an inclined plane. Since strike is the bearing of a horizontal line, we can read the bearing of either of its ends; thus, 000o and 180o are the same strike. Dip is the inclination of an inclined plane. For linear fabric such as hingeline, axis, or lineation, we use trend and plunge to represent the bearing and inclination. Notice that horizontal planes don't have any strike because they don't intersect the horizontal along a line.

Trend is the bearing of a linear object measured in the direction to which the line is inclined down. Plunge is the amount of the inclination of the linear feature. Thus, both trend and plunge are scalars; together they define the line vector. For example, a 060o, 30o (also written as 30o, 060o, or 30o, N60oE) is a pair of trend/plunge (direction/magnitude) or plunge/trend, which means that a line plunges 30o down below the horizontal in the 060o direction. Linear objects can also be defined by their pitch on a specific plane. Notice that vertical lines don't have any definable trend, and that the trend of a non-horizontal linear object must be read from a reference (e.g., N) to the direction that the line plunges. Thus, a trend of 000o and 180o are not the same thing (contrast this with strike!). In practice, it is extremely difficult, if not highly error-prone, to measure the trend of steeply plunging linear object; in such cases we use pitch. Notice that the trend of any line on a vertical plane is the same as the strike of that plane (a useful geometric fact). Pitch is the acute angle measured on a plane, from the strike of the plane that contains the line, toward the line (the sense is important!). For example, a pitch of 40oSW (read as: 40o from SW) means that the line is pitching 40o from (not to!) the SW end of the strike line of a plane that contains the line. Notice that pitch generally is not a horizontal or vertical angle, except for horizontal and vertical planes containing linear features. Pitch is an alternative to trend and plunge, although, sometimes, it is the only practical way of measuring a line correctly, especially if the line is steeply plunging.

Measuring the attitude of linear objects

Measuring trend and plunge: If the linear object is below our line of sight, open the sighting arm and the lid, and align the open, long slot of the arm parallel to the linear feature. If the linear feature is above our head (e.g., on a bedding above us), stand under the object and align the linear feature with the black axial line on the mirror on the lid of the compass. In either case, level the bull's eye, round level while aligning. If the linear object is plunging, only one of the needles (the north-seeking or the south-seeking) indicates the true trend of the linear feature. This is a case where many inexperienced geologists can make a common, critical mistake! Some people have the habit of only reading the north-seeking (white) needle of the compass, or vice versa, which is an error-prone practice. When using the Brunton compass we should be color-blind, and only read the direction of the needle that is correctly indicating the direction to which the line is plunging (down, not up!). Thus, the trend of only one of the needles is correct when reading a line. To figure out which one, we should be aware of the local geographic directions, that is, know the direction of north or south in the field at all times. For example, if we are measuring a linear object plunging down to the south (S or somewhere in the SE or SW quadrants), we must read the trend indicating any one of these southern directions (e.g., 120 o or S60 o E), and not the diametrically opposite northern directions (i.e., 300 o or N60oW) indicated by the opposite end of the needle. For a plunging line, 120 o and 300 o are not equivalent; only one is the true down direction (120 o or S60 o E in this case). The true direction of the trend may be indicated by the white or the black needle; the color depends on how we hold the compass (sighting arm away or toward our body), and on which way we are facing in the field (looking north or south). Therefore to avoid a common mistake, no matter how you are holding the compass or which way you are facing; just know where the geographic N or S is in the field, and ask yourself this question: Which way is the line going down (i.e., plunging)? If it is plunging to around the N, then read the needle (white or black) that points to N or in the NE or NW quadrants and not the opposite directions. This is the easiest and most practical way of correctly measuring a line. Of course, if a linear feature is non-plunging (a special case), we have the freedom of reading either the white or the black needle, because the line is horizontal (both ends are the same).

Example: We are measuring the crest of a ripple mark which is roughly trending somewhere around north (we know which way is N in the field because we have the compass!). The crest of the ripple mark is plunging, and lies on a bedding, which is dipping. Align the compass's sighting arm with the crest and then read either the direction indicated by the white or the black needle that points somewhere to the north. Thus, if the black needle points to N20oW and the black needle points to the S20oE, we must read the black needle. Don't wrongly assume that the white needle gives you the north readings; a common misconception.

Measuring vertical angles, height, and distance

To measure vertical angles, fold the lid and use the compass as was described for measuring the plunge of lines, i.e., with the clinometer. The vertical angle (q) can then be used to calculate the height (h) of an object (e.g., wall, tower, mountain peak) using the equation h = x tanq, if we know the distance (x) to the object. We can also use the trigonometric functions to calculate the horizontal distance (x) from point A to an object located at point B as follows. Walk from point A to another point C such that AC is perpendicular to line AB. This is done by taking a bearing at 90o to the bearing of AB with the compass. Use a tape or a measured pace (if pace spacing is known). In practice, we define AC to be 10 meters; or walk from A to C by 10 meters with our pace. Read a bearing from point C to point B. Subtracting the two bearings gives the angle b between AB and CB. Now we have a right angle triangle ABC with AC = 10 m, AB = x, and a known angle b. Use the equation tanb = AC/AB = 10m/x, and calculate x in meters.

