================================================================================ LSS BACKGROUND CORRECTION — AAR Generated Using: select_background_casts.py ntu_background_correction.py ================================================================================ Methodology The relative measure of light backscattering above ambient, dNTU, was calculated in two steps: (1) Background profile construction - a depth-dependent background LSS(v) profile was constructed using five low level "background" casts that were determined using a scoring method detailed below. Said background casts were binned into 10 m depth intervals, smoothed with a 50 m centered rolling mean, and interpolated to the exact depth of each observation. (2) dNTU calculations - Background profile value were subtracted from each raw CTDLSS measurement at the corresponding depth (dNTU = CTD_LSS − background_LSS(depth)) STEP 1 - BACKGROUND CAST SELECTION AND PROFILE CONSTRUTION ================================================================================ SCORING METHOD -------------- Every cast with >= 50 observations below 500 m was scored using deep-water CTDLSS statistics: score = LSS_mean + LSS_std + 0.3 * LSS_max LSS_mean : The average deep-water scattering. A high mean suggests the cast sampled water that is persistently particle-rich LSS_std : Cast-to-cast variability. High standard deviation indicates the cast passed through layers of increased particle load. A noisy cast with a high std produces an unreliable background estimate. LSS_max : The peak value encountered during the cast. Weighted at 0.3 (rather than 1.0) so that a single brief spike — from a passing biological layer or instrument noise — does not unfairly discard an otherwise clean cast. The score is the sum of these three terms. Lower score = cleaner cast. SELECTION CRITERIA ------------------ Auto-selected : top 4 lowest-scoring casts across the full dataset Force-included: Station 7 (all casts) — this is the deepest profile in the survey, reaching 3215 m. It anchors the background profile at the greatest depths. Without it, the background profile would be unconstrained below the shallowest background cast's maximum depth, producing NaN delta_NTU values in the target depth range. SELECTED CASTS (5 total — stations [7, 10, 47, 86, 123]) ---------------------------------------------------------------------------- stn_cast Station Lat Lon MaxDep(m) LSS_mean LSS_std LSS_max Score Note S10C1 10 -62.9960 157.0030 2343 0.00642 0.00132 0.01595 0.01252 auto-selected S123C1 123 -62.4928 157.2640 1564 0.00681 0.00115 0.01782 0.01331 auto-selected S47C1 47 -63.0003 162.9990 2002 0.00624 0.00124 0.01996 0.01347 auto-selected S86C1 86 -62.6600 160.0030 2002 0.00672 0.00176 0.01663 0.01347 auto-selected S7C1 7 -62.3310 155.4970 3216 0.00649 0.00227 0.04229 0.02144 FORCED — Station 7 (deepest cast) BACKGROUND PROFILE CONSTRUCTION -------------------------------- Input : background_casts.csv — 5 casts from stations [7, 10, 47, 86, 123] Step 1 — Depth binning Raw CTD data (~1 m resolution) is binned into 10 m depth bins. For each bin, the mean CTDLSS is computed across all observations from all background casts. Bins with fewer than 5 observations are discarded as statistically unreliable. Step 2 — Smoothing A 5-bin centered rolling mean is applied to the binned profile, equivalent to a 50 m moving average. This reduces bin-to-bin noise while preserving the broad depth