Standar Prosedur Pengambilan Data Seismik 2-D (Land)

Stations Receivers and Sources

Positioning nomenclature for 2D seismic surveying and processing is based on receiver (REC) stations (STN). Source (SOU) positions are relative to these and can have an offset and skid. Offset is the separation between the REC line and SOU line; this occurs when the receivers are placed to one side of the survey so that the source can move along the survey line without damaging any of the recording equipment. Skid is when the source is displaced along the survey line such that the shot point is positioned in between two receiver stations. Example; for a 4 m receiver separated survey using an Accelerated Weight Drop shooting between every receiver, the source will have a 1 m offset and 2 m skid.  This is graphically shown below in Figure 1.

The survey line should be Marked out as in Figure 2 with a 4 numeral STN number on each peg. The first numeral represents the Line number and the following 3 numbers are sequential starting with the first STN at the start of the line (SOL).

Figure 1. Schematic showing the offset of source and receiver lines and skid of sources with respect to the receivers

File nomenclature

File naming of shot records should be descriptive of where the shot was taken. As current survey design requires multiple spreads with repeated shot stations and reverse shooting; the following 8 character File Name is suggested;

 Area – first  two characters; (e.g. B1, B2)

Spread – third Character; (a, b, c, d…)

Forward/Reverse shooting – fourth character; (F or R)

Line and SHT number – last 4 characters (2008 = Line 2 shot 8)

e.g. B2aF3012.sg2 is shot 12, on line 3, spread a) Area B2, and shot in the Forward direction.

Seismic line preparation and marking

Positioning, marking and laying out of the seismic line should be in accordance with standard seismic station nomenclature defined above.

	Task	Procedure
1	Prepare survey line	Use GPS to Clearly identify and mark the start of the survey line (SOL). This is the from the first shot station not geophone Clear the survey line keeping this as straight as possible and along planned path. Any deviations from the planned line due to terrain should be smooth bends not sharp corners. Use intermediate flags to back shoot and guide the line Clearly mark the end of line (EOL)
2	Mark stations	Use a survey chain or rope with knots spaced 2 m apart to mark out 2 or 4 meter stations (STN). Suggest survey rope with 22 x 2 m markings Mark from STN 1 to 11, drag the chain through to STN 11 and repeat for STN 11 to 21, STN 21 to 31 … STN markers should be placed to the side of the survey line so they are not knocked over or need to be moved when progressing up and down the line. The markers should be on the high side of the survey path if working on a slope. At least every second STN should be labelled with the STN number.
3	Prepare geophone positions	The first Geophone STN should be identified (STN 1013 if using standard half spread shot offset) and a hole dug at the survey marker position. This hole should be sufficiently deep enough to remove the spongy soil and root mass so the geophone can be placed into consolidated soil. Replace the survey marker after the hole has been dug. Remember this hole is to be dug to the side of the survey path so geophones and survey markers are not in the way when moving the Accelerated Weight Drop (AWD). **The geophones and receiver lines can now be laid out whilst a team prepares the shot pads**
4	Prepare shot pads	Shot pads are to be prepared half way between each survey marker. The pads are to be dug deep enough to remove the top spongy soil and root mass. The ground needs to be level and sufficiently large enough to accommodate the AWD tripod.
5	Lay receiver line	Geophones should be placed at all STN for the first spread (STN 1013 to 1036) Geophones need to be vertical By hand, push the geophone vertically into the soil then carefully with the heel of the boot press firmly into the ground. Geophones and cables should be placed to the side of the track out of the way of the AWD. Geophones should be “planted” so that the top of the geophone is level with the surface of the ground or deeper to restrict wind noise. Lay receiver cable to the side of the survey path clear of the shot pads and connect the geophones.
6	CHECK STN MARKERS	Prior to the start of data collection, the GEOPHYSICIST and / or DANMARE linesman should check all survey markers, geophones and shot STN are correct for each shooting pattern (spread). The survey set up should then be described and recorded on the field data sheet e.g. LINE 1, 24 channels, 4 m spacing, FAS = 13, LAS = 36, FSS = 0.5, LSS = 48.5.
7	Total Station surveying	All geophone stations and shot locations need to be accurately surveyed due to the extreme topographical variations. It is essential that once a geophone is planted and data has been recorded, that it and its corresponding STN Marker are not moved at any time during the survey. The STN Marker is to remain at the geophone location until the end of the seismic survey and its position has been surveyed with either a Total Station or DGPS with high elevation accuracy. Geophone STN should be recorded as e.g. REC1013 to REC1096 Shot STN should be recorded as e.g. SOU1001 to SOU1108 (EOL)

Equipment recommendations

MU recommendations;

1. Deployment of a larger number of channels to achieve greater offsets as higher offsets have better S/N ratio
2. Acquire new cables (water proof) and more geophones.
3. Extra cables and geophones for deploying at least one spread ahead of acquisition.

