Document:

EXHIBIT 4.2

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                            W. R. BERKLEY CORPORATION
                                       TO

                        THE BANK OF NEW YORK, as Trustee

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                         FIRST SUPPLEMENTAL INDENTURE TO
                        INDENTURE DATED FEBRUARY 14, 2003
                            (SENIOR DEBT SECURITIES)

                          Dated as of February 14, 2003

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                          5.875% Senior Notes due 2013

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                                TABLE OF CONTENTS

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                                                                            ----

                                    ARTICLE I

                       Relation to Indenture; Definitions

Section 1.1.   RELATION TO INDENTURE...........................................1
Section 1.2.   DEFINITIONS.....................................................1

                                ARTICLE II

                         The Series of Securities

Section 2.1.   TITLE OF THE SECURITIES.........................................2
Section 2.2.   LIMITATION ON AGGREGATE PRINCIPAL AMOUNT........................2
Section 2.3.   PRINCIPAL PAYMENT DATE..........................................2
Section 2.4.   INTEREST AND INTEREST RATES.....................................2
Section 2.5.   PLACE OF PAYMENT................................................3
Section 2.6.   REDEMPTION......................................................3
Section 2.7.   DENOMINATION....................................................5
Section 2.8.   CURRENCY........................................................5
Section 2.9.   FORM OF NOTES...................................................5
Section 2.10.  REGISTRAR AND PAYING AGENT FOR THE NOTES........................5
Section 2.11.  SINKING FUND OBLIGATIONS........................................5
Section 2.12.  DEFEASANCE AND COVENANT DEFEASANCE..............................5
Section 2.13.  PAYMENT OF TAXES................................................5
Section 2.14.  LIMITATION ON LIENS ON STOCK OF PRINCIPAL SUBSIDIARIES..........5
Section 2.15.  LIMITATIONS ON ISSUE OR DISPOSITION OF COMMON STOCK OF
               PRINCIPAL SUBSIDIARIES..........................................6
Section 2.16.  IMMEDIATELY AVAILABLE FUNDS.....................................6

                                   ARTICLE III

                            Miscellaneous Provisions

Section 3.1.   TRUSTEE NOT RESPONSIBLE FOR RECITALS............................6
Section 3.2.   PAYMENT OF EXPENSES UPON RESIGNATION OR REMOVAL.................7
Section 3.3.   ADOPTION, RATIFICATION AND CONFIRMATION.........................7
Section 3.4.   COUNTERPARTS....................................................7
Section 3.5.   GOVERNING LAW...................................................7

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                            W. R. BERKLEY CORPORATION

                         FIRST SUPPLEMENTAL INDENTURE TO
                        INDENTURE DATED FEBRUARY 14, 2003
                            (SENIOR DEBT SECURITIES)

                                  $200,000,000

                          5.875% Senior Notes due 2013

     FIRST SUPPLEMENTAL INDENTURE, dated as of February 14, 2003 between W. R.
BERKLEY CORPORATION, a Delaware corporation (the "Company"), and THE BANK OF NEW
YORK, a trust company organized under the laws of the State of New York, as
Trustee (the "Trustee").

                                    RECITALS

     The Company has heretofore executed and delivered to the Trustee an
indenture for senior debt securities, dated as of February 14, 2003 (the
"Indenture"), providing for the issuance from time to time of series of the
Company's Securities.

     Section 3.1 of the Indenture provides for various matters with respect to
any series of Securities issued under the Indenture to be established in an
indenture supplemental to the Indenture.

     Section 9.1(4) of the Indenture provides for the Company and the Trustee to
enter into an indenture supplemental to the Indenture to establish the form or
terms of Securities of any series as provided by Sections 2.1 and 3.1 of the
Indenture.

     NOW, THEREFORE, THIS FIRST SUPPLEMENTAL INDENTURE WITNESSETH:

     For and in consideration of the premises and the issuance of the series of
Securities provided for herein, it is mutually agreed, for the equal and
proportionate benefit of all Holders of the Securities of such series, as
follows:

                                   ARTICLE I

                       Relation to Indenture; Definitions

     Section 1.1. RELATION TO INDENTURE. This First Supplemental Indenture
constitutes an integral part of the Indenture.

     Section 1.2. DEFINITIONS. For all purposes of this First Supplemental
Indenture:

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     (a) Capitalized terms used herein without definition shall have the
meanings specified in the Indenture;

     (b) All references herein to Articles and Sections, unless otherwise
specified, refer to the corresponding Articles and Sections of this First
Supplemental Indenture; and

     (c) The terms "herein," "hereof," "hereunder" and other words of similar
import refer to this First Supplemental Indenture.

     (d) "Fair Value," when used with respect to Common Stock, means the fair
value thereof as determined in good faith by the Board of Directors.

                                   ARTICLE II

                            The Series of Securities

     Section 2.1. TITLE OF THE SECURITIES. There shall be a series of Securities
designated the "5.875% Senior Notes due 2013" (the "Notes").

     Section 2.2. LIMITATION ON AGGREGATE PRINCIPAL AMOUNT. The aggregate
principal amount of the Notes shall initially be limited to $200,000,000. The
Company may, without the consent of the Holders of the Notes, issue additional
Securities having the same interest rate, maturity date and other terms as
described in the related prospectus supplement and prospectus. Any additional
Securities, together with the Notes offered by the related prospectus
supplement, will constitute a single series of Securities under the Indenture.
No additional Securities may be issued if an Event of Default under the
Indenture has occurred and is continuing with respect to the Securities.

     Section 2.3. PRINCIPAL PAYMENT DATE. The principal amount of the Notes
outstanding (together with any accrued and unpaid interest) shall be payable in
a single installment on February 15, 2013, which date shall be the Stated
Maturity of the Notes Outstanding.

     Section 2.4. INTEREST AND INTEREST RATES. The rate of interest on each Note
shall be 5.875% per annum, accruing from February 14, 2003, or from the most
recent interest payment date (each such date, an "Interest Payment Date") to
which interest has been paid or duly provided for, payable semiannually in
arrears on February 15 and August 15 of each year commencing August 15, 2003
until the principal thereof shall have become due and payable, and until the
principal thereof is paid or duly provided for or made available for payment.
The amount of interest payable on any Interest Payment Date shall be computed on
the basis of a 360-day year of twelve 30-day months. The amount of interest
payable for any partial period shall be computed on the basis of the actual
number of days elapsed in a 360-day year of twelve 30-day months. In the event
that any date on which interest is payable on any Note is not a Business Day,
then payment of interest payable on such date will be made on the next
succeeding day that is a Business Day (and without any interest or other payment
in respect of any such delay). The interest installment so payable in respect of
any Note, and punctually paid or duly provided for, on any Interest Payment Date
will, as provided in the Indenture, be paid to

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the person in whose name such Note (or one or more Predecessor Securities) is
registered at the close of business on February 1 or August 1 prior to such
Interest Payment Date. Any such interest installment not punctually paid or duly
provided for in respect of any Note shall forthwith cease to be payable to the
registered Holder on such Regular Record Date and may either be paid to the
Person in whose name such Note (or one or more Predecessor Securities) is
registered at the close of business on a Special Record Date to be fixed by the
Trustee for the payment of such Defaulted Interest, notice whereof shall be
given to the Holders of the Notes not less than 10 days prior to such Special
Record Date, or be paid at any time in any other lawful manner not inconsistent
with the requirements of any securities exchange on which the Notes may be
listed, and upon such notice as may be required by such exchange, all as more
fully provided in the Indenture.