If the linear object of interest is steeply plunging, it is better to use pitch instead of the trend and plunge. Measuring pitch is only possible if the linear feature lies on a physical plane. For example, if a set of slickenlines (striations) plunges (e.g., around S) on a fault, we measure the striations as follows. First, measure the plane (i.e., fault) that contains the linear features (see next section for this). Next, measure the pitch of the striations on the fault plane as follows. The Brunton compass has a circular, high relief ring on its back, which is designed for measuring pitch. Open the compass (the arm and lid opened completely) and align the edge of the lid and box with the line while the whole ring on the back of the compass touches the fault. If the clinometer, barrel-shaped level is not centered in this position, gently move the box off the plane and slightly turn the clinometer, and lay the box back on the plane while aligning the edge with the line. If the clinometer is not centered, repeat these steps several times until the clinometer is leveled while the edge of the box is parallel to the line, and the circle behind the compass is completely lying on the plane. This is a trial and error process that requires some practice to master.

Using the compass as hand level on a Jacob Staff

The compass can be used as hand level, mounted on a Jacob Staff to measure the true stratigraphic thickness of a lithostratigraphic unit (e.g., member, formation) as follows. Measure the true dip of the layers and set the clinometer at that angle. Mount the compass vertically (as in reading the plunge) on the Jacob Staff with the lid half closed; making sure that the clinometer is set at the measured dip angle. Start at the lower contact of a stratigraphic unit. Tilt the staff in the direction of the dip of the beds, and look inside the mirror until the clinometer level is centered. At this position, look through the sighting tip and through the sighting window. Identify a point (e.g., a brush, a piece of rock) on the ground where the line of sight intersects the ground. Move the base of the Jacob staff to that point. Use a counter and register the number of times (n) it takes to go from the basal contact of the lithostratigraphic unit to its top. At the upper contact, multiply the length of the Jacob Staff, which is 1.5 meter, by 'n', to get the true stratigraphic thickness of the unit.

Measuring the attitude of planes

If the plane is flat, smooth, and non-magnetic, the easiest way to measure the strike and dip of the plane is to touch the edge of the box (not all the rectangular side!) with the plane while centering the circular, bull's eye level. This will generate a horizontal line parallel to the edge of the box on the plane of interest. We have the freedom of reading either of the two needles; it does not matter which one we read! (e.g., 140o and 320o are the same strikes). This is the strike of the plane; we can mark the strike line on the plane with a pencil drawn parallel to the edge of the box. In special cases such as when taking oriented samples of rock for structural analysis, we need to distinguish and mark only one of these two ends of the horizontal line with an arrow (preferably with half arrow tip).

After the strike is measured, the magnitude of the dip of the plane is measured by putting the entire rectangular side of the box perpendicular to the strike line and centering the clinometer level. The general, dominant direction of the dip is identified geographically by checking the down-dip direction (by centering the bull's eye level and finding out where the principal geographic directions are). In North America, the format is strike, dip and dip direction (e.g., 050 o, 30 o NW), because that is the sequence of measuring the attitude of a plane with the Brunton compass. Silva and other similar compasses allow easier measurement of the dip direction before or without identifying the strike direction. Thus, in Europe and other places, the format may be dip amount, dip direction, which is a vector (e.g., 30o, 320o).

If the plane of interest is not flat, and lies in front of us at the level of our line of sight, we must use eye-level sighting as follows. Stretch the sighting arm and bend the sighting tip. Close one of the eyes, and move sideways while looking onto the edge of the inclined plane. Stop moving if further moving exposes the surface of the plane. In this position, we are looking edgewise along the plane. Fold the compass lid until we see the edge of the inclined plane in the sighting window through the lid. Hold the compass as follows: Put the two thumbs under the sighting arm on the box; the two index fingers on the edge of the lid, and the center fingers behind the horizontally positioned box. Adjust the lid with the index fingers until the bull's eye level is apparent in the mirror. Center the bull's eye level, and intersect the edge of the plane with the black line in the sighting window (don't try to align the black line with the edge (because it tilts the box) unless the plane is vertical). Hold your breath, and read the bearing indicated by the black or white needle (whichever is apparent in the mirror), that is, the strike of the plane in the mirror without moving the box or going off level. While in the same position, read the amount of dip by aligning the flat edge of the box with the edge of the plane. Determine the direction of the dip by inspection; give the principal direction of inclination from the geographic space as described above.

If the plane of interest is vertical and the terrain is horizontal (a special case), stand directly above the edge of the plane and read the trend of the edge of the plane as the strike. Some inexperienced geologists assume that they can determine the strike of an inclined, non-vertical plane in this way. The technique of standing above the edge of a plane does not work if the plane is non-vertical and/or the top surface is not horizontal. This is because the intersection of a non-vertical plane and non-horizontal plane is not a horizontal line, and thus cannot be a strike! In such cases, we need to directly measure the strike by either eye-level sighting or by touching as described above.

Measuring the attitude of a plane with the two-line technique

The two-line technique is a very useful and precise method of measuring subhorizontal and gently dipping planes. Such low-dip planes are very common, and cannot accurately be measured by measuring the strike and dip. If the exposed surface of a plane is small, we drop two sharpened pencils on the plane at high angles to each other. Point the sharp ends of the pencils down-plunge. Measure the trend and plunge of the two lines (l1 and l2). If the subhorizontal plane is large and extensive (e.g., a basalt layer), we define two long lines with two persons. The two persons stand at two points and shoot at each other to determine the trend and plunge of the line connecting them (l1). Take the average of the two readings. They repeat this for a second line by constructing a second line (l2). To determine the orientation of the plane that contains the two lines, plot the lines as two points on the stereonet, and align them on the same great circle. Read the strike and dip of the great circle.