Geophones and takeout cables with water proof screw thread connectors (Photo 1) can be manufactured from an Indian cable supplier (http://www).  An accurate wiring diagram and parts list will need to be drawn up to commission the work.

As the GEODE is a modular system, any number of GEODE 24 seismographs can be added to extend the channel count. Recommend to expand the current system immediately to 48 and up to 96 channels in the future. Currently available second hand, are two GEODE 24 seismographs.

Current equipment modification suggestions;

• Accelerated Weight Drop (AWD) pull release rope needs to be replaced. Currently very thin (but strong) and digs into operators hand during pulling. Could have simple garden hose or some handle introduced.
• AWD is difficult to manoeuvre/manhandle from place to place especially in steep wet terrain. Suggestions;
  • Wrap legs and mast with high density foam to pad shoulders whilst carrying
  • More crew
  • Carry handles
• AWD - wire rope got caught 3-4 times between the pulley wheel and guard - the cable is now kinked and needs re-termination or be replaced with an alternative.
• Adjustable legs need to have split pins replaced so legs can be adjusted in the field and lock nuts need to be used on the AWD to prevent self dismantling.
• Striker plate needs a handle so it can be pulled out of the mud and moved easily.
• Field crew need a good set of cutters and multi-meter (one which can test capacitance; nano farads) at the trigger.
• Connections at trigger extension cable and trigger (including spares) should be a water proof (waterproof screw type) set up for only one possible connection polarity. This would allow quick testing and changing of triggers with little/no training.
• Trigger extension break out box should also have the connectors which can only be connected in one way to protect polarity (waterproof screw type). Immediately the –ve polarity banana socket (currently coloured with faded permanent marker pen) should be replaced with the correct black socket.
• Although the GEODE is waterproof a waterproof container (e.g. modified ESKIE) to protect water ingress into the external connectors could be made.

Suggestion from Reddy to make new weight drop and employ two on the survey, shooting in from both sides; Shoot 1, shoot 2 and then move both. This would definitely be useful on spreads with larger channel counts. Also allows continuous shooting if there are any issues with an AWD.

Field Practice and Quality Control Procedures

 Seismic operator, at least one weight drop operator and one linesman should be maintained as permanent crew members. They will provide training to any new crew member.

1. 1. Geophysicist is to QA/QC during stacking and shot moves and put SHOOTING FIRST. If there are only limited issues with the final stack (e.g. 1 or 2 reversed geophones, poor coupling of 1 or 2 geophones, low SNR ...) the problems should be fixed during the moving of the AWD with co-ordination with the linesman and by increasing stacks at the next position. If the problem reoccurs in the next shot record, the AWD should stand down until the problem is resolved and a new stack acquired.
   2. Linesman should be responsible for ensuring correct; marking of the line, shot stations are correct and correct any problems with the geophones (poor coupling, reversed channels ...) during recording.

• AWD operator to be responsible for safe operation and moving of AWD, rearming the AWD immediately after each stack and moving the AWD in a timely manner. 48 shot moves per day should be achievable.

2. Re-arm the AWD almost immediately (5 seconds) after the shot is acquired. Saving 10 - 30 seconds each stack can considerably increase production.
3. During the period of rearming the AWD the Geophysicist is to examine the stack and confirm its quality. If necessary (excessive noise, miss-fire ...) then the data should be saved and a new record collected. Saving and clearing data half way through stacking is better than at the end of the stack. Keep all data and document shots which have been shot twice.
4. If the operator is uncertain of the signal strength (depth of penetration) it is necessary to conduct tests before production or possibly even during line production if the ground conditions change considerably. Tests should include stacks for 4, 6, 8, and 12 stacks.
   1. Program the GEODE to record 12 stacks and show every stack;
   2. Carefully monitor the improvement in the shot record as stacks are required;

• At some point visually there will be no improvement in the shot record as more stacks are acquired. This is the minimum stack count which should be used.