structure of the background. min_periods=1 ensures edge bins are not dropped. Step 3 — Interpolation The smoothed 10 m mean profile is linearly interpolated to the exact depth of every observation in the full dataset. No extrapolation is performed — observations shallower than 5 m or deeper than 3215 m receive NaN. ================================================================================ PART 2 — dNTU CALCULATION ================================================================================ Applying the background LSS depth profile (derived in Part 1) to the full CTD dataset to compute a background-corrected scattering anomaly: dNTU = CTDLSS - background_LSS(depth) NOTE: Negative raw anomalies (CTDLSS < background) are clipped to 0. These below-ambient values reflect natural variability around the background mean and carry no hydrothermal significance. Retaining them as negatives would complicate interpretation and visualization without adding scientific value. ================================================================================ ALL CASTS RANKED BY SCORE (lowest = cleanest) Deep water > 500 m | >= 50 obs required | 154 casts scored Used_as_BG = YES if cast is in the selected background ================================================================================ Rank stn_cast Station MaxDep(m) LSS_mean LSS_std LSS_max LSS_p95 Score Used_as_BG 1 S10C1 10 2343 0.00642 0.00132 0.01595 0.00863 0.01252 YES 2 S123C1 123 1564 0.00681 0.00115 0.01782 0.00857 0.01331 YES 3 S47C1 47 2002 0.00624 0.00124 0.01996 0.00811 0.01347 YES 4 S86C1 86 2002 0.00672 0.00176 0.01663 0.00987 0.01347 YES 5 S87C1 87 2002 0.00605 0.00117 0.02098 0.00795 0.01351 6 S29C1 29 2001 0.00838 0.00171 0.01679 0.01121 0.01513 7 S138C1 138 2002 0.00667 0.00121 0.02456 0.00847 0.01524 8 S85C1 85 2003 0.00663 0.00185 0.02373 0.00948 0.01560 9 S42C2 42 1365 0.00698 0.00183 0.02336 0.00973 0.01582 10 S118C1 118 1198 0.00821 0.00169 0.01984 0.01140 0.01586 11 S88C1 88 2002 0.00651 0.00139 0.02767 0.00871 0.01620 12 S44C1 44 1850 0.00709 0.00211 0.02419 0.01059 0.01646 13 S113C1 113 1009 0.00863 0.00150 0.02118 0.01082 0.01649 14 S109C1 109 1001 0.00947 0.00100 0.02028 0.01066 0.01655 15 S40C1 40 706 0.00945 0.00172 0.01913 0.01242 0.01691 16 S122C1 122 681 0.00883 0.00148 0.02227 0.01075 0.01700 17 S46C1 46 2001 0.00808 0.00171 0.02566 0.01110 0.01748 18 S28C2 28 2002 0.00792 0.00195 0.02658 0.01121 0.01785 19 S126C1 126 1001 0.01035 0.00159 0.02103 0.01296 0.01826 20 S43C1 43 1481 0.00694 0.00189 0.03190 0.00987 0.01840 21 S119C1 119 1032 0.00851 0.00169 0.02756 0.01129 0.01847 22 S115C1 115 1013 0.00860 0.00200 0.02830 0.01124 0.01908 23 S13C1 13 851 0.01036 0.00199 0.02246 0.01433 0.01909 24 S39C1 39 1527 0.00761 0.00228 0.03220 0.01091 0.01955 25 S106C1 106 1002 0.01055 0.00213 0.02755 0.01424 0.02094 26 S121C1 121 958 0.00858 0.00198 0.03502 0.01066 0.02107 27 S24C1 24 1461 0.00901 0.00226 0.03341 0.01328 0.02130 28 S100C2 100 2001 0.00906 0.00254 0.03245 0.01344 0.02133 29 S11C3 11 1654 0.00918 0.00217 0.03345 0.01299 0.02139 30 S7C1 7 3216 0.00649 0.00227 0.04229 0.01078 0.02144 YES 31 S112C1 112 1076 0.01051 0.00160 0.03169 0.01247 0.02162 32 S114C1 114 580 0.01257 0.00224 0.02574 0.01602 0.02254 33 S48C1 48 2002 0.00711 0.00261 0.04342 0.01087 0.02274 34 S89C3 89 2052 0.00987 0.00254 0.03516 0.01342 0.02296 35 S72C1 72 697 0.01225 0.00241 0.02775 0.01658 0.02299 36 S49C1 49 2002 0.00726 0.00273 0.04344 0.01091 