Although the two-line technique is the best way to determine the attitude of subhorizontal or gently dipping layers, the attitude of small, gently dipping planes (e.g., bedding at the hinge zone of a mesoscopic folds) can be determined by trial and error as follows. Measure the dip of the layer where we think is near the maximum inclination (true dip); center the clinometer level. Remember the dip value. While the rectangular side of the box is still completely touching the layer, slowly turn the compass and reread the dip. If the dip is less than the previous reading, then we are going away from the maximum inclination, and our previous reading was closer to the true dip. Move toward the first position and go to the opposite direction. Repeat the process until we identify the maximum inclination which is the true dip. When the true dip orientation and magnitude is registered, measure the strike of the plane perpendicular to this line.

Using the compass for the two-point problem

Sometimes we may be located at a contact of a horizontal layer (e.g., a basalt layer or a bed) on a hill, and be interested to locate a point of the same elevation on an adjacent hill. To do this, we set the clinometer at the 0o mark, and flip the compass sideways (vertically) as described for measuring the plunge. Look through the hole of the sighting tip, through the sighting window, and center the clinometer level (which is set to 0o) by moving the box up or down and looking in the mirror (without turning the clinometer). When the level is centered, locate a point on the other hill at the intersection of your line of sight and the ground. That point has the same elevation as the point of our position.

This technique is also handy in determining the strike of a layer. Just set the clinometer at the 0o mark; stand on the layer, look along the layer, and center the clinometer without turning the clinometer. After the level is centered, locate a point on the sloping layer along your horizontal line of sight. Now that we know the strike line (the horizontal line), we need to read its bearing either by eye-level or waist-level sighting. While in the same position, read the dip of the layer across the strike line using the clinometer as described for measuring the dip.



 

posted by Geology on 06:29

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The concept of domain of Brunton

One of the objectives of studying a complexly deformed area (e.g., refolded folds) is to identify domains (subareas) within which the fabric data of, for example, folds, lineations, foliations are homogeneous. This means, for example, that the hingelines and/or fold axes, and the poles to the axial planes (or axial planar foliation, if it exists) of all minor folds define maxima (i.e., cluster distribution), with the mean axis lying on the mean axial plane. The boundaries of the domains are identified (mapped) by locating the adjacent stations at which specific fabric data are homogeneous. The homogeneity in each domain reflects two major facts: (1) Homogeneity of the strain which results in equal extension in a strain field in which the axes of the maximum principal extension are parallel at every point, keeping originally-parallel lines and planes parallel, and straight lines straight. (2) Homogeneity of the rock, i.e., rock properties is the same at each point in the rock continuum during the deformation. Geologists cannot produce a useful map of, or obtain useful information from, moderately- to highly-deformed areas, without knowing how to use the compass to collect the fabric data and to delineate the domain boundaries. Thus, we need to know how to measure linear and planar objects of all kinds, such as sedimentological and structural fabric elements, and map lithostratigraphic boundaries such as contacts.

 

posted by Geology on 06:26

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Setting the declination of Brunton

Geologists use the compass for mapping and measuring linear and planar objects. The magnetic declination is set by turning the brass screw on the side of the compass box. For a west declination of say 16o (i.e., declination is 16o west of true north), turn the card west, i.e., counterclockwise (by turning the screw) so that the index pin points to 16o on the side of the card marked with 'W' in the quad scale, or 344o in azimuth scale. For an east declination of 16o, turn the card east (i.e., clockwise), so that the index pin points to 16o on the side of the card marked with 'E' in the quad scale, or 016o in azimuth scale.

 

posted by Geology on 06:25

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Determining the magnetic declination of Brunton

If the compass needle points east or west of the true north, the offset is called east or west declination, respectively. The standard is to use the magnetic north (MN) as a reference for declination, even in the southern hemisphere. To determine the magnetic declination in a study area we can use: (1) Published topographic maps; some maps display an out-of-date declination indicated by the angle between two arrows pointing to the magnetic north (MN) and true north (GN). (2) Published or online isogonic charts, which are available at:


(3) Online calculator to determine the latest magnetic declination for a given location (latitude and longitude) and year

or

 

posted by Geology on 06:24

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Bearing from Brunton

The direction of a line on the ground is given by the bearing of the line, which is the horizontal angle between the line and a reference, commonly north in the quadrant scale, or 000o (marked as 0o on the card) in the azimuth scale. The reference, however, can also be the south (S) in the quadrant scale, when reading the bearing (i.e., trend) of south-trending linear objects. The position of 'E' and 'W' are reversed on the circular card; 'E' lies left of the 0o mark (i.e., at 9 o'clock), and 'W' is to the right of the 0o (i.e., 3 o'clock) mark on the card. The reversal is designed to make the correct reading of the bearing possible. To appreciate this fact, notice that the north-seeking end of the needle always stays pointing north even when the compass dial is rotated. For example, to read a bearing of 045o, we level the dial and then turn right of north, but the north-seeking end of the needle turns to the left of 0o, which is actually east on the dial; so we read a correct bearing.
The vertical angle between the magnetic vectors relative to the level (horizontal) ground is the magnetic inclination, which varies with latitude; it is 90° at the north magnetic pole, and 0° at the magnetic equator

Measuring the bearing of a line between two points

Commonly we want to measure the trend and plunge of a line connecting two points, e.g., a line connecting a person and another person, or another landmark (e.g., house, tower, smoke stack). To do this we can either use the eye-level or waist-level sighting. The eye-level sighting was described above. For the waist-level sighting, we put the lid against our body, and tilt the lid while holding the box horizontally by centering the bull's eye level. Position the target on the black line on the mirror, and after centering the round level, read the trend.

Measure the plunge of this line as follows. Flip the compass (box is vertical) while the lid and sighting arm are folded. Look through the hole in the sighting tip and through the sighting window, and then center the clinometer level while shooting to a specific point on the target. If the two persons have the same height, intersect the other person's eyes with the black line on the sighting window. If we are sighting (shooting) to another person who is shorter than we are, say by 5 cm, then we shoot 5 cm above that person's eye level (at forehead or head level). If the other person is taller by 5 cm, then we shoot to the mouth level of that person.