5. First weight drop thump should not be recorded as it is used only to compress the ground (inelastic effect). The crew members operating AWD should do that as soon as the AWD is erected. When the AWD is mounted and ready for the first shot the operator should be notified immediately.
6. While recording is in progress, two or more people are preparing the shot pads (remove the soft layer until firm flat ground is obtained), marking out and preparing the next geophone spread (removing soft root and soil mass) to avoid any delay.
7. Have at least one full spread positioned ahead of the active spread. That will enable continuous recording rather than current lengthy breaks in acquisition.
8. Geophones need to be;
   1. Planted vertically;
   2. Pressed all the way into the ground with the heal of a boot until the geophone is flush with ground surface;

• Check geophone coupling by gently pulling on the geophone string or trying to move geophone by hand left and right. If it comes out, re-plant it in a better place.

9. Positioning errors have to be kept below 1/8 of the geophone or shot separation.

Four Spread shooting and stack tables.

 The minimum number of stacks at different offsets should be maintained as per the tables below. In poor SNR areas stacks should be increased as per the QC procedure.

FILE can be 1001, 1002... if the GEODE does not allow Alpha-numeric file masks (e.g. B2aF)

table 1. Shooting and stack tables for spreads 1 and 2 forward then reverse shooting

SPREAD 1: FAS 13, LAS 36, FSS 1001, LSS1048				SPREAD 2: FAS 25, LAS 48, FSS 1013, LSS1060
STN	SHOT #	FILE	STACKS	STN	SHOT #	FILE	STACKS
1	1	B2aF1001	8	60	49	B2bR1049	8
2	2	B2aF1002	8	59	50	B2bR1050	8
3	3	B2aF1003	8	58	51	B2bR1051	8
4	4	B2aF1004	8	57	52	B2bR1052	8
5	5	B2aF1005	8	56	53	B2bR1053	8
6	6	B2aF1006	7	55	54	B2bR1054	7
7	7	B2aF1007	7	54	55	B2bR1055	7
8	8	B2aF1008	7	53	56	B2bR1056	7
9	9	B2aF1009	7	52	57	B2bR1057	7
10	10	B2aF1010	7	51	58	B2bR1058	7
11	11	B2aF1011	6	50	59	B2bR1059	6
12	12	B2aF1012	6	49	60	B2bR1060	6
13	13	B2aF1013	6	48	61	B2bR1061	6
14	14	B2aF1014	6	47	62	B2bR1062	6
15	15	B2aF1015	6	46	63	B2bR1063	6
16	16	B2aF1016	5	45	64	B2bR1064	5
17	17	B2aF1017	5	44	65	B2bR1065	5
18	18	B2aF1018	5	43	66	B2bR1066	5
19	19	B2aF1019	5	42	67	B2bR1067	5
20	20	B2aF1020	5	41	68	B2bR1068	5
21	21	B2aF1021	5	40	69	B2bR1069	4
22	22	B2aF1022	5	39	70	B2bR1070	4
23	23	B2aF1023	5	38	71	B2bR1071	4
24	24	B2aF1024	5	37	72	B2bR1072	4
25	25	B2aF1025	5	36	73	B2bR1073	4
26	26	B2aF1026	5	35	74	B2bR1074	4
27	27	B2aF1027	5	34	75	B2bR1075	4
28	28	B2aF1028	5	33	76	B2bR1076	4
29	29	B2aF1029	5	32	77	B2bR1077	5
30	30	B2aF1030	5	31	78	B2bR1078	5
31	31	B2aF1031	5	30	79	B2bR1079	5
32	32	B2aF1032	5	29	80	B2bR1080	5
33	33	B2aF1033	5	28	81	B2bR1081	5
34	34	B2aF1034	6	27	82	B2bR1082	6
35	35	B2aF1035	6	26	83	B2bR1083	6
36	36	B2aF1036	6	25	84	B2bR1084	6
37	37	B2aF1037	6	24	85	B2bR1085	6
38	38	B2aF1038	6	23	86	B2bR1086	6
39	39	B2aF1039	7	22	87	B2bR1087	7
40	40	B2aF1040	7	21	88	B2bR1088	7
41	41	B2aF1041	7	20	89	B2bR1089	7
42	42	B2aF1042	7	19	90	B2bR1090	7
43	43	B2aF1043	7	18	91	B2bR1091	7
44	44	B2aF1044	8	17	92	B2bR1092	8
45	45	B2aF1045	8	16	93	B2bR1093	8
46	46	B2aF1046	8	15	94	B2bR1094	8
47	47	B2aF1047	8	14	95	B2bR1095	8
48	48	B2aF1048	8	13	96	B2bR1096	8