0.02302 37 S69C3 69 2002 0.00959 0.00271 0.03717 0.01326 0.02345 38 S27C1 27 1461 0.00911 0.00261 0.04024 0.01318 0.02379 39 S143C1 143 2368 0.00733 0.00221 0.05047 0.01058 0.02469 40 S111C1 111 684 0.01237 0.00231 0.03426 0.01471 0.02495 41 S117C1 117 584 0.01283 0.00307 0.03220 0.01785 0.02556 42 S73C2 73 1061 0.01068 0.00282 0.04035 0.01483 0.02561 43 S98C3 98 2002 0.00905 0.00335 0.04403 0.01382 0.02561 44 S52C1 52 2003 0.00904 0.00335 0.04413 0.01434 0.02563 45 S84C1 84 2002 0.00648 0.00218 0.05661 0.00942 0.02564 46 S96C2 96 834 0.01270 0.00268 0.03425 0.01706 0.02565 47 S14C1 14 1106 0.00984 0.00286 0.04409 0.01465 0.02593 48 S61C1 61 2001 0.00940 0.00220 0.05344 0.01177 0.02764 49 S70C1 70 2001 0.00913 0.00333 0.05090 0.01363 0.02774 50 S23C1 23 1487 0.00921 0.00286 0.05348 0.01345 0.02811 51 S133C1 133 2002 0.01232 0.00367 0.04152 0.01909 0.02844 52 S129C1 129 1846 0.01112 0.00300 0.05183 0.01545 0.02967 53 S108C1 108 1001 0.01110 0.00281 0.05311 0.01527 0.02984 54 S137C1 137 2187 0.00711 0.00202 0.07028 0.00920 0.03021 55 S6C2 6 2003 0.01025 0.00365 0.05482 0.01642 0.03035 56 S12C2 12 1510 0.00986 0.00283 0.06513 0.01396 0.03223 57 S97C2 97 1187 0.01110 0.00337 0.05955 0.01533 0.03234 58 S101C3 101 2002 0.00963 0.00385 0.06436 0.01473 0.03279 59 S18C1 18 1655 0.01205 0.00376 0.05898 0.01764 0.03350 60 S37C1 37 2003 0.01077 0.00436 0.06147 0.01769 0.03357 61 S50C1 50 1269 0.00887 0.00365 0.07076 0.01259 0.03375 62 S110C1 110 1001 0.00986 0.00287 0.07072 0.01118 0.03395 63 S75C1 75 2003 0.01107 0.00296 0.06937 0.01504 0.03484 64 S8C1 8 2001 0.00680 0.00266 0.08487 0.00988 0.03492 65 S4C1 4 2662 0.01015 0.00543 0.06464 0.01955 0.03497 66 S116C1 116 954 0.01207 0.00422 0.06282 0.01638 0.03514 67 S68C2 68 1423 0.00998 0.00359 0.07313 0.01385 0.03551 68 S30C2 30 2002 0.00889 0.00394 0.07599 0.01478 0.03562 69 S25C1 25 1208 0.00960 0.00391 0.07607 0.01330 0.03633 70 S103C1 103 2001 0.01097 0.00347 0.07423 0.01485 0.03671 71 S21C1 21 2001 0.00844 0.00287 0.08501 0.01124 0.03681 72 S99C2 99 2002 0.00871 0.00306 0.08356 0.01249 0.03684 73 S128C1 128 2033 0.01030 0.00309 0.08198 0.01397 0.03798 74 S71C1 71 1995 0.00995 0.00434 0.07937 0.01388 0.03810 75 S144C1 144 2003 0.01033 0.00373 0.08163 0.01525 0.03855 76 S103C3 103 2002 0.01179 0.00527 0.07260 0.01859 0.03884 77 S5C4 5 1003 0.01546 0.00691 0.05850 0.02836 0.03992 78 S67C2 67 2001 0.00978 0.00473 0.08623 0.01557 0.04038 79 S77C1 77 2502 0.01330 0.00468 0.07631 0.02056 0.04088 80 S54C1 54 1978 0.01099 0.00498 0.08445 0.01731 0.04131 81 S2C3 2 2504 0.01090 0.00489 0.08594 0.01785 0.04158 82 S107C1 107 1002 0.01096 0.00445 0.09025 0.01467 0.04249 83 S22C2 22 1851 0.00977 0.00414 0.10781 0.01500 0.04625 84 S9C2 9 2001 0.00904 0.00351 0.11264 0.01239 0.04634 85 S38C1 38 2000 0.00746 0.00492 0.11342 0.01045 0.04641 86 S66C2 66 2001 0.01030 0.00564 0.10461 0.01693 0.04732 87 S145C1 145 2312 0.01492 0.01432 0.06124 0.04974 0.04761 88 S33C1 33 1986 0.00852 0.00356 0.12371 0.01209 0.04920 89 S124C1 124 2067 0.02135 0.01024 0.06173 0.03926 0.05010 90 S83C3 83 1210 0.00759 0.00659 0.12295 0.00939 0.05107 91 S142C1 142 2262 0.02233 0.01285 0.05534 0.04952 0.05178 92 S94C2 94 2001 0.00929 0.00468 0.12612 0.01382 0.05180 93 S15C2 15 2002 0.01130 0.00602 0.11565 0.01961 0.05201 94 S31C2 31 2002 0.00834 0.00509 0.13006 0.01224 0.05245 95 S5C3 5 1004 0.01535 0.00799 0.09875 0.02610 0.05297 96 S42C1 42 1156 0.01016 0.00540 0.13196 0.01464 0.05515 97 S140C1 