 

posted by Geology on 06:22

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Main Parts of Brunton Compass

Brunton compasses have three main parts, box, sighting arm, and lid. The box contains most of the components: the needle; bull's eye level (round level to read horizontal angles); clinometer level (barrel-shaped) and clinometer scale (for reading vertical angles); damping mechanism (to more efficiently stabilizing the needle); lift pin (to lock the needle); side brass screw and index pin (to set and display the declination); graduated circle or card (to read the bearing). The needle has two ends: the north-seeking end (commonly white in genuine Brunton compasses, labeled 'N' in others), and the black, south-seeking end. The north-seeking end of the needle is pulled down in the northern hemisphere where the magnetic inclination is downward. An additional small weight attached to the south-seeking end of the needle provides proper balancing of the needle. The weight needs to be reversed if using the compass in the southern hemisphere where the magnetic inclination is upward.

The lid, attached to the box with a hinge, contains the mirror with the axial line and oval sighting window (for waist- and eye-level sighting), and the sight. The long sighting arm, attached to the box with a hinge, has a long, oval rectangular cutout or slot (for reading linear objects), and a tiltable sighting tip, which is used for aligning the line of sight. The circle card of the Brunton compass is designed in two traditional scales. The azimuth scale uses three digits, with north at 000o or 360o, and south at 180o. The quadrant scale uses an alphanumeric notation (e.g., N60oE, S20oW) with the card graduated in four 90o quadrants (NE, SE, SW, NW); north and south lie at the two upper and lower 0o marks, respectively.


 

posted by Geology on 06:21

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List of Common Drilling Terms

  • To cease producing oil or gas from a well.
  • Pressure exerted by a formation exceeding normal pressure for any given depth.
  • To inject HCl into a calcareous formation under pressure, that causes, enlargement of fissures and improvement of permeability characteristics.
  • A large valve installed above the ram preventers.
  • The space between drill string and casing or open hole.
  • American petroleum Institute; founded in 1920, this organisation aims for standardisation in the oil field.
  • To unscrew one threaded section from another as with pipe.
  • Ba S04, a mineral used to weight up drilling fluid.
  • 42 US. Gallon = 158.97 litres 1m3 = 6.2897 bbls.
  • Fishing accessory run above a bit or mill to recover small pieces of junk.
  • Device for breaking out the bit from the string.
  • Equipment installed to prevent the uncontrolled Blow-out Preventer escape of gas oil or salt water from the well.
  • To unscrew one section of pipe from another generally during pulling of pipe. The tongs are used in this operation.
  • A record of the diameter of the wellbore indicating washout or enlargements due to casings.
  • To control a blow-out by placing a very strong valve on the well bore.
  • A short heavy hollow cylindrical steel- concrete section with a rounded bottom placed on the end of the casing shoe. Also called a guide shoe.
  • A spool- shaped statement on each end of the draw works used for hoisting, breaking and tightening around the drill floor.
  • A thin wire line used for lifting heavy equipment around the rig, powered by the cathead.
  • A pit, beneath the drill floor, to give additional clearance between floor and wellhead to accommodate the BOP’s and to drain the area. To “jet the cellar” is to drain this pit.
  • An acoustic or sonic logging method recording Cement the quality of the bond between the casing and well bore.
  • The arrangement of piping and chokes through that the drilling mud is circulated when the BOP’s are closed.
  • The control valves, pressure gauges and chokes assembled at the top of a well to control oil/gas flow.
  • The drilling line from crown-block Sheave to the anchor, that does not move.
  • The inclination of the wellbore From the vertical, in degrees.
  • A small enclosure on the rig floor used to house driller, records and equipment.
  • Sudden increase or decrease in penetration rate.
  • To remove residual cement with bit.
  • A well that has no hydrocarbons or has uneconomic quantity of them..
  • A set of clamps that grip a stand, or column of casing, tubing, drill pipe or sucker rods so that the stand can be raised or lowered into the hole.
  • An extra- thick wall at the threaded end of a drill pipe or tubing. it has a thicker diameter at each end.
  • The end the drilling line that is affixed to the reel of the draw works, that travels faster than any other part of the line.
  • A rack that supports the tops of the stands in the derrick.
  • An object left in the well that need be recovered.
  • A method to stimulate production from a poorly permeable zone by pressuring open the fissures jacking them open with beads or such like, then releasing this pressure.
  • A drilling mud with entrained formation gas, causing reduced weight.
  • A single length of drill pipe, casing or drill collar.
  • A short sub placed between kelly and drill pipe to save excessive wear on the kelly threads.
  • A pneumatically operated device mound on the top of the kelly that turns the kelly. useful in making up pipe.
  • To attach the elevators to a section of pipe to pull it, or run it, into hole.
  • A platform on derrick from which the derrick man works while tripping.
  • An opening in the rig floor, pipe lined, that singles are placed in before making up.
  • The mud engineer.
  • To assemble the BOP stack onto the well.
  • To pierce the casing and cement for the purpose of allowing formation fluids to enter the production piping.
  • To trip string out of the hole.
  • Either a line hole in the rig floor on that the Kelly is kept during trips, or a hole of smaller diameter drilled at the bottom of the main hole.
  • To trip out, then into the hole.
  • To trip pipe into the hole.
  • A grooved pulley.
  • To drill around a blocked well bore by kicking off a new hole at an angle to the original.
  • To remove worn fast line, and slip more line in from the anchor point so moving the dead line around.
  • A temporary platform erected in the derrick for use while casing.
  • The connected joints of pipe racked in the derrick.
  • Drill string, casing or tubing that has become immovable in the hole.
  • A short length of pipe, threaded at each end, used to adapt different pats of the drill string that otherwise would not connect, or else to perform a specialist function e.g. junk sub, kelly saver sub.
  • Abbreviation of total depth - the end of the well.
  • Under gauge hole section through which it is difficult to pull the drill string. Or a well about that information is restricted.
  • The large wrenches used for making up or breaking out drill pipe.
  • Moving the drill string up and down in the hole whilst not rotating to prevent sticking.
A short trip up into casing then back to bottom to clean out the hole, to check for gauge, and to reduce the danger of getting stuck.