     Section 2.5. PLACE OF PAYMENT. The Place of Payment where the Notes may be
presented or surrendered for payment, where the Notes may be surrendered for
registration of transfer or exchange and where notices and demand to or upon the
Company in respect of the Notes and the Indenture may be served shall be the
Corporate Trust Office of the Trustee.

     Section 2.6. REDEMPTION.

     (a) The Company may redeem the Notes, in whole or in part, at any time at a
Redemption Price equal to the greater of (i) 100% of the principal amount of
such Securities to be redeemed or (ii) an amount, as determined by an
Independent Investment Banker, equal to the sum of the present values of the
remaining scheduled payments of principal of and interest on the securities to
be redeemed (not including any portion of such payments of interest accrued as
of the date of redemption) discounted to the Redemption Date on a semiannual
basis (assuming a 360-day year consisting of twelve 30-day months) at the
Adjusted Treasury Rate, plus 25 basis points, plus, in either of the above
cases, accrued and unpaid interest thereon to, but not including, the Redemption
Date.

     (b) For the purposes of this Section 2.6,

     "Adjusted Treasury Rate" means, with respect to any Redemption Date:

     -    the yield, under the heading which represents the average for the
          immediately preceding week, appearing in the most recently published
          statistical release designated "H.15(519)" published by the Board of
          Governors of the Federal Reserve System (or any successor publication
          which is published weekly by the Board of Governors of the Federal
          Reserve System and which establishes yields on actively traded United
          States Treasury securities adjusted to constant maturity) under the
          caption "Treasury Constant Maturities," for the maturity corresponding
          to the Comparable Treasury Issue. If no maturity is within three
          months before or after the Remaining Life, yields for the two
          published maturities most closely corresponding to the Comparable
          Treasury Issue shall be determined and the Adjusted Treasury Rate
          shall be interpolated or

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          extrapolated from such yields on a straight line basis, rounding to
          the nearest month; or

     -    if such release (or any successor release) is not published during the
          week preceding the calculation date or does not contain such yields,
          the rate per annum equal to the semi-annual equivalent yield to
          maturity of the Comparable Treasury Issue, calculated using a price
          for the Comparable Treasury Issue (expressed as a percentage of its
          principal amount) equal to the Comparable Treasury Price for such
          Redemption Date.

     The Adjusted Treasury Rate shall be calculated on the third Business Day
preceding the Redemption Date.

     "Comparable Treasury Issue" means the United States Treasury security
selected by an Independent Investment Banker as having a maturity comparable to
the remaining term of the notes to be redeemed that would be utilized, at the
time of selection and in accordance with customary financial practice, in
pricing new issues of corporate debt securities of comparable maturity to the
remaining term of such securities ("Remaining Life").

     "Comparable Treasury Price" means (i) the average of three Reference
Treasury Dealer Quotations for such Redemption Date, after excluding the highest
and lowest Reference Treasury Dealer Quotations, or (ii) if the Independent
Investment Banker obtains fewer than three such Reference Treasury Dealer
Quotations, the average of all such quotations.

     "Independent Investment Banker" means one of the Reference Treasury Dealers
appointed by us.

     "Reference Treasury Dealer" means:

     -    each of Morgan Stanley & Co. Incorporated and Merrill Lynch, Pierce,
          Fenner & Smith Incorporated and their respective successors; provided
          that, if any of the foregoing ceases to be a primary U.S. Government
          securities dealer in the United States (a "Primary Treasury Dealer"),
          the Company shall substitute therefor another Primary Treasury Dealer;
          and

     -    any other Primary Treasury Dealer selected by the Company.

     "Reference Treasury Dealer Quotations" means, with respect to each
Reference Treasury Dealer and any Redemption Date, the average, as determined by
the Independent Investment Banker, of the bid and asked prices for the
Comparable Treasury Issue (expressed in each case as a percentage of its
principal amount) quoted in writing to the Independent Investment Banker at 5:00
p.m., New York City Time, on the third Business Day preceding such Redemption
Date.

     The Company will mail a notice of redemption at least 30 days but not more
than 60 days before the Redemption Date to each holder of the notes to be
redeemed. If less than all

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of the notes are to be redeemed, the trustee will select, by such method as it
will deem fair and appropriate, including pro rata or by lot, the notes to be
redeemed in whole or in part.

     Unless the Company defaults in payment of the Redemption Price, on and
after the Redemption Date, interest will cease to accrue on the notes or
portions thereof called for redemption.

     Section 2.7. DENOMINATION. The Notes shall be issuable only in registered
form without coupons and in denominations of $1,000 and integral multiples
thereof.

     Section 2.8. CURRENCY. Principal and interest on the Notes shall be payable
in such coin or currency of the United States of America that at the time of
payment is legal tender for payment of public and private debts.

     Section 2.9. FORM OF NOTES. The Notes shall be substantially in the form
attached as EXHIBIT A hereto.

     Section 2.10. REGISTRAR AND PAYING AGENT FOR THE NOTES. The Trustee shall
serve initially as Registrar and Paying Agent for the Notes.

     Section 2.11. SINKING FUND OBLIGATIONS. The Company has no obligation to
redeem or purchase any Notes pursuant to any sinking fund or analogous
requirement or upon the happening of a specified event or at the option of a
Holder thereof.

     Section 2.12. DEFEASANCE AND COVENANT DEFEASANCE. The Company has elected
to have both Section 4.2(2) of the Indenture (relating to defeasance) and
Section 4.2(3) (relating to covenant defeasance) applied to the Notes.

     Section 2.13. PAYMENT OF TAXES. The Company will pay or discharge or cause
to be paid or discharged, before the same shall become delinquent, all taxes,
assessments and governmental charges levied or imposed upon the Company or any
Subsidiary or upon the income, profits or property of the Company or any
Subsidiary, and lawful claims for labor, materials and supplies, which, if
unpaid, might by law become a lien upon the property of the Company or any
Subsidiary; provided, however, that the Company shall not be required to pay or
discharge or cause to be paid or discharged any such tax, assessment or
governmental charge whose amount, applicability or validity is being contested
in good faith by appropriate proceedings or where the failure to effect such
payment is not adverse in any material respect to the Holders of the Notes.

     Section 2.14. LIMITATION ON LIENS ON STOCK OF PRINCIPAL SUBSIDIARIES. The
Company will not, and it will not permit any Subsidiary of the Company to, at
any time directly or indirectly create, assume, incur or permit to exist any
Indebtedness secured by a pledge, lien or other encumbrance (any pledge, lien or
other encumbrance being hereinafter in this Section referred to as a "lien") on
the voting securities of Principal Subsidiaries, or the voting securities of a
Subsidiary that owns, directly or indirectly, the voting securities of any of
the Principal Subsidiaries without making effective provision whereby the Notes
then Outstanding (and, if the Company so elects, any other Indebtedness of the
Company

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that is not subordinate to the Notes and with respect to which the governing
instruments require, or pursuant to which the Company is otherwise obligated or
required, to provide such security) shall be equally and ratably secured with
such secured Indebtedness so long as such other Indebtedness shall be secured.
For purposes of this Section 2.14 only, "Indebtedness", in addition to those
items specified in Section 1.1 of the Indenture, shall include any obligation
of, or any such obligation guaranteed by, any Person for the payment of amounts
due under a swap agreement or other similar instrument or agreement or foreign
currency hedge exchange or similar instrument or agreement.