140 2315 0.02121 0.01215 0.08153 0.04333 0.05782 98 S5C1 5 2183 0.00903 0.00622 0.14264 0.01586 0.05805 99 S53C1 53 2002 0.01074 0.00631 0.13950 0.01676 0.05890 100 S95C2 95 1540 0.01094 0.00747 0.13538 0.01745 0.05902 101 S136C1 136 2062 0.00770 0.00449 0.15736 0.01249 0.05940 102 S139C1 139 2103 0.02333 0.01676 0.06548 0.04981 0.05974 103 S81C2 81 2001 0.01985 0.00904 0.10326 0.03614 0.05987 104 S60C1 60 2001 0.01034 0.00725 0.14201 0.01580 0.06020 105 S132C1 132 2074 0.02548 0.01527 0.06854 0.05167 0.06131 106 S35C2 35 2000 0.01361 0.00871 0.13381 0.02394 0.06246 107 S59C1 59 2002 0.01172 0.00802 0.14361 0.01985 0.06282 108 S64C1 64 2001 0.01237 0.00719 0.14561 0.02122 0.06324 109 S32C1 32 2002 0.00898 0.00536 0.16999 0.01442 0.06534 110 S105C2 105 2002 0.01440 0.00723 0.14596 0.02243 0.06542 111 S51C1 51 2001 0.00872 0.00545 0.17279 0.01332 0.06601 112 S135C1 135 2086 0.01852 0.02078 0.08922 0.07689 0.06607 113 S146C1 146 2407 0.00858 0.00577 0.17373 0.01797 0.06647 114 S19C2 19 2002 0.01287 0.00944 0.15193 0.02284 0.06789 115 S16C2 16 1966 0.01277 0.00763 0.15844 0.02122 0.06793 116 S124C2 124 2064 0.03181 0.01427 0.07292 0.05928 0.06796 117 S92C3 92 2001 0.01434 0.00863 0.15133 0.02366 0.06837 118 S41C1 41 790 0.01129 0.01031 0.16347 0.02164 0.07064 119 S45C1 45 800 0.00923 0.01008 0.17645 0.01255 0.07224 120 S91C1 91 2002 0.01075 0.00701 0.18589 0.01733 0.07353 121 S130C1 130 2176 0.02549 0.02019 0.10458 0.07827 0.07705 122 S134C1 134 2072 0.02785 0.02487 0.08207 0.07249 0.07734 123 S20C2 20 2001 0.01231 0.01043 0.18346 0.02180 0.07778 124 S93C2 93 2002 0.01144 0.00821 0.20426 0.01829 0.08093 125 S82C3 82 1300 0.03671 0.02386 0.07277 0.06697 0.08240 126 S34C1 34 2001 0.01343 0.00879 0.20712 0.02269 0.08435 127 S55C2 55 1876 0.01604 0.01109 0.19320 0.02979 0.08509 128 S90C1 90 2001 0.01262 0.00960 0.21193 0.01986 0.08580 129 S74C1 74 1141 0.01695 0.01078 0.19401 0.02755 0.08594 130 S76C1 76 2702 0.01394 0.00765 0.21872 0.02076 0.08721 131 S58C1 58 1546 0.01369 0.01060 0.21219 0.02298 0.08795 132 S141C1 141 2090 0.04520 0.01824 0.08728 0.06572 0.08963 133 S125C1 125 2101 0.04605 0.01888 0.08316 0.06882 0.08988 134 S36C1 36 2001 0.01396 0.01118 0.22035 0.02587 0.09125 135 S120C1 120 982 0.00892 0.01076 0.24369 0.01037 0.09279 136 S82C2 82 1846 0.03158 0.02857 0.14032 0.08348 0.10225 137 S17C2 17 2003 0.01190 0.00953 0.27406 0.01978 0.10365 138 S63C1 63 2002 0.01389 0.01198 0.28417 0.02129 0.11113 139 S57C1 57 2003 0.01293 0.01253 0.30002 0.02265 0.11547 140 S125C5 125 2101 0.05903 0.02547 0.10872 0.08637 0.11712 141 S55C3 55 1851 0.01587 0.01520 0.33189 0.02890 0.13064 142 S3C1 3 2186 0.01068 0.01280 0.35901 0.01919 0.13118 143 S79C2 79 2002 0.01466 0.01660 0.34723 0.02572 0.13543 144 S82C4 82 1840 0.03086 0.02972 0.25298 0.08362 0.13647 145 S104C3 104 2001 0.01280 0.01425 0.37784 0.02101 0.14041 146 S65C2 65 2001 0.01016 0.01477 0.42551 0.01485 0.15259 147 S62C1 62 2001 0.01578 0.01320 0.43515 0.02498 0.15953 148 S131C1 131 2102 0.06614 0.02357 0.24651 0.08773 0.16367 149 S80C3 80 2001 0.01605 0.02316 0.48266 0.02791 0.18401 150 S56C1 56 2002 0.01478 0.02183 0.51728 0.02498 0.19179 151 S127C1 127 1730 0.02216 0.03537 0.48571 0.05268 0.20324 152 S78C2 78 1760 0.04013 0.13074 1.73765 0.08461 0.69216 153 S102C2 102 1325 0.01506 0.10520 3.04828 0.01667 1.03474 154 S26C1 26 960 0.19550 0.48063 2.49606 1.40023 1.42495 ================================================================================