 

posted by Geology on 04:30

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Lag

The Lag
The calculation and practical application of the lag is of primary importance in mud logging and relates to the all data that the mud transmits to the surface. The mud actually carries the information that we require from the bit depth to the surface and the time that the mud takes to get from the bit to surface is the basic calculation made. The factors that affect the time or lag of the mud are the flow rate of the mud, the configuration of the well; the sizes and depths of the different hole sections and the drill string sections’ dimentions.
Lag definitions:
· Lag time is the time the mud takes to travel inside the hole between two specified depth points.
· The time taken between the surface to the bottom of the hole is called ”lag down“ or “Lag in”.
· The time taken between the bottom of the hole to the surface is called ”lag-up“ or “bottoms’up”.
· The surface to surface time is called “Complete cycle” or In/Out time. It is more practical to calculate lag in terms of pump strokes as the flow rate is not necessarily constant.
To calculate the lag the hole dimensions must be known as well as the drill string dimension. Most holes have at least two section of different diameters and towards the end of the well may will have more (riser, casing liner, and open hole). Added to this is the fact that the drill string will usually have sections of different diameters (drill pipe, heavyweight drill pipe and drill collars, etc).
Two techniques may be applied to calculating the annular volume,
These are :-
·         In The first method, the lengths and the dimentions of each section of the annulus are determined, the volumes are calculated and totalized. The The lag equations are applied to determin the equivalent times and strokes.
  • The second method involves calculating the volume of the hole and the
volume of the drill string (metal and internal capacity) and then subtracting the values from each other to determine the lag time and strokes for the whole well.
      The first method is the one preferred because it informs the logger of the exact nature of the various annular sections and their individual volumes. This also helps in the calculations of the annular pressure drops. With the use of the off-line mudlogging units the increase with depth should be calculated for a given length of hole by calculating the annular volume of the hole (bit diameter) filled with drill pipe. This should be added to the total annular volume to update the lag calculation to the current depth. For the On - Line logging unit this is automatically calculated and added as the depth increases. The lag, as already mentioned, is most accurately counted in pump strokes. The annulus volume divided by the pump output per stroke will give the number of strokes needed to displace the mud up the annulus. It is possible to monitor the lag in time units although this practice is much less accurate and very prone to errors. The most common error is to fail to keep a record of the amount of time that the pumps are off due to connections etc. The lag will be delayed by this amount.
LAG EQUATIONS
A. Converting Barrels è Gallons:
Gallons (gal) = Barrels î 42
B. Converting Gallons è Barrels:
Barrels (bbl) = Gallons · 42
C. Calculating Pipe Volume:
Pipe. Volume (bbl)
(Pipe / Collar ID2 Length(ft)
1029
=
) ´
C. Calculating Annular Volume:
Ann. Volume (bbl)
(Hole / Casing ID2 Pipe / Collar OD2 Length(ft)
1029
=
- ) ´
D. Calculating lag-in strokes:
Lag in strokes
Annular Volume bbl
Pump Output bbl stk
- =
( )
( / )
E. Calculating lag-in minutes:
Lag in utes
Lag in strokes
PumpRate spm
- =
-
min
( )
F. Converting Meter è Ft:
Feet (Ft) = Meter î 3.281
G. Converting Cu. inch è bbl:
1 bbl = 9702 cu. inch
H. Converting gcc èppg:
ppg = gcc x 8.33
Lag Correction
Using the correct lag is vital to the geologist so that samples and hydrocarbon shows are described at the correct depth from which they came. If the open hole section is in gauge; then the actual lag will be the same as the calculated lag. This is rarely the case in practice as most of the salt sections and some shale sequences tend to become washed out. Therefore carbide lag checks should be run frequently to determine the actual lag. The procedure for carbide lag is to wrap a quantity of fine carbide in paper towel and place it inside the pipe at a connection. The action of water on the carbide will release acetylene gas which on circulating out of the system will be detected by the gas detector. Since the gas has to travel down the pipe to the bit and then to the surface, it is necessary to calculate the following:
1. The number of strokes from the surface to the bit inside the pipe.
2. The total number of strokes from starting up the pump until the gas arrives at the surface.
3. Subtract 1 from 2
The resulting number of strokes is the actual lag time. From this it is possible to estimate the amount of washout in the hole. Apart from making regular carbide lag checks, a check should be made if for any reason the lag becomes suspect; for example the cuttings do not correspond with the drill rates from which they are supposed to come, or connection gas does not appear at the correct time. If for some reason , carbide is not available a perfectly good lag check can be obtained by using rice or lentil. The main disadvantage of this is that it is necessary to stay and watch the shakers when the rice is due appear, or it could well be missed. Rates of travel up the annulus differ for gas and cuttings as the cuttings will tend to slip back due to slip velocity. Slip velocity depends on the cuttings size density the mud properties flow rate and hole size.
Lag calculation example:
GIVEN:
Pump information
pump output = 0.123 bbls/stroke
pump rate = 75 spm
Drill string information
Drillpipe :                                          5” OD 4.276” ID length 8075’
Heavy-weight Drillpipe:                    5” OD 3.000” ID Length 275’
Drill collars:                                      8” OD 2.813” ID Length 650
Hole information
Casing :                                          13 3/8” OD 12.415” ID Length 3500
Open hole :                                    12.25” OD
TD :                                                 900’
CALCULATE:
a. Calculate the volume of mud in the Drillpipe, Heavy-weight and collars.
b. Calculate the annular volume for each annular section.
c. Add the section annular volumes to give the total annular volume.
d. Calculate the lag in minutes.

