Magnetic data were postprocessed following the methodology:
1)Initial Processing in MagMap2000
-Download base files from INTERMAGNET observatory (
http://www.intermagnet.org/apps/dl_data_prel_e.php)
-Despike base files and visually inspect for dropouts
-Apply bias to DIURNAL (base) readings for GMT to local PDT + longitude diff. from base to survey = 7hrs + 0.22hrs =-24,408sec **NOTE: 0.0666667hr/deg longitude correction
-Import .survey files, despike
2)Import to Oasis
-Remove periodic sparker spike from raw G882 and G882_diurnal channel using a non-linear filter GX = nlfilt
(ex. filter width = 3 data points, filter tolerance = 0.3 nt)
3)Calculated IGRF for G882 data
4)Back calculated Base Magnetometer measurements
G882filt-DIURNAL_filt = Fresno_base
5)Calculated IGRF value at base station FRN = 48909.6nT
6)Calculated Total Field (Residual) Anomaly using the following equation:
A(x) = [F(x,t) - I(x)] - [FB(t) - IB]
where:
A(x)is total field anomaly at position x
F(x,t) is the G882 reading at time t and position x
I(x) is IGRF/DGRF at the G882
FB(t) is the base station reading at time t
IB is the IGRF/DGRF at the base station
7)Performed tie line leveling
8) Manually edited a subset of lines (+6 nT)
9) Applied a decorrugation filter "L" lines only (500m line spacing, line azimuth=30 degrees)
Following methodology at Geosoft -
http://www.geosoft.com/media/uploads/resources/technical-notes/microleveling%20tech%20note.pdf
FILTER PARAMETERS for "L" lines
Butterworth filter:
cutoff wavelength = 2000 (4 x line spacing)
filter order = 8 (suggested)
high-pass
Directional Cosine filter:
centre direction = 30 (shiptrack azimuth)
degree of cosine function = 0.5 (suggested)