Tables 2. Shooting and Stack tables for Spreads 3 and 4 forward then reverse shooting

SPREAD 3: FAS 37, LAS 60, FSS 1025, LSS1072				SPREAD 4: FAS 49, LAS 72, FSS 1037, LSS1084
STN	SHOT #	FILE	STACKS	STN	SHOT #	FILE	STACKS
25	97	B2aF1097	8	84	145	B2bR1145	8
26	98	B2aF1098	8	83	146	B2bR1146	8
27	99	B2aF1099	8	82	147	B2bR1147	8
28	100	B2aF1100	8	81	148	B2bR1148	8
29	101	B2aF1101	8	80	149	B2bR1149	8
30	102	B2aF1102	7	79	150	B2bR1150	7
31	103	B2aF1103	7	78	151	B2bR1151	7
32	104	B2aF1104	7	77	152	B2bR1152	7
33	105	B2aF1105	7	76	153	B2bR1153	7
34	106	B2aF1106	7	75	154	B2bR1154	7
35	107	B2aF1107	6	74	155	B2bR1155	6
36	108	B2aF1108	6	73	156	B2bR1156	6
37	109	B2aF1109	6	72	157	B2bR1157	6
38	110	B2aF1110	6	71	158	B2bR1158	6
39	111	B2aF1111	6	70	159	B2bR1159	6
40	112	B2aF1112	5	69	160	B2bR1160	5
41	113	B2aF1113	5	68	161	B2bR1161	5
42	114	B2aF1114	5	67	162	B2bR1162	5
43	115	B2aF1115	5	66	163	B2bR1163	5
44	116	B2aF1116	5	65	164	B2bR1164	5
45	117	B2aF1117	4	64	165	B2bR1165	4
46	118	B2aF1118	4	63	166	B2bR1166	4
47	119	B2aF1119	4	62	167	B2bR1167	4
48	120	B2aF1120	4	61	168	B2bR1168	4
49	121	B2aF1121	4	60	169	B2bR1169	4
50	122	B2aF1122	4	59	170	B2bR1170	4
51	123	B2aF1123	4	58	171	B2bR1171	4
52	124	B2aF1124	4	57	172	B2bR1172	4
53	125	B2aF1125	5	56	173	B2bR1173	5
54	126	B2aF1126	5	55	174	B2bR1174	5
55	127	B2aF1127	5	54	175	B2bR1175	5
56	128	B2aF1128	5	53	176	B2bR1176	5
57	129	B2aF1129	5	52	177	B2bR1177	5
58	130	B2aF1130	6	51	178	B2bR1178	6
59	131	B2aF1131	6	50	179	B2bR1179	6
60	132	B2aF1132	6	49	180	B2bR1180	6
61	133	B2aF1133	6	48	181	B2bR1181	6
62	134	B2aF1134	6	47	182	B2bR1182	6
63	135	B2aF1135	7	46	183	B2bR1183	7
64	136	B2aF1136	7	45	184	B2bR1184	7
65	137	B2aF1137	7	44	185	B2bR1185	7
66	138	B2aF1138	7	43	186	B2bR1186	7
67	139	B2aF1139	7	42	187	B2bR1187	7
68	140	B2aF1140	8	41	188	B2bR1188	8
69	141	B2aF1141	8	40	189	B2bR1189	8
70	142	B2aF1142	8	39	190	B2bR1190	8
71	143	B2aF1143	8	38	191	B2bR1191	8
72	144	B2aF1144	8	37	192	B2bR1192	8

Observations and daily log

The field observer’s notes must represent everything which went on in the field so issues with data, equipment and procedures can be identified. Observations of the following need to be made:

1. Time on site and off site
2. Crew numbers and progress rates.
3. All data parameters including the location of files
4. Quick sketch of spread layout including topography with respect to STN numbers FAS, LAS, FSS and LSS.
5. Numbers of stacks and relative noise sources
6. Problems /delays caused by equipment e.g. Trigger or AWD issues.