 

posted by Geology on 04:30

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Borehole Geophysics and Petrophysics

Downhole seismic systems
The GSC operates both sidewell-locking geophones and hydrophone arrays for measurement of compressional and shear wave velocities in overburden materials. These units have been constructed in-house using commercial hydrophone and geophone elements. Surface seismic sources commonly used are the . buffalo gun. for P waves and the polarized horizontal hammer on a weighted plate for S waves. Recording is done on an multichannel engineering seismograph. A depth capability of 300 m is possible for both wave velocities.
All systems are operated in a cased borehole, preferably one which is not open or screened at the bottom.

P wave velocity log
The downhole P wave velocity log is derived using either a 12- or 24-channel hydrophone array. This array is moved incrementally either up or down the borehole; a surface source (commonly a 12-gauge Buffalo gun fired in a shallow hole) is placed close to the borehole (3 to 6 m to one side, at 1 m depth).
The spacing between hydrophones is fixed at 0.5 meters; hence incremental vertical moves of the array in the order of 1 m between source records will yield considerable redundancy of hydrophone locations. Travel-times between source and receivers are individually picked for each shot record. The data redundancy is used to obtain best estimates of interval velocities over short vertical intervals (Hunter et al., 1998). For this compilation plots of P wave velocities are given at intervals of 0.5 meters downhole. Usually 3 pt (over 1 m vertically) or 5 pt (over 2 m vertically) velocity fit results are shown.
Compressional (P) wave velocities are strongly affected by the presence or absence of pore-water. Low velocities are exhibited above the water table and in areas of the borehole where gas exists in the pore space. Most normally consolidated water-saturated soils have velocities close to that of water (1480 m/s). Overconsolidation of water-saturated soils ( with resulting reduction of porosity) is indicated by somewhat higher velocities (e.g. a compacted coarse-grained basal till can yield velocities of 2500-3500 m/s. Lithification to rock, or ice-bonding of soils, results in velocities which may range between 2500-5500 m/s. Empirical relationships between soil porosity and P wave velocities have been developed.
 

 

posted by Geology on 04:29

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Field Instruments and Field Methods in Geology

Field Instruments

The main field instruments used by geologists include the Brunton compass (and/or Silva compass), tape measures, and plane table and alidade. The Brunton compass is a compact device that permits compass bearings to be made upon linear features (including strike lines) and lines connecting any two points. The Brunton may also function as a protractor (when placed upon a map) and as a device for measuring structural dip and vertical angles (using its internal clinometer).

The Silva compass is somewhat similar, except that it does not have a bubble level or adjustable clinometer, so a task like measuring a vertical angle is not possible, and strike and dip measurements may not be as accurately made as with a Brunton. This Silva does not come in a rugged case, as does the Brunton, but its design as a flat, blade shape allows it to be used for map work more easily than a Brunton.

The geologist's tape measure is usually the reel-in variety, which is marked in meters and feet. A typical tape length is 100 ft (30.5 m).

The plane table and alidade are surveying devices used to measure distance and relative elevation. The plane table sits atop a tripod and a geological or topographic map under construction would be taped to its top.

The alidade is a telescopic device that can be moved over the map surface as sightings are made. This device allows measure of horizontal distance and elevation. Horizontal distance and elevation of a point on the earth's surface is obtained by viewing through the alidade sight, a rod with a printed scale upon it (called a stadia). Data recorded during this observation are used for recording distance and elevation of the surveyed point upon a map.

The field instruments above would accompany most fully equipped field geologists on an expedition of mapping and sample collection. The geologist would typically also carry along a field notebook, hand lens, hammer, acid bottle, knife, shovels or trowels, sample bags, pens and pencils, aerial photographs and satellite imagery, maps and literature, camping equipment, and a camera. In the modern era, these materials could also be supplemented by a global positioning satellite system (GPS) receiver (for determining location and retracing routes), laptop computers, digital cameras, and portable geophysical equipment (including a gravimeter, altimeter, magnetic susceptibility meter, etc.). Occasionally, a geologist will bring along power tools for cutting or drilling rock or plaster, and burlap for wrapping delicate samples such as fossil bones.

Field Methods

Field methods in geology may be broken down into four main groups: (1) obtaining and marking samples and describing and measuring where they came from in an outcrop; (2) measuring and recording orientation (i.e., altitude) of strata or other planar features; (3) measuring dimensions (height and width); and (4) constructing geologic and topographic maps.

