Borehole roblems 43 4 Borehole roblems Horizontal Level Ground In order to determine the subsurface geology of an area, boreholes are sunk at convenient places in areas such as cultivated lands, forests deserts, alluvium, etc. The surface is completely covered and the outcrops are very few. Such boreholes reveal the presence of economic deposits of coal, petroleum, etc. The subsurface geological formations, rock types and their dip and strike can be determined from such borehole data, which render very valuable information for plans to exploit the hidden treasures. Boreholes Sunk on Horizontal Ground Example: 1. Three boreholes are sunk at three points of an equilateral triangle whose sides are 480 m each. Is West of Q and R is North of midpoint Q? Boreholes and R-reach the upper surface of a rich coal seam at,, and 260 m depths respectively. (a) Determine the attitude (Dip and strike) of the coal seam. (b) Another borehole is sunk at S, midpoint of QR. Determine at what depth the borehole S reaches the coal seam. Depth Diagram (Not to Scale) rocedure: Construct an equilateral triangle with a suitable scale. Show the positions of the boreholes. The coal seam is reached at point and Q at and. So the coal seam dips from to Q. To determine the inclination (gradient) along Q construct through sketch depth diagram and determine the gradient it is 120 m in 480 m. So it is 1 in 4. Similarly construct the depth diagram along R it is 160 m in 480 m i.e. 1 in 3. Take convenient scale and mark 4 units along Q and 3 units along R from. They are A and B. Join AB and extend. It is the true strike direction (TSD). Draw a perpendicular to AB from. It cuts AB at C. Measure C it is 2.85 i.e. the gradient is 1 in 2.85. It is true dip. Gradient of Q 120/480 = 1 in 4 Gradient of R 160/480 = 1 in 3
44 Applied Engineering Geology racticals Q R 260 m 120 m 1 in 4 1 in 3 160 m Fig. 4.1 To determine the direction of true dip, measure the angles CQ = 45. So direction of true dip is the complementary angle from North direction so (90 45 ) 45. So it is N45 E or NE. True dip 1 in 2.85 along NE. Strike = SE and NW. To determine the depth at which the borehole S reaches the coal seam, join S it intersects AB line (true strike direction) at T. Measure T with units selected it is 3 cm. So the gradient along T is 1 in 3. Measure S it is 4.2 cm = 420 m. Depth = Horizontal distance S Gradient + Depth of borehole at = 420 1/3 + 100 = 140 + 100 = 240 m To check whether this calculation is correct or not, let us find out the gradient of coal seam along QR. Draw depth diagram. The Gradient is 1 in 12 from Q, QS is 240 m. Q 480 m R 480 m 1 in 12 Fig. 4.2
Borehole roblems 45 Depth = (Horizontal distance Gradient) + Depth of Borehole Q = 240 1/12 + 220 = 20 + 220 = 240 m Scale 1 cm = Gradient 1 cm = 1 unit. R B S N C T 1 in 2.85 TSD A Q Fig. 4.3 2. Three boreholes are sunk at SW, SE, and NW corners of square level ground. The side of the square is 150 m long. The boreholes are X, Y, Z respectively. The boreholes meet the coal seam at 15 m, in X, 45 m in Y, and 60 m in Z. (a) Determine the attitude of the coal seam. (b) Fourth borehole is proposed at, the NE corner of the square land. Calculate at what depth, the borehole encounters the coal seam. Depth Diagram (Not to Scale) Y X Z 15 M 150 m 150 m 15 m 45 m 1 in 5 1 in 3 65 m 30 M 50 m Gradient of XY = 30/150 =1 in 5 Gradient of XZ = 50/150 = 1 in 3 Fig. 4.4
46 Applied Engineering Geology racticals Scale 1 cm = 30 m Gradient Scale 1cm = 1 unit Z A N 1 in 2.6 1 in 2.7 T 710 M TST X 150 metres Fig. 4.5 Y B True depth = 1 in 2.55 along North 30 E Strike = N60 W and S60 E Depth of unknown point = Horizontal distance Gradient + Depth of borehole at minimum depth = 216 1/2.7 + 15 = 80 + 15 = 95 m EXERCISE 1. Four boreholes are proposed at A, B, C, D at corners of a featureless square land. The sides of the square land are 360 m long. A is West of B and D is South of B. A coal seam is encountered in at 160 m, in B at 60 m and in D at 240 m. (a) Determine the attitude of the coal seam. (b) Another borehole is proposed at C. Calculate at what depth it reaches the coal seam. [Ans. (a) True dip 1 in 1.65 along S35 W, Strike S55 E and N 55 W; (b) Depth at borehole C = 360 m] 2. Three boreholes are sunk at the corners of an isosceles triangle. The base AB is East West 400 m. A is West of B, C borehole is 500 m from A and B and North of midpoint AB. The boreholes touch the oil-bearing stratum in A at 30 m, in B 80 m and in C 130 m. (a) Determine the attitude of the oil-bearing stratum. (b) Another borehole is proposed at midpoint of BC. Calculate at what depth the same oil bearing stratum is met. [Ans. (a) True dip 1 in 4.8 along N23 E, Strike N67 W and S67 E; (b) Depth at midpoint BC = 105 m] (Contd.)
Borehole roblems 47 3. Boreholes are sunk at the corners of a scalene triangle on a featureless ground. AB is 500 m, BC is 600 m and AC is 400 m. B is West of C and A is on the Northern side of BC. The boreholes A, B and C meet a coal seam at 150 m, 350 m and 250 m respectively. (a) Determine the true dip and strike of the coal seam. (b) A borehole is proposed on BC line due south of A. Calculate at what depth the same coal seam is met in the proposed borehole. [Ans. (a) True dip 1 in 2.25 along S23 W, Strike S67 E and N67 W; (b) Depth in proposed borehole 285 m] 4. Three boreholes are sunk at three points ABC of an equilateral triangle whose sides are 600 m each. B is West of C and A is North of midpoint of BC. Boreholes ABC reach the coal seam at 20 m, 50 m and 60 m depth respectively. Determine (a) The attitude of the coal seam. (b) Another borehole is at D at a distance of 1000 m from A. Determine at what depth the borehole D reaches the coal seam. [Ans. Depth in proposed borehole is 89 m]