     If the Company shall hereafter be required to secure the Notes equally and
ratably with any other Indebtedness pursuant to this Section, (i) the Company
will promptly deliver to the Trustee an Officer's Certificate stating that the
foregoing covenant has been complied with, and an Opinion of Counsel stating
that in the opinion of such counsel the foregoing covenant has been complied
with and that any instruments executed by the Company or any Subsidiary of the
Company in the performance of the foregoing covenant comply with the
requirements of the foregoing covenant and (ii) the Trustee is hereby authorized
to enter into an indenture or agreement supplemental hereto and to take such
action, if any, as it may deem advisable to enable it to enforce the rights of
the holders of the Notes so secured.

     Section 2.15. LIMITATIONS ON ISSUE OR DISPOSITION OF COMMON STOCK OF
PRINCIPAL SUBSIDIARIES. As long as any of the Notes remain outstanding, the
Company will not, and will not permit any Subsidiary to, issue, sell, assign,
transfer or otherwise dispose of, directly or indirectly, any of the Common
Stock of any Principal Subsidiary (except to the Company or to one or more
Subsidiaries or for the purpose of qualifying directors); provided, however,
that this covenant shall not apply if (i) the issuance, sale, assignment,
transfer or other disposition is required to comply with the order of a court or
regulatory authority of competent jurisdiction, other than an order issued at
the request of the Company or of one of its Subsidiaries; (ii) the entire Common
Stock of a Principal Subsidiary then owned by the Company or by its Subsidiaries
is disposed of in a single transaction or in a series of related transactions,
for consideration consisting of cash or other property which is at least equal
to the Fair Value of such Common Stock; or (iii) after giving effect to the
issuance, sale, assignment, transfer or other disposition, the Company and its
Subsidiaries would own directly or indirectly at least 80% of the issued and
outstanding Common Stock of such Principal Subsidiary and such issuance, sale,
assignment, transfer or other disposition is made for consideration consisting
of cash or other property which is at least equal to the Fair Value of such
Common Stock.

     Section 2.16. IMMEDIATELY AVAILABLE FUNDS. All payments of principal and
interest shall be made in immediately available funds.

                                  ARTICLE III

                            Miscellaneous Provisions

     Section 3.1. TRUSTEE NOT RESPONSIBLE FOR RECITALS. The recitals herein
contained are made by the Company and not by the Trustee, and the Trustee
assumes no

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responsibility for the correctness thereof. The Trustee makes no representation
as to the validity or sufficiency of this First Supplemental Indenture.

     Section 3.2. PAYMENT OF EXPENSES UPON RESIGNATION OR REMOVAL. Upon
termination of this First Supplemental Indenture or the Indenture or the removal
or resignation of the Trustee, unless otherwise stated, the Company shall pay to
the Trustee all amounts accrued to the date of such termination, removal or
resignation.

     Section 3.3. ADOPTION, RATIFICATION AND CONFIRMATION. The Indenture, as
supplemented and amended by this First Supplemental Indenture, is in all
respects hereby adopted, ratified and confirmed.

     Section 3.4. COUNTERPARTS. This First Supplemental Indenture may be
executed in any number of counterparts, each of which shall be an original, but
such counterparts shall together constitute but one and the same instrument.

     Section 3.5. GOVERNING LAW. THIS FIRST SUPPLEMENTAL INDENTURE AND EACH NOTE
SHALL BE DEEMED TO BE A CONTRACT MADE UNDER THE LAWS OF THE STATE OF NEW YORK
AND SHALL BE GOVERNED BY AND CONSTRUED IN ACCORDANCE WITH THE LAWS OF THE STATE
OF NEW YORK WITHOUT REGARD TO CONFLICTS OF LAWS PRINCIPLES THEREOF.

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     IN WITNESS WHEREOF, the parties hereto have caused this First Supplemental
Indenture to be duly executed on the day and year first above written.

                                        W. R. BERKLEY CORPORATION

                                        By:
                                            ------------------------------
                                            Name:
                                            Title:

                                        THE BANK OF NEW YORK, as Trustee

                                        By:
                                            ------------------------------
                                            Name:
                                            Title:

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                                                                       EXHIBIT A

                             (FORM OF FACE OF NOTE)

     This Note is a global Note within the meaning of the Indenture hereinafter
referred to and is registered in the name of a Depository or a nominee of a
Depository. This Note is exchangeable for Securities registered in the name of a
person other than the Depository or its nominee only in the limited
circumstances described in the Indenture, and no transfer of this Note (other
than a transfer of this Note as a whole by the Depository to a nominee of the
Depository or by a nominee of the Depository to the Depository or another
nominee of the Depository) may be registered except in limited circumstances.

     Unless this Note is presented by an authorized representative of The
Depository Trust Company (55 Water Street, New York, New York) to the issuer or
its agent for registration of transfer, exchange or payment, and any Note issued
is registered in the name of Cede & Co. or such other name as requested by an
authorized representative of The Depository Trust Company and any payment hereon
is made to Cede & Co., ANY TRANSFER, PLEDGE OR OTHER USE HEREOF FOR VALUE OR
OTHERWISE BY A PERSON IS WRONGFUL since the registered owner hereof, Cede & Co.,
has an interest herein.

Certificate No. 1                                                  $ 200,000,000
Dated: February 14, 2003                                     CUSIP No. 084423AJ1

                            W. R. BERKLEY CORPORATION

                          5.875% Senior Notes due 2013

     W. R. BERKLEY CORPORATION, a Delaware corporation (the "Company," which
term includes any successor corporation under the Indenture hereinafter referred
to), for value received, hereby promises to pay to CEDE & CO. or registered
assigns, the principal sum of TWO HUNDRED MILLION DOLLARS AND NO CENTS
($200,000,000.00) on February 15, 2013. The Company further promises to pay
interest on said principal sum outstanding from February 14, 2003, or from the
most recent interest payment date (each such date, an "Interest Payment Date")
to which interest has been paid or duly provided for, semiannually (subject to
deferral as set forth herein) in arrears on February 15 and August 15 of each
year commencing August 15, 2003 at the rate of 5.875% per annum, until the
principal hereof shall have become due and payable and, until the principal
hereof is paid or duly provided for or made available for payment. The amount of
interest payable on any Interest Payment Date shall be computed on the basis of
a 360-day year of twelve 30-day months. The amount of interest payable for any
partial period shall be computed on the basis of the number of actual days
elapsed in a 360-day year of twelve 30-day months. In the event that any date on
which interest is payable on this Note is not a Business Day, then payment of
interest payable on such date will be made on the next succeeding day that is a
Business Day (and without any interest or other payment in respect of any such
delay). A "Business Day," with respect to any Place of Payment or other
location, shall mean any day other than a Saturday, Sunday or other day on which
banking institutions in such Place of Payment or other location are authorized
or obligated by law, regulation or executive

                                      A-1

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order to close. The interest installment so payable, and punctually paid or duly
provided for, on any Interest Payment Date will, as provided in the Indenture,
be paid to the Person in whose name this Note (or one or more Predecessor
Securities) is registered at the close of business on February 1 or August 1
prior to such Interest Payment Date. Any such interest installment not
punctually paid or duly provided for shall forthwith cease to be payable to the
registered Holder on such Regular Record Date and may either be paid to the
Person in whose name this Note (or one or more Predecessor Securities) is
registered at the close of business on a Special Record Date to be fixed by the
Trustee for the payment of such Defaulted Interest, notice whereof shall be
given to the Holder of this Note not less than 10 days prior to such Special
Record Date, or be paid at any time in any other lawful manner not inconsistent
with the requirements of any securities exchange on which this Note may be
listed, and upon such notice as may be required by such exchange, all as more
fully provided in the Indenture.