Obtaining and marking samples and describing and measuring where they originate in an outcrop requires observational skills and patience to record all information that might be obtained at one outcrop. Typically, the thickness of strata at an outcrop is recorded in a notebook where the layers are drawn to scale and described as to rock type, grain size, fossil content, color, sedimentary structures, and other attributes. Thickness of strata is measured using a tape measure or a Jacob's staff, which is a long stick made for sighting intervals of equal stratigraphic thickness (usually 5 ft, or 1.5 m). In the field notebook, detail is given about sampling locations and where photographs of the rocks are made. Samples are marked with an arrow indicating 'up' direction and labeled with a number which relates to the notebook number for the outcrop plus a number relating to feet or meters above the base of the stratigraphic section at that location. The same process is followed at each locale. Later, this information is compiled into a measured and described section for each outcrop, which may be used for correlation between outcrops. In terrains where igneous and metamorphic rocks occur, it is usually not so important for sample information to be recorded about the up direction and elevation above the base of outcrop.

Measuring and recording orientation (i.e., attitude) of strata or other planar features is another important field activity that relates to understanding geological structures and to the making of geological maps. Strike, dip direction, and dip magnitude of rock layers and other planar geological features (e.g., foliation) are obtained in as many places as possible within a study area in order to understand completely all the geological structures (i.e., folds and fault patterns) of an area. Analysis of geological structures can help geologists interpret the conditions of deformation of rocks in an area. Generally, the geologist tries to obtain as many orientation or attitude measurements as possible in the field area being studied.

Measuring dimensions (height and width) of an area or of features in an area, is an important aspect of many geological studies. This may be done as an estimate by using the moveable clinometer in a Brunton compass and employing trigonometric relationships to compute the height or width. For example, if one uses a tape measure (or number of foot paces, if the average foot pace of the observer is known) to measure distance to a cliff wall, and then uses the clinometer in his Brunton to measure angle between his eye level and the top of the cliff, a computation of cliff height can be made. In this instance, the cliff height is equal to the person's eye height plus the product of the horizontal distance to the cliff times the tangent of the sighted angle. Geologists sometimes make simple maps, called "pace and compass maps," using the Brunton compass to take bearings and his measured pace length as a distance measure.

Constructing geologic and topographic maps is another field activity that occupies geologists. Geological maps are made by using a base map or set of aerial photographs to record the observed rock type (preferably a measured and described section, as noted above) and rock attitude at numerous locales in the study area. It is the task of the geologist to ultimately fashion a geologic map that is the simplest interpretation of all the surficial data about rock type and rock attitude in the area. Topographic maps are made by plane table and alidade, as noted above, and these may form the base map for geological mapping studies because surficial elevation is important in interpreting physical relationships between rock formations.

Other types of geological field work include reconnaissance studies of areas where detailed mapping is yet to be done, geological sample analysis conducted on-site at drilling operations, geophysical studies (where the objective is to collect data such as gravity strength, magnetic characteristics, etc.), surface- and groundwater studies (where the emphasis is upon water distribution, quality, and its relationship to geologic features), economic geology studies (where mines and excavations are studied and areas explored for the value of potential new mining), engineering geology field work (where studies assess the impact of human disturbance upon rock and soil stability), and many others.

 

posted by Geology on 04:21

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Geological Brunton (Part-II)


How we use brunton?

The Brunton may be adjusted for declination angle according to one's location on the Earth. It is used to get directional degree measurements (azimuth) through use of the Earth's magnetic field.

Holding the compass at waist-height, the user looks down into the mirror and lines up the target, needle, and guide line that is on the mirror. Once all three are lined up and the compass is level, the reading for that azimuth can be made.

Arguably the most frequent use for the Brunton in the field is the calculation of the strike and dip of geological features (faults, contacts, foliation, sedimentary strata, etc.).

If next to the feature, the strike is measured by leveling (with the bull's eye level) the compass along the plane being measured.

Dip is taken by laying the side of the compass perpendicular to the strike measurement and rotating horizontal level until the bubble is stable and the reading has been made.

If properly used and if field conditions allow, additional features of the compass allow users to measure such geological attributes from a distance

Suggestion

I suggest you get hold of a simple compass having an edge parallel to the N-S orientation (or devise it yourself). Put that edge along the dipping bed and note the reading, and find out in which quadrant the strike falls. The best way to note Strike is with reference to North(In a Brunton the  circular scale is reversed for direct reading), e.g 100 degrees N means not exactly in the East but 10 degrees more. As you know the dip of a bed is with reference to the horizontal ground. Find the maximum dip direction
by pouring some water on the dipping surface. Take a simple Level with a bubble (as with masons) and put its one end vertically against the dipping bed in line with the flow direction of the poured water, make it horizontal (parallel to the ground) and see what angle it makes with the bed. The angle between your Level and the rock surface will give the dip angle of the bed. You may not know the exact degrees of angle of dip, but for exercise purposes, note down qualitative readings as low, medium low, medium high, high etc etc. This make-shift exercise will make you understand the concept.



 

posted by Geology on 07:41

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Geological Brunton (Part-1)

Introduction

The Brunton  is a specialized instrument used widely by those needing to make accurate degree and angle measurements in the field. These people are primarily geologists.

Compasses work because the earth acts like a giant bar magnet. Motions in the liquid nickel-iron core of the earth induce a magnetic field with a north and south pole. Magnetic lines of force connect the earth's north and south magnetic poles as show below:




Compasses work because a magnetized compass needle will align itself with the earth's magnetic lines of force and point approximately north. I said approximately because you'll note in the figure above that the north and south magnetic poles don't exactly align with the earth's axis of rotation which defines the north and south geographic poles. We'll discuss this important fact shortly.

Structural geologists use compasses to create geologic maps and measure the orientations of geologic structures. Therefore, in order to do structural geology research in the field, you need to know how a compass works and how to properly use one.

Azimuths vs. Quadrants

There are three common ways to express a direction with a compass. The first is to simply estimate your direction as north, east, south, or west. If you want a little more precision, you can use northeast, northwest, southeast, or southwest as well. For this purpose, the compass below would work fine.


