Schmucker, 1970 (Scripps) - MTNet

Schmucker: Geomagnetic Variations 93
-6
T l' = 16 • 10 emu· cm (7.2a)
as mean total conductivity of the surface cover at sea and
-6
T L = 0.4 . 10 emu· cm (7.2b)
on land. These values correspond to 4000 m and 100 m of seawater, respectively.
The land value is also equivalent to 4000 m of rock formations with a
resistivity of 10 II • m, which seems to be a reasonable continental average.
Deep conductivities in the upper mantle are represented by a perfect substitute
conductor at the frequency-dependent depth h* as introduced in section
5.5, 6. The evaluation of the coastal anomaly led to estimates for h* in California
as listed in table 2 for various frequencies. They are compared with
corresponding values obtained from Lahiri and Price's model "d" and other
distributions in table 3.
7.2 Coastal Anomaly in California
This anomaly has been studied in great detail on land, but its continuation
toward the open ocean remained largely unknown. The illustrations in figures
40 to 45 summarize the results of the interpretation attempted here for
frequencies between 0.5 and 4.0 cph. The empirical zp and hp-profiles have
been obtained by projecting the transfer values between anomalous and normal
parts (tabs. 6 -10) upon the direction of the various profiles perpendicular
to the coast (eq. 3.26). Hence, zp represents the complex ratio Za/B where
B refers to the normal part of the horizontal disturbance vector which is perpendicular
to the coast. The presence of a minute normal part in Z has been
ignored throughout the data reduction (cf. sec. 3.11).
Details about the model calculations which led to the computed curves will
be. discussed separately in section 7.3. It may suffice to state that the
models are 2 -dimensional and that they consist of a 'thin surface cover of
variable total conductivity above nonconducting matter and a perfectly conducting
substratum at the depth h*. The total conductivity merges into T 1> at
sea and into T L on land (eq. 7.2). Near the coast T is chosen according to
the change in water depth and two inland anomalies have been added to account
for the effect of the San Joaquin valley and of the Sierra Nevada on the
distribution of superficial eddy currents.
We infer from the extreme edge model in figure 38 that the slope of the
zp-profiles from the coastal peak toward inland is determined by the depth to
the underlying perfect conductor. Hence, h* as the only free parameter of
the models is also a very critical one which can be adjusted by trial and
error to yield the best possible fit between the observed and the computed zpprofiles.
This has been done, and the resulting h * -values are listed in table
2 together with the corresponding Q-ratios of equation 6.5 in reference to the
relative strength of superficial eddy currents on land and at sea.