     The principal of (and premium, if any) and the interest on this Note shall
be payable at the office or agency of the Company maintained for that purpose in
the United States in such coin or currency of the United States of America that
at the time of payment is legal tender for payment of public and private debts;
PROVIDED, HOWEVER, that payment of interest may be made at the option of the
Company by check mailed to the registered Holder at such address as shall appear
in the Security Register. Notwithstanding the foregoing, so long as the Holder
of this Note is Cede & Co., the payment of the principal of (and premium, if
any) and interest on this Note will be made at such place and to such account as
may be designated by Cede & Co. All payments of principal and interest hereunder
shall be made in immediately available funds.

     Reference is hereby made to the further provisions of this Note set forth
on the reverse hereof, which further provisions shall for all purposes have the
same effect as if set forth at this place.

     Unless the certificate of authentication hereon has been executed by the
Trustee referred to on the reverse hereof by manual signature, this Note shall
not be entitled to any benefit under the Indenture or be valid for any purpose.

                                      A-2

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     IN WITNESS WHEREOF, the Company has caused this instrument to be executed.

                                        W. R. BERKLEY CORPORATION

                                        By:
                                            ------------------------------
                                            Name:
                                            Title:

                          CERTIFICATE OF AUTHENTICATION

     This is one of the Securities of the series designated herein referred to
in the within-mentioned Indenture.

Dated: February 14, 2003

THE BANK OF NEW YORK,
as Trustee

By:
    ------------------------------
     Authorized Signatory

                                      A-3

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                            (FORM OF REVERSE OF NOTE)

     This Note is one of a duly authorized issue of securities of the Company,
designated as its 5.875% Senior Notes due 2013 (herein referred to as the
"Securities"), issued under and pursuant to an Indenture, dated as of February
14, 2003 between the Company and The Bank of New York, as Trustee (herein called
the "Trustee," which term includes any successor trustee under the Indenture),
as supplemented by the First Supplemental Indenture dated as of February 14,
2003, between the Company and the Trustee (the Indenture as so supplemented, the
"Indenture"), to which Indenture and all indentures supplemental thereto
reference is hereby made for a description of the rights, limitations of rights,
obligations, duties and immunities thereunder of the Trustee, the Company and
the Holders of the Securities, and of the terms upon which the Securities are,
and are to be, authenticated and delivered.

     All terms used in this Note that are defined in the Indenture shall have
the meanings assigned to them in the Indenture.

     The Company may redeem the Securities, in whole or in part, at any time at
a Redemption Price equal to the greater of (i) 100% of the principal amount of
such Securities to be redeemed or (ii) an amount, as determined by an
Independent Investment Banker, the sum of the present values of the remaining
scheduled payments of principal of and interest thereon on the securities to be
redeemed (not including any portion of such payments of interest accrued to the
date of redemption) discounted to the Redemption Date on a semiannual basis
assuming a 360-day year consisting of twelve 30-day months) at the Adjusted
Treasury Rate, plus 25 basis points, plus, in either of the above cases, accrued
and unpaid interest thereon to the Redemption Date.

     "Adjusted Treasury Rate" means, with respect to any Redemption Date:

     -    the yield, under the heading which represents the average for the
          immediately preceding week, appearing in the most recently published
          statistical release designated "H.15(519)" published by the Board of
          Governors of the Federal Reserve System (or any successor publication
          which is published weekly by the Board of Governors of the Federal
          Reserve System and which establishes yields on actively traded United
          States Treasury securities adjusted to constant maturity) under the
          caption "Treasury Constant Maturities," for the maturity corresponding
          to the Comparable Treasury Issue. If no maturity is within three
          months before or after the Remaining Life, yields for the two
          published maturities most closely corresponding to the Comparable
          Treasury Issue shall be determined and the Adjusted Treasury Rate
          shall be interpolated or extrapolated from such yields on a straight
          line basis, rounding to the nearest month; or

     -    if such release (or any successor release) is not published during the
          week preceding the calculation date or does not contain such yields,
          the rate per annum equal to the semiannual equivalent yield to
          maturity of the

                                      A-4

<PAGE>

          Comparable Treasury Issue, calculated using a price for the Comparable
          Treasury Issue (expressed as a percentage of its principal amount)
          equal to the Comparable Treasury Price for such Redemption Date.

     The Adjusted Treasury Rate shall be calculated on the third Business Day
preceding the Redemption Date.

     "Comparable Treasury Issue" means the United States Treasury security
selected by an Independent Investment Banker as having a maturity comparable to
the remaining term of the securities to be redeemed that would be used, at the
time of selection and in accordance with customary financial practice, in
pricing new issues of corporate debt securities of comparable maturity to the
remaining term of such securities ("Remaining Life").

     "Comparable Treasury Price" means (i) the average of three Reference
Treasury Dealer Quotations for such Redemption Date, after excluding the highest
and lowest Reference Treasury Dealer Quotations, or (ii) if the Independent
Investment Banker obtains fewer than three such Reference Treasury Dealer
Quotations, the average of all such quotations.

     "Independent Investment Banker" means one of the Reference Treasury Dealers
appointed by us.

     "Reference Treasury Dealer" means:

     -    each of Morgan Stanley & Co. Incorporated and Merrill Lynch, Pierce,
          Fenner & Smith Incorporated, and their respective successors;
          provided, however, that if any of the foregoing shall cease to be a
          primary U.S. Government securities dealer in the United States (a
          "Primary Treasury Dealer"), the Company shall substitute therefor
          another Primary Treasury Dealer; and

     -    any other Primary Treasury Dealer selected by the Company.

     "Reference Treasury Dealer Quotations" means, with respect to each
Reference Treasury Dealer and any Redemption Date, the average, as determined by
the Independent Investment Banker, of the bid and asked prices for the
Comparable Treasury Issue (expressed in each case as a percentage of its
principal amount) quoted in writing to the Independent Investment Banker at 5:00
p.m., New York City Time, on the third Business Day preceding such Redemption
Date.

     The Company will mail a notice of redemption at least 30 days but not more
than 60 days before the Redemption Date to each holder of the securities to be
redeemed. If less than all of the securities are to be redeemed, the Trustee
will select, by such method as it will deem fair and appropriate, including pro
rata or by lot, the securities to be redeemed in whole or in part.

                                      A-5
<PAGE>

     Unless we default in payment of the Redemption Price, on and after the
Redemption Date, interest will cease to accrue on the securities or portions
thereof called for redemption.

     If an Event of Default with respect to Securities of this series shall
occur and be continuing, the principal of the Securities of this series may be
declared due and payable in the manner, with the effect and subject to the
conditions provided in the Indenture.

     The Indenture contains provisions for satisfaction, discharge and
defeasance at any time of the entire indebtedness of this Note upon compliance
by the Company with certain conditions set forth in the Indenture.

     The Indenture permits, with certain exceptions as therein provided, the
amendment thereof and the modification of the rights and obligations of the
Company and the rights of the Holders of the Securities of each series to be
affected under the Indenture at any time by the Company and the Trustee with the
consent of the Holders of a majority in principal amount of the Securities of
each series at the time Outstanding of each series to be affected. The Indenture
also contains provisions permitting Holders of specified percentages in
principal amount of the Securities of each series at the time Outstanding, on
behalf of the Holders of all Securities of such series, to waive compliance by
the Company with certain provisions of the Indenture and certain past defaults
under the Indenture and their consequences. Any such consent or waiver by the
Holder of this Note shall be conclusive and binding upon such Holder and upon
all future Holders of this Note and of any Note issued upon the registration of
transfer hereof or in exchange therefor or in lieu hereof, whether or not
notation of such consent or waiver is made upon this Note. No reference herein
to the Indenture and no provision of this Note or of the Indenture (other than
Section 4.2 of the Indenture) shall alter or impair the obligation of the
Company to pay the principal and interest on the Note at the times, place and
rate, and in the coin or currency, herein prescribed.