Let's take a moment to make sure we understand how such a compass works. Do you know why, for example, that E (east) and W (west) appear reversed on the compass above?

The compass needle is magnetic and aligns itself with the earth's magnetic field such that the white end of the needle points toward the north magnetic pole (we'll talk about the difference between the geographic north pole and the magnetic north pole shortly). Let's hold a compass while we're facing north. The arrow and the N on the compass will line up as shown below on the left.


Now let's turn and face northeast. The compass needle doesn't move, it always points north, and now it's located half-way between the N and the E (northeast) on the compass. Let's keep turning and face east. The compass needle is still pointing toward the north but now it lines up with the E on the compass indicating that we're facing east. See why the E and W on compass faces are reversed?

While such a compass may be fine for casual hiking, it obviously isn't very exact and people making measurements (like structural geologists) need to have more accurate data. Compasses used by structural geologists commonly come in two forms -- those using the azimuthal notation and those using the quadrant notation. Let's examine each of these in turn..

In azimuthal notation, a circle is divided up from 0° to 359° in a clockwise direction. North is 0°, east is 90°, south is 180°, and west is 270°.



In transfering this to the face of a compass, the numbers increase from 0° to 359° in a counter-clockwise direction (to remain consistent with the reversal of E and W on compasses).



This is called azimuthal notation. If you were facing east, you would say that the azimuth was 90°. If you were facing south, you would say that the azimuth was 180°.

Once again, in azimuth notation, your direction is specified as an angle between 0° and 359° where north is 0°, east in 90°, south is 180°, and west is 270°.

Now let's look at quadrant notation, an alternative to azimuth notation.

Quadrant notation divides the compass face up into quadrants (think of cutting a pie into four equal pieces). There is a northeast, southeast, southwest, and northwest quadrant. Angles are measured east or west of north and east or west of south as shown below.


In quadrant notation, you specify a direction as being a certain number of degrees east or west of north or east or west of south (depending upon which quadrant it is). A few examples should clear up any confusion.

Northeast is 45° east of north, southeast is 45° east of south, and southwest is 45° west of south.

Some directions may be specified in two ways. East is 90° east of north or as 90° east of south. Similarly for west (it's 90° west of north or south). North is 0° east of north or 0° west of north. Similarly for south (it's 0° east or west of south).

The correct way to write a quadrant notation is N or S (for north or south), followed by an angle, followed by E or W (for east or west). Therefore, N45°W is 45° west of north or northwest.

Below is a compass face set up for quadrant notation and indicating, coincidentally, N45°W.


Converting between azimuth and quadrant notation is pretty straightforward as long as you remember how each are defined. Look at the figure below to compare the two methods.


I personally prefer azimuthal notation because it's the easiest to enter into computer programs which, for example, will plot the data in a stereographic projection (which we'll learn about shortly). Others prefer quadrant notation, and there's nothing wrong with it, but just make sure that whatever method you do use (and that will probably depend on the type of compass you have) is used consistently. In other words, don't keep switching from one method to another when recording data or you'll quickly become confused.

Magnetic Declination

Many people don't realize that a compass needle does not usually point due north, but at some angle east or west of north. This is because the earth's geographic pole (the axis about which it rotates) is not in the same place as its magnetic pole (the place where the magnetic lines of force emerge from the earth). The direction to the earth's geographic pole is called true north and the direction to the earth's magnetic pole is called magnetic north.

An expeditionby the Geological Survey of Canada in 1994 determined that the average position of the north magnetic pole for that year was 78.3° N, 104.0° W (near Ellef Ringnes Island in the Canadian Arctic). They also determined that the magnetic pole was moving approximately 15 km per year.

So, if you're using a compass somewhere on the surface of the earth, you have to account for this difference between magnetic north (where your compass is pointing) and true north (which you need to know). The angle between true north and magnetic north is called the magnetic declinationand changes with your location and, at any one location, with time.

How do we determine what the magnetic declination is?

There are some neat computer programswhich calculate the magnetic declination at a given latitude and longitude on the earth's surface. What most geologists do, however, is simply consult a recent topographic map for their field area available published by the United States Geologic Survey (and available in many sporting goods stores catering to hikers or hunters) or, if they're working outside of the United States, the USGS also publishes inexpensive mapsshowing the magnetic declination around the world.

On a typical topographic map, the magnetic declination is indicated as shown below:


The star indicates true north (toward the top of virtually all maps) and the MN indicates the direction to the north magnetic pole from the center of the map. In this case, the north magnetic pole is 15° west of true north. This means that your compass, unless you correct it, will point 15° west of true north.

There are simple ways to correct for the magnetic declination on most compasses. The Brunton compass, which will be discussed shortly, has an index pin at the north end of the compass ring (the ring around the face of the compass with the azimuths printed on it). When the compass is set for a 0° magnetic declination, the index pin is aligned with zero.



There is a brass setscrew on the side of the compass which moves the compass ring either clockwise or counter-clockwise. The only trick in correcting for magnetic declination is to remember which way to turn the compass ring for east and west declinations.

For a magnetic declination 15° east of true north, you would turn the compass ring such that the index pin was over 15° (i.e. 15° to the east side of north).


For a 15° west declination, you would turn the compass ring such that the index pin was over 345° (i.e. 15° to the west side of north).


Let's suppose, for example, that you've set your compass for a 15° east declination. You can check to see if the setting is correct by orienting your compass such that the white end of the needle is at 0°. Rotate the compass 15° east and if the white end of the needle is pointing in the same direction of the sighting arm for the compass you're in business.




 

posted by Geology on 07:32

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