     As provided in the Indenture and subject to certain limitations therein set
forth, the transfer of this Note is registrable in the Security Register, upon
surrender of this Note for registration of transfer at the office or agency of
the Company maintained under Section 10.2 of the Indenture duly endorsed by, or
accompanied by a written instrument of transfer, in form satisfactory to the
Company and the Security Registrar, duly executed by the Holder hereof or his or
her attorney duly authorized in writing, and thereupon one or more new
Securities of this series, of authorized denominations and for the same
aggregate principal amount, will be issued to the designated transferee or
transferees. No service charge shall be made for any such registration of
transfer or exchange, but the Company may require payment of a sum sufficient to
cover any tax or other governmental charge payable in connection therewith.

     Prior to due presentment of this Note for registration of transfer, the
Company, the Trustee and any agent of the Company or the Trustee may treat the
Person in whose name this Note is registered as the owner hereof for all
purposes, whether or not this Note be overdue, and neither the Company, the
Trustee nor any such agent shall be affected by notice to the contrary.

                                      A-6

<PAGE>

     This global Note is exchangeable for Securities in definitive form only
under certain limited circumstances set forth in the Indenture. Securities of
this series so issued are issuable only in registered form without coupons in
denominations of $1,000 and any integral multiple thereof. As provided in the
Indenture and subject to certain limitations herein and therein set forth,
Securities of this series so issued are exchangeable for a like aggregate
principal amount of Securities of this series of a different authorized
denomination, as requested by the Holder surrendering the same.

     The Company and, by its acceptance of this Note or a beneficial interest
therein, the Holder of, and any Person that acquires a beneficial interest in,
this Note agree that for United States federal, state and local tax purposes it
is intended that this Note constitute indebtedness.

     THE INTERNAL LAWS OF THE STATE OF NEW YORK SHALL GOVERN THE INDENTURE AND
THE SECURITIES WITHOUT REGARD TO CONFLICT OF LAW PROVISIONS THEREOF.

                                      A-7<PAGE>

                                                                    Exhibit 10.1

                        THIRD AMENDMENT TO LOAN AGREEMENT

         THIS THIRD AMENDMENT TO LOAN AGREEMENT (this "Amendment"), dated as of
January 14, 2003, is between CONCORD TECHNOLOGIES, LP, a Texas limited
partnership ("Concord"), GEOSPACE ENGINEERING RESOURCES INTERNATIONAL, LP, a
Texas limited partnership ("Engineering"), GEO SPACE, LP, a Texas limited
partnership ("Geo Space"), OYO INSTRUMENTS, LP, a Texas limited partnership
("Instruments"), and OYOG OPERATIONS, LP, a Texas limited partnership
("Operations", and together with Concord, Engineering, Geo Space and
Instruments, the "Borrowers"), jointly and severally, and SOUTHWEST BANK OF
TEXAS, N.A., a national banking association ("Lender").

                                    RECITALS:

         A. Borrowers and Lender entered into that certain Loan Agreement dated
as of February 16, 2001, as amended by First Amendment to Loan Agreement dated
as of February 17, 2001 and Second Amendment to Loan Agreement dated as of
January 15, 2002 (the "Agreement").

         B. Pursuant to the Agreement, OYOG, LLC, a Delaware limited liability
company, OYO Geospace Corporation, a Delaware corporation, and OYOG Limited
Partner, LLC, a Nevada limited liability company ("Guarantors") executed those
certain Guaranty Agreements dated as of January 15, 2002 (the "Guaranty
Agreements") pursuant to which Guarantors guaranteed to Lender the payment and
performance of the Obligations (as defined in the Agreement).

         C. Borrowers and Lender now desire to amend the Agreement as herein set
forth.

         NOW, THEREFORE, in consideration of the premises herein contained and
other good and valuable consideration, the receipt and sufficiency of which are
hereby acknowledged, the parties hereto agree as follows:

                                   ARTICLE I.

                                   Definitions

         Section I.1. Definitions. Capitalized terms used in this Amendment, to
the extent not otherwise defined herein, shall have the meanings given to such
terms in the Agreement, as amended hereby.

                                       -1-

<PAGE>

                                   ARTICLE II.

                                   Amendments

         Section II.1. Amendment to Certain Definitions. (a) The definition of
each of the following terms contained in Section 1.1 of the Agreement is amended
to read in its respective entirety as follows:

             "Borrowing Base" means, at any particular time, an amount equal to
          the sum of (a) eighty percent (80%) of Eligible Accounts plus (b) the
          lesser of (i) twenty-five percent (25%) of Eligible Inventory and (ii)
          $5,000,000.00 plus (c) eighty percent (80%) of Eligible Notes.

             "EBITDA" means for Parent and its Subsidiaries, on a consolidated
          basis, for any period, the sum of (a) Net Income for such period, plus
          (b) depreciation, amortization and other non cash charges for such
          period, plus (c) Interest Expense for such period, plus (d) income tax
          expense for such period, plus (e) for the fiscal quarter ended June
          30, 2002, the $1,246,000.00 non-cash write-off of Borrower's advance
          to Labelon Corporation.

             "Termination Date-A" means 11:00 a.m., Houston, Texas time on
          January 13, 2004, or such earlier date on which the Commitment-A
          terminates as provided in this Agreement.

         (b) The following definitions shall be added to Section 1.1 of the
Agreement in proper alphabetical order:

             "Advance Request Forms" means the Advance Request Form-A and the
          Advance Request Form-B.

             "Advances" means Advance-A and Advance-B.

             "Applicable Rate" means (a) during the period that an Advance is a
          LIBOR Advance, the sum of the LIBOR Rate plus two and one-half percent
          (2.50%), and (b) during the period that an Advance is a Prime Rate
          Advance, the Prime Rate.

             "Continue", "Continuation" and "Continued" shall refer to
          continuation pursuant to Section 3.7 of an Advance as an Advance of
          the same Type from one Interest Period to the next Interest Period.

                                       -2-

<PAGE>

             "Convert", "Conversion" and "Converted" shall refer to a conversion
          pursuant to Section 3.7 or 3.8 of one Type of Advance into another
          Type of Advance.

             "Dollar," "Dollars" and "$" means currency of the United States of
          America which is at the time of payment legal tender for the payment
          of public and private debts in the United States of America.

             "Interest Period" means with respect to any LIBOR Advance, each
          period commencing on the date such Advance is made or the date such
          Advance is Converted from an Advance of another Type or, in the case
          of each subsequent, successive Interest Period applicable to a LIBOR
          Advance, each period commencing on the last day of the immediately
          preceding Interest Period with respect to such LIBOR Advance, and in
          each case ending on the thirtieth (30th), sixtieth (60th) or ninetieth
          (90th) day thereafter as provided in Section 2.5 or 3.7; provided,
          however, that (a) each Interest Period which would otherwise end on a
          day which is not a Business Day shall end on the next succeeding
          Business Day, (b) any Interest Period for any LIBOR Advance which
          would otherwise extend beyond the Termination Dates, shall end on the
          Termination Dates, (c) no more than five (5) Interest Periods for the
          Advances shall be in effect at the same time and (d) no Interest
          Period shall have a duration of less than thirty (30) days, and, if
          the Interest Period for any LIBOR Advance would otherwise be a shorter
          period, such Advance shall be a Prime Rate Advance.

             "LIBOR Advances" means Advances the interest rate on which are
          determined on the basis of the rates referred to in the definition of
          "LIBOR Rate".

             "LIBOR Rate" means, for any LIBOR Advance, for any Interest Period
          therefor, the rate per annum offered for Dollar deposits in an amount
          comparable to the principal amount of such LIBOR Advance for a period
          of time equal to such Interest Period as of 11:00 A.M. City of London,
          England time two (2) London Business Days prior to the first date of
          such Interest Period as shown on the display designated as "British
          Bankers Association Interest Settlement Rates" on the Bloomberg system
          ("Bloomberg"); provided, however, that if such rate is not available
          on Bloomberg then such offered rate shall be otherwise independently
          determined by Lender from an alternate, substantially similar
          independent source available to Lender and recognized in the banking
          industry.

                                       -3-

<PAGE>

             "London Business Day" means any day other than a Saturday, Sunday
          or a day on which banking institutions are generally authorized or
          obligated by laws or executive order to close in the City of London,
          England.

             "Prime Rate Advances" means Advances that bear interest at rates
          based upon the Prime Rate.

             "Reserve Requirement" means the aggregate maximum reserve
          percentages (including any marginal, special, supplemental or
          emergency reserves, and expressed as a decimal) established by the
          Federal Reserve Board or any other United States banking authority to
          which Lender is subject for "Eurocurrency Liabilities" (as defined in
          Regulation D of the Board of Governors of the Federal Reserve System).
          Such reserve percentages shall include, without limitation, those
          imposed under Regulation D of the Board of Governors of the Federal
          Reserve System.

             "Type" means the type of Advance (i.e. Prime Rate Advance or LIBOR
          Advance).

          Section II.2. Amendment to Section 2.3. Section 2.3 contained in the
Agreement is amended to read in its entirety as follows:

             Section 2.3. Repayment of Principal and Interest; Extension. (a)
          Accrued and unpaid interest on the Advances-A (and, therefore, Note-A)
          shall be due and payable as follows:

                  (i)   in the case of each Advance-A which is a Prime Rate
             Advance, on the first day of each month, commencing February 1,
             2003;

                  (ii)  in the case of each Advance-A which is a LIBOR Advance,
             on the last day of each Interest Period therefor;

                  (iii) upon the payment or prepayment (mandatory or optional)
             of any Advance-A or the Conversion of any Advance-A (but only on
             the principal amount so paid, prepaid, or Converted); and

                  (iv)  on the Termination Date-A.

             (b)  The principal of the Advances-A shall be due and payable on
          the Termination Date-A.

                                       -4-

<PAGE>

             (c) Prior to the Termination Date-A, Lender will review such
          matters as it may deem appropriate in its sole discretion and may, in
          its sole and absolute discretion, determine whether to extend the
          Termination Date-A.

         Section II.3. Amendment to Section 2.4. Section 2.4 of the Agreement is
amended to read in its entirety as follows:

             Section 2.4. Interest. The unpaid principal amount of the
          Advances-A shall bear interest prior to maturity at a varying rate per
          annum equal from day to day to the lesser of (a) the Maximum Rate or
          (b) the Applicable Rate in effect from day to day, and each change in
          the rate of interest charged on the Advances-A shall become effective,
          without notice to any Borrower, on the effective date of each change
          in the Applicable Rate or the Maximum Rate, as the case may be;
          provided, however, if at any time the rate of interest specified in
          clause (b) preceding shall exceed the Maximum Rate, thereby causing
          the interest on the Advances-A to be limited to the Maximum Rate, then
          any subsequent reduction in the Applicable Rate shall not reduce the
          rate of interest on the Advances-A below the Maximum Rate until the
          aggregate amount of interest actually accrued on the Advances-A equals
          the amount of interest which would have accrued on the Advances-A if
          the interest rate specified in clause (b) preceding had at all times
          been in effect. If an Event of Default has occurred and is continuing,
          all principal of the Advances-A shall bear interest at the Default
          Rate.

         Section II.4. Amendment to Section 2.5. Section 2.5 contained in the
Agreement is amended to read in its entirety as follows:

             Section 2.5. Requests for Advances-A. (a) As long as the Autopay
          Agreement is in effect, Advances-A which are Prime Rate Advances may
          be made as provided in the Autopay Agreement, and Borrowers shall not
          be required to request an Advance-A directly from Lender by means of
          an Advance Request Form-A.

             (b) The provisions of this paragraph shall apply to all requests
          for Advances-A which are to be LIBOR Advances. The provisions of this
          paragraph shall also apply to Advances-A which are to be Prime Rate
          Advances if Borrowers so choose, or if the Autopay Agreement is not in
          effect, or if the Available Amount (as defined in the Autopay
          Agreement) is, or has been declared to be, equal to zero. Borrowers
          shall request each Advance-A by delivering to Lender an Advance
          Request Form-A (i) stating the amount of the Advance-A, (ii) stating
          the date on which Borrowers desire that the Advance-A be funded
          (provided that without any date specified, an Advance-A that is to be
          a Prime Rate Advance shall be funded on the next Business Day and an
          Advance-A that is to be a LIBOR Advance shall be funded on the
          succeeding third Business Day following such Advance Request Form-A),
          (iii)

                                       -5-

<PAGE>

          stating the Type of the Advance-A, and (iv) if such Advance-A is a
          LIBOR Advance, designating the Interest Period thereof. Such documents
          shall be delivered to Lender at least (i) one (1) Business Day before
          the date on which Borrowers desire that the Advance-A be funded in the
          case of each Advance-A which is to be a Prime Rate Advance and (ii) at
          least three (3) Business Days before the date on which Borrowers
          desire that the Advance-A be funded in the case of each Advance-A
          which is to be a LIBOR Advance; provided that no Advance-A which is a
          LIBOR Advance may be in an amount which is less that $500,000.00.
          Borrowers at any time may redesignate the amounts of, and Convert and
          Continue the Advances-A, but only to be effective from and after the
          end of the Interest Period therefor if an Advance-A is to be Continued
          as, or Converted from, a LIBOR Advance, and subject to the terms and
          provisions of this Agreement, including Sections 3.7, 3.8 and 3.9
          hereof. Prior to making any Advance-A, Lender may require that
          Borrowers deliver a Borrowing Base Certificate dated a recent date
          acceptable to Lender evidencing that the amount of the outstanding
          Advances-A plus the requested Advance-A plus the Letter of Credit
          Liabilities is less than the lesser of (i) the Commitment-A or (ii)
          the Borrowing Base.

         Section II.5. Amendment to Section 3.6. The phrase "in this Section
3.7" contained in Section 3.6 of the Agreement shall be amended to read "in this
Section 3.6" each time such phrase appears in Section 3.6.

         Section II.6. Addition of Sections 3.7, 3.8 and 3.9. Sections 3.7, 3.8
and 3.9 shall be added to the Agreement and shall read in their entirety as
follows:

             Section 3.7. Conversions and Continuations. Borrowers shall have
          the right from time to time to Convert any Advance from one Type of
          Advance into another Type of Advance or to Continue any LIBOR Advance
          as a LIBOR Advance by giving Lender written notice at least one (1)
          Business Day before Conversion into a Prime Rate Advance and at least
          three (3) Business Days before Conversion into or Continuation of a
          LIBOR Advance, specifying (a) the Conversion or Continuation date, and
          (b) in the case of Conversions, the Type of Advance to be Converted
          into; provided that (w) no more than five (5) Interest Periods may be
          in effect for the LIBOR Advances at any time, (x) no LIBOR Advance may
          be in an amount which is less than $500,000.00, (y) a LIBOR Advance
          may only be Converted on the last day of the Interest Period therefor,
          and (z) except for Conversions to Prime Rate Advances, Lender shall
          have no obligation to make any Conversions while an Event of Default
          has occurred and is continuing. All notices under this Section shall
          be irrevocable and shall be given not later than 11:00 A.M. Houston,
          Texas time

                                       -6-

<PAGE>

          on the day which is not less than the number of Business Days
          specified above for such notice. If Borrowers shall fail to give
          Lender the notice specified above for Continuation or Conversion of
          any LIBOR Advance prior to the end of the Interest Period with respect
          thereto, such LIBOR Advance shall automatically be Converted into a
          Prime Rate Advance on the last day of such Interest Period.

             Section 3.8. Illegality, Impossibility, Regulatory Change and
          Compensation. In the event that (a) it becomes unlawful for Lender to
          honor its obligation to make LIBOR Advances hereunder or to maintain
          LIBOR Advances hereunder, (b) Lender determines that (i) quotations of
          interest rates for the relevant deposits referred to in the definition
          of "LIBOR Rate" are not being provided in the relative amounts or for
          the relative maturities for determining the interest rates borne by
          the LIBOR Advances as provided in this Agreement or (ii) such
          quotations do not accurately reflect Lender's costs in connection
          therewith, or (c) a Regulatory Change (including the imposition of a
          Reserve Requirement) occurs which changes Lender's basis of taxation
          with respect to LIBOR Advances or imposes reserve, capital or other
          requirements with respect thereto, then Lender shall notify Borrowers
          of any such event. Following such notice, upon written notice to
          Borrowers showing the computation of such amounts in reasonable
          detail, Borrowers shall promptly pay to Lender such amounts as Lender
          may determine (which determination shall be conclusive provided such
          determination is made on a reasonable basis) to be necessary to
          compensate Lender for any increased costs incurred by Lender after
          such notice or decreases in amounts receivable by Lender after such
          notice which Lender determines are attributable to any event described
          in clauses (a), (b) or (c) above. For so long as such event or
          circumstance shall continue in effect, the obligation of Lender to
          make or Continue LIBOR Advances or to Convert Prime Rate Advances to
          LIBOR Advances shall terminate, and (i) all future Advances shall be
          Prime Rate Advances and (ii) all outstanding Advances which are LIBOR
          Advances shall be Converted to Prime Rate Advances on the last day of
          the current Interest Period therefor.

             Section 3.9. Compensation for Prepayment or Failure to Borrow. Upon
          (a) any prepayment or Conversion of any LIBOR Advance on a day other
          than the last day of an Interest Period therefor or (b) the failure by
          Borrowers to borrow as provided in an Advance Request Form delivered
          to Lender, Convert or prepay a LIBOR Advance on any date required
          hereby, Borrowers shall pay to Lender a fee in an amount reasonably
          determined by Lender equal to funding losses actually incurred by
          Lender as a result of such event, but not more than one percent
          (1.00%) of the principal amount of the

                                       -7-

<PAGE>

          Advance times a fraction, the numerator of which is the number of days
          remaining in the Interest Period and the denominator of which is 365.

          Section II.7. Amendment to Section 7.1. Clause (b) contained in
Section 7.1 of the  Agreement is amended to read in its entirety as follows:

             (b) Quarterly Financial Statements - Parent. As soon as available,
          and in any event within sixty (60) days after the end of each of the
          first three quarters of each fiscal year of Parent, a copy of the
          consolidated financial statements of Parent and its Subsidiaries as of
          the end of such fiscal quarter and for the portion of the fiscal year
          then ended, on SEC Form 10-Q, containing, on a consolidated basis,
          balance sheets, statements of income and cash flows in each case
          setting forth in comparative form the figures for the corresponding
          period of the preceding fiscal year, all in reasonable detail,
          prepared in accordance with GAAP and reviewed by independent certified
          public accountants acceptable to Lender.

          Section II.8. Amendment to Section 8.1. Section 8.1 contained in the
Agreement is amended to read in its entirety as follows:

             Section 8.1. Debt. No Borrower will incur, create, assume or permit
          to exist, nor will it permit any Guarantor or any Subsidiary to incur,
          create, assume, or permit to exist, any Debt, except (a) Debt to
          Lender, (b) Debt of Borrowers, Guarantors and their Subsidiaries in an
          aggregate principal amount which does not exceed $3,000,000.00
          outstanding at any time, (c) Debt described in Schedule 8.1(c), (d)
          Subordinated Debt, (e) accounts payable in the ordinary course of
          business, and (f) Debt arising from the endorsement of instruments for
          collection in the ordinary course of business.

          Section II.9. Amendment to Exhibits. (a) Exhibit "A" to the Agreement
(Note-A) is amended to conform in its entirety to Annex "A" to this Amendment,
(b) Exhibit "L" to the Agreement (Borrowing Base Certificate) is amended to
conform in its entirety to Annex "B" to this Amendment and (c) Exhibit "M" to
the Agreement (No Default Certificate) is amended to conform in its entirety to
Annex "C" to this Amendment.

                                       -8-

<PAGE>

                                  ARTICLE III.

                              Conditions Precedent

          Section III.1. Conditions. The effectiveness of this Amendment is
subject to the receipt by Lender of the following in form and substance
satisfactory to Lender:

             (a) Certificate - Borrowers. A certificate of the Secretary or the
          Assistant Secretary (or another officer acceptable to Lender) of each
          Borrower certifying resolutions of the board of directors of the
          General Partner as general partner of each Borrower which authorize
          the execution, delivery and performance by each Borrower of this
          Amendment and the other Loan Documents to which such Person is or is
          to be a party.

             (b) Governmental Certificates - General Partner and Each Borrower.
          Certificates issued by the appropriate government official of the
          state of organization of General Partner and each Borrower as to the
          existence and good standing of such Person, and certificates of
          existence and good standing of General Partner as a foreign entity in
          the state of Texas.

             (c) Note-A. Note-A executed by Borrowers.

             (d) Additional Information. Such additional documents, instruments
          and information as Lender may request.

          Section III.2. Additional Conditions. The effectiveness of this
Amendment is also subject to the satisfaction of the additional conditions
precedent that (a) the representations and warranties contained herein and in
all other Loan Documents, as amended hereby, shall be true and correct as of the
date hereof as if made on the date hereof, (b) all proceedings, corporate or
otherwise, taken in connection with the transactions contemplated by this
Amendment and all documents, instruments and other legal matters incident
thereto shall be satisfactory to Lender, and (c) no Event of Default or
Unmatured Event of Default shall have occurred and be continuing.

                                       -9-

<PAGE>

                                   ARTICLE IV.

                 Ratifications, Representations, and Warranties

         Section IV.1. Ratifications. The terms and provisions set forth in this
Amendment shall modify and supersede all inconsistent terms and provisions set
forth in the Agreement and except as expressly modified and superseded by this
Amendment, the terms and provisions of the Agreement are ratified and confirmed
and shall continue in full force and effect. Borrowers and Lender agree that the
Agreement as amended hereby shall continue to be the legal, valid and binding
obligation of such Persons enforceable against such Persons in accordance with
its terms.

         Section IV.2. Representations, Warranties and Agreements. Each Borrower
hereby represents and warrants to Lender that (a) the execution, delivery, and
performance of this Amendment and any and all other Loan Documents executed or
delivered in connection herewith have been authorized by all requisite action on
the part of such Borrower and General Partner and will not violate the
Organizational Documents of such Borrower, (b) the representations and
warranties contained in the Agreement as amended hereby, and all other Loan
Documents are true and correct on and as of the date hereof as though made on
and as of the date hereof, (c) no Event of Default or Unmatured Event of Default
has occurred and is continuing, (d) Borrower is in full compliance with all
covenants and agreements contained in the Agreement as amended hereby, (e)
Borrower is indebted to Lender pursuant to the terms of the Notes, as the same
may have been renewed, modified, extended and rearranged, including, without
limitation, renewals, modifications and extensions made pursuant to this
Amendment, (f) the liens, security interests, encumbrances and assignments
created and evidenced by the Loan Documents are, respectively, valid and
subsisting liens, security interests, encumbrances and assignments and secure
the Notes as the same may have been renewed, modified or rearranged, including,
without limitation, renewals, modifications and extensions made pursuant to this
Amendment and (g) Borrower has no claims, credits, offsets, defenses or
counterclaims arising from the Loan Documents or Lender's performance under the
Loan Documents.

                                      -10-

<PAGE>

                                   ARTICLE V.

                                  Miscellaneous

         Section V.1. Survival of Representations and Warranties. All
representations and warranties made in this Amendment or any other Loan
Documents including any Loan Document furnished in connection with this
Amendment shall fully survive the execution and delivery of this Amendment and
the other Loan Documents, and no investigation by Lender or any closing shall
affect the representations and warranties or the right of Lender to rely on
them.

         Section V.2. Reference to Agreement. Each of the Loan Documents,
including the Agreement and any and all other agreements, documents, or
instruments now or hereafter executed and delivered pursuant to the terms hereof
or pursuant to the terms of the Agreement, as amended hereby, are hereby amended
so that any reference in such Loan Documents to the Agreement shall mean a
reference to the Agreement, as amended hereby.

         Section V.3. Expenses of Lender. As provided in the Agreement,
Borrowers agree to pay on demand all reasonable costs and expenses incurred by
Lender in connection with the preparation, negotiation and execution of this
Amendment and the other documents and instruments executed pursuant hereto,
including, without limitation, the reasonable costs and fees of Lender's legal
counsel.

         Section V.4. Severability. Any provision of this Amendment held by a
court of competent jurisdiction to be invalid or unenforceable shall not impair
or invalidate the remainder of this Amendment and the effect thereof shall be
confined to the provision so held to be invalid or unenforceable.

         Section V.5. APPLICABLE LAW. THIS AMENDMENT AND ALL OTHER LOAN
DOCUMENTS EXECUTED PURSUANT HERETO SHALL BE DEEMED TO HAVE BEEN MADE AND TO BE
PERFORMABLE IN HOUSTON, HARRIS COUNTY, TEXAS AND SHALL BE GOVERNED BY AND
CONSTRUED IN ACCORDANCE WITH THE LAWS OF THE STATE OF TEXAS.

         Section V.6. Successors and Assigns. This Amendment is binding upon and
shall inure to the benefit of Lender and Borrowers and their respective
successors and assigns, except no Borrower may assign or transfer any of its
rights or obligations hereunder without the prior written consent of Lender.

         Section V.7. Counterparts. This Amendment may be executed in one or
more counterparts, each of which when so executed shall be deemed to be an

                                      -11-

<PAGE>

original, but all of which when taken together shall constitute one and the same
instrument.

         Section V.8. Effect of Waiver. No consent or waiver, express or
implied, by Lender to or for any breach of or deviation from any covenant,
condition or duty by Borrowers shall be deemed a consent or waiver to or of any
other breach of the same or any other covenant, condition or duty.

         Section V.9. Headings. The headings, captions, and arrangements used in
this Amendment are for convenience only and shall not affect the interpretation
of this Amendment.

         Section V.10. ENTIRE AGREEMENT. THIS AMENDMENT AND ALL OTHER
INSTRUMENTS, DOCUMENTS, AND AGREEMENTS EXECUTED AND DELIVERED IN CONNECTION WITH
THIS AMENDMENT EMBODY THE FINAL, ENTIRE AGREEMENT AMONG THE PARTIES HERETO WITH
RESPECT TO THE SUBJECT MATTER HEREOF AND THEREOF AND SUPERSEDE ANY AND ALL PRIOR
COMMITMENTS, AGREEMENTS, REPRESENTATIONS AND UNDERSTANDINGS, WHETHER WRITTEN OR
ORAL, RELATING TO THE SUBJECT MATTER HEREOF AND THEREOF, AND MAY NOT BE
CONTRADICTED OR VARIED BY EVIDENCE OF PRIOR, CONTEMPORANEOUS OR SUBSEQUENT ORAL
AGREEMENTS OR DISCUSSIONS OF THE PARTIES HERETO. THERE ARE NO UNWRITTEN ORAL
AGREEMENTS AMONG THE PARTIES HERETO.

         Executed as of the date first written above.

                                          BORROWERS:

                                          CONCORD TECHNOLOGIES, LP

                                          By: OYOG, LLC, its general partner

                                              By:   /s/ Thomas T. McEntire
                                                 -------------------------------
                                                    Thomas T. McEntire
                                                    Vice President and
                                                    Chief Financial Officer

                                      -12-

<PAGE>

                                    GEOSPACE ENGINEERING RESOURCES
                                    INTERNATIONAL, LP

                                    By: OYOG, LLC, its general partner

                                        By:  /s/ Thomas T. McEntire
                                           -------------------------------------
                                             Thomas T. McEntire
                                             Vice President and
                                             Chief Financial Officer

                                    GEO SPACE, LP

                                    By: OYOG, LLC, its general partner

                                        By:  /s/ Thomas T. McEntire
                                           -------------------------------------
                                             Thomas T. McEntire
                                             Vice President and
                                             Chief Financial Officer

                                    OYO INSTRUMENTS, LP

                                    By: OYOG, LLC, its general partner

                                        By:  /s/ Thomas T. McEntire
                                           -------------------------------------
                                             Thomas T. McEntire
                                             Vice President and
                                             Chief Financial Officer

                                      -13-

<PAGE>

                                    OYOG OPERATIONS, LP

                                    By: OYOG, LLC, its general partner

                                        By:      /s/ Thomas T. McEntire
                                           ------------------------------------
                                                 Thomas T. McEntire
                                                 Vice President and
                                                 Chief Financial Officer

                                    LENDER:

                                    SOUTHWEST BANK OF TEXAS, N.A.

                                    By: /s/ Edward K. Bowdon
                                       ----------------------------------------
                                        Edward K. Bowdon
                                        Vice President

         Each of the undersigned Guarantors hereby consents and agrees to this
Amendment and agrees that the Guaranty Agreement executed by such person shall
remain in full force and effect and shall continue to be the legal, valid and
binding obligations of such Guarantor, enforceable against such Guarantor in
accordance with its terms and shall evidence such Guarantor's guaranty of
Note-A, as renewed and extended from time to time, including, without
limitation, the renewal and extension evidenced by Note-A in substantially the
form of Annex "A" attached hereto.

                                    OYOG, LLC

                                    By: /s/ Thomas T. McEntire
                                       ----------------------------------------
                                        Thomas T. McEntire
                                        Vice President and
                                        Chief Financial Officer

                                      -14-

<PAGE>

                                    OYOG LIMITED PARTNER, LLC

                                    By: /s/ Thomas T. McEntire
                                       ----------------------------------------
                                        Thomas T. McEntire
                                        Manager

                                    OYO GEOSPACE CORPORATION

                                    By: /s/ Thomas T. McEntire
                                       ----------------------------------------
                                        Thomas T. McEntire
                                        Vice President and
                                        Chief Financial Officer

                